Besides the littoral species Ochrosia parviflora, Hensl., that ranges

over most of the archipelagoes of the Pacific from the Solomon Islands

to Tahiti, but is not found in Hawaii, we have in the Pacific, O.

elliptica, Lab., of New Caledonia and Fiji; another species of New

Guinea and the Ladrones; and one or two inland species of Hawaii.

Ochrosia parviflora was familiar to me on Keeling Atoll, in the coral

islets of the Solomon Group, and on the islets and coasts of certain

parts of Fiji. Its fruits, which are dispersed by the currents, were

found amongst the stranded drift of the Keeling and Fijian beaches.

Although usually a coast-tree in Fiji, it came under my notice in one

locality growing inland; and it is a very suggestive circumstance in

connection with the inland species of Hawaii, that in Tahiti this tree

is only described by the French botanists as growing in the mountains at

elevations of 700 to 800 metres above the sea, it having for some reason

abandoned the beach. The process which we thus see in operation in

Tahiti is completed in Hawaii, and we there find a peculiar inland

species far away in the interior of the islands which is placed by

Schumann in the same section of the genus with the littoral O.

parviflora, that is not, however, found in the group. It may be remarked

that Gray describes only one species from Hawaii, O. sandwicensis, but

Schumann makes two species of it—one, O. compta, Sch., peculiar to the

group and referred to the same section as O. parviflora; the other, the

original species of Gray, which he considers as probably a variety of O.

borbonica. These determinations of the German botanist, who had no

theory to serve, are especially interesting. It is with the littoral

trees now missing from the Hawaiian beaches that he compares the inland

species of the group, trees now chiefly characteristic the one of the

Indian Ocean and the other of the South Pacific; and we can scarcely

doubt that originally one littoral tree ranged over both oceans.

Hillebrand describes Ochrosia sandwicensis of Gray as a shrub or small

tree, 6 to 12 feet in height, growing in the open woods of the lower and

middle regions on all the islands. Its dry ellipsoid fruit is two inches

(5 cm.) long, and possesses a thin suberose covering on one side and a

very thick woody endocarp, one-quarter to one-third of an inch (6 to 8

mm.) in depth. The other species which he characterises as a variety is

not so generally distributed in the group. We have to explain not only

how the original species reached the group, but also how they have been

distributed over the islands. The currents could scarcely have

transported the fruits as we now see them. Those of O. sandwicensis have

only a trace of a buoyant covering, and, judging from some fruits that I

examined, they could possess little or no floating power. Even the most

enthusiastic advocate of dispersal by birds must pause here; and there

remains the view, supported by evidence of a striking character, that

the inland Hawaiian species are derived from littoral species that,

having been originally brought by the currents, like O. parviflora in

Fiji, abandoned the beach and took to the mountains, where they have

become differentiated.

It is probable that the lesson of Ochrosia in Hawaii can be applied to

one or two of the other Hawaiian “difficulties,” and that plants that

now set at defiance all the attempts of the student of dispersal to

explain their occurrence in this group may have commenced their

existence in these islands as littoral species brought originally by the

currents and afterwards driven off the beach. One of the greatest

enigmas of the Hawaiian flora is connected with another small

Apocynaceous tree peculiar to the group and described by Hillebrand as

Vallesia macrocarpa and by other Hawaiian botanists as a species of

Ochrosia. Schumann, however, places it in a new genus, Pteralyxia, near

to Alyxia, a genus already in the islands. However this may be, its dry

drupaceous fruits two inches (5 cm.) in length, and its pyrenes almost

as long, could never have been transported as such by the birds of our

own time; and if they could have been carried in the stomach of a bird

given to the dietetic humours of the cassowary, such birds in their

trans-oceanic passages would have left some trace behind in the groups

of the mid-Pacific. In our perplexity we read again the lesson of

Ochrosia.

_Summary of Chapter_ (see end of Chapter XVI.).

CHAPTER XVI

THE RELATION BETWEEN LITTORAL AND INLAND PLANTS

(_continued_)

The Fijian difficulty.—Inland species of a genus possessing

fruits not known to have any means of dispersal through

agencies now at work in the Pacific.—Pandanus.—Its remarkable

distribution in oceanic groups.—To be attributed perhaps

to extinct Columbæ or extinct Struthious

birds.—Barringtonia.—Guettarda.—Eugenia.—Drymispermum.—Acacia

laurifolia.—Conclusions to be drawn from the discussion.—Summary of

chapters XIV, XV, XVI.

SECTION IV

HERE we deal with two genera, Pandanus and Barringtonia, where inland

endemic species occur in the same group with the wide-ranging coast

species, but possess fruits concerning which it is either difficult or

almost impossible to suggest a mode of dispersal by existing agencies.

This section is especially concerned with Fiji, and represents the

peculiar “Fijian difficulty” that is illustrated by other genera as—for

instance, the Coniferous genus Dammara—which are not in any sense

littoral. Further investigation is, however, requisite in the case of

Barringtonia, and to a less degree with Pandanus; and I can only here

point to the general indications of the data at my disposal. We have in

these genera to assume either that the inland species are derived from

the coast species, or that the seeds were brought by one of the extinct

birds of the Western Pacific, by a megapode or by one of the Columbæ, or

by some Struthious bird like the moa or the cassowary, or, if these two

assumptions fail, that there has been a continental connection through

the islands to the westward with the mainland beyond.

PANDANUS.

I take this genus first because the recent monograph on the Pandanaceæ

by Dr. Warburg (Engler’s _Das Pflanzenreich_, 1900) enables me to tread

on relatively safe ground in making my deductions. The three genera of

the order, Freycinetia, Pandanus, and Sararanga, each tell their own

story; and in each and all of them I have taken an especial interest

from the standpoint of their dispersal. Freycinetia is fully discussed

in Chapter XXV., and presents no difficulties respecting its dispersal.

In the discovery of Sararanga the author has had a share. It was first

established by Mr. Hemsley from specimens sent by me to Kew in 1885; and

it has received from the botanist the name given to it by the natives of

the islands of Bougainville Straits in the Solomon Group, where I first

collected it. It contains only one species and was also discovered by

Dr. Beccari, the celebrated Italian botanist, in Jobie Island, New

Guinea. From the other two genera of the order, Pandanus and

Freycinetia, it stands quite apart; and it apparently presents us with a

relic of some ancient flora on the western borders of the Pacific. Its

fleshy drupes (one-half to three-quarters of an inch in size) inclosing

several small osseous pyrenes seem suited for dispersal by birds; and it

is not at first sight easy to understand why its distribution should be

so limited, unless this is connected with its diœcious habit (see

Guppy’s _Solomon Islands_, p. 302; _Journ. Linn. Soc. Bot._ vol. xxx.;

and Warburg’s monograph).

It is, however, with the genus Pandanus that we are here especially

concerned. If the advocate of the previous continental connections of

Fiji and the groups around were to look for evidence in support of his

views, he apparently could not do better than take this genus. Whilst P.

odoratissimus, the littoral species of tropical Asia and Malaya, is

found on the coasts of almost all the Pacific islands from Fiji to

Tahiti and northward to Hawaii, it is only in the archipelagoes of the

Western Pacific, namely, in Fiji and Samoa, that inland endemic species

have been found. (Such species occur also in the more western islands

not dealt with here—New Caledonia, Solomon Islands, &c.) Not even in

Hawaii, with all its botanical evidence of antiquity, has an inland

endemic species been found, although the coast species extends miles

inland, and for nearly 2,000 feet up the mountain slopes. When, however,

we turn to Fiji and Samoa, we find in each group two endemic inland

species. To endeavour to connect the inland species of Fiji and Samoa

with the widespread littoral Pandanus odoratissimus, that owes its

dispersal largely to the currents, is out of the question, at least for

the student of plant-dispersal, since they belong to different sections

of the genus, and in their characters are often far removed (see Note

58).

As regards the agency of birds, it is of course possible that

fruit-pigeons that can disperse the “stones” of Canarium and Elæocarpus

could transport the smaller drupes of Pandanus to oceanic islands like

the Fijis, Samoa, and the Mascarene Islands; and in Note 58 reference is

made to the size of the drupes of the endemic species of Pandanus in

those groups. But my difficulty is that I have not come upon any record

of birds eating these fruits; and I should imagine that amongst living

birds only those like the cassowary and its kin would prefer such a kind

of diet; whilst the only pigeon that could have ever attempted it must

have been able to swallow pebbles like the dodo. It is remarkable that

the Mascarene Islands, the home of the extinct Columbæ, possess more

endemic species of Pandanus than any other groups.

Dr. Warburg points out that, with the exception of some three or four

species dispersed by the currents (P. dubius, P. leram, P. polycephalus,

P. odoratissimus), almost all the species (156 in number) are very

restricted in their areas. When we look at his table of the distribution

of the genus we notice that, excepting the islands of the Hawaiian and

Tahitian regions, nearly all the elevated or mountainous islands of the

tropical and subtropical latitudes of the Indian and Pacific oceans have

their peculiar species, whether in the case of Mauritius, Rodriguez,

Réunion, and the Seychelles in the one ocean, or of Lord Howe Island,

New Caledonia, Fiji, and Samoa in the other. The student here hesitates

even to raise the question of present plant-dispersal in the face of

such evidence of isolation all over the area of the genus. He is almost

inclined to evade the issue and to place the matter beside that of the

dying or extinct Columbæ that have been found in some of these islands,

as in Mauritius, Rodriguez, Réunion, and Samoa.

For reasons above given in the instance of Fiji and Samoa, it would seem

futile to attempt to connect in their origin the inland with the coast

species; and it may be inferred that, excepting the few dispersed by the

currents, the species are in the main inland in their stations. Those

peculiar to Fiji, for instance, occur in the swampy forests of the lower

regions of the interior, as well as high up towards the mountain

summits. When traversing the Fijian forests I often used to speculate on

the modes of dispersal of the plants familiar to me; but the sight of a

strange Pandanus usually brought my speculations to a close. Many of the

enigmas of insular floras would be solved if we could interpret aright

the 156 species of Pandanus that are enumerated and described by Dr.

Warburg in his monograph. Observers like myself obtain little peeps into

the conditions of existence of these interesting plants; and the

travelled botanist, who becomes a systematist in his later years,

attains to a far more extensive view, yet even he can only penetrate the

mystery for a little way.

It is doubtful whether Pandanus odoratissimus, the shore-tree of the

tropical beaches of the islands of the Pacific and Indian oceans, of

Australia, Malaya, and Southern Asia, can aid us much in any one

locality, since its distribution has no doubt been often assisted by

man. Yet it is probable that the currents have played a predominant part

in its dispersal. Its fruits occur commonly in beach-drift, both in the

Indian and Pacific oceans, and are often incrusted with serpulæ,

polyzoa, and cirripedes. At certain seasons the currents bring them to

Keeling Atoll in abundance. When, however, we come to inquire why it is

that this beach species is the only representative of the genus in

Hawaii and Tahiti, we are met with the possibility of its having been

introduced by the aborigines. The tree is almost as useful to a

Polynesian as the coco-nut palm, and it has been cultivated by him in

some of the atoll-groups, as in the Marshall and in the Radack

archipelagoes. In Chapter VII. good reasons are advanced for regarding

it as an aboriginal introduction into Hawaii. When, therefore, we learn

that in the group just named it extends from the sea-coast to nearly

2,000 feet above the sea, that in Samoa it may at times be found at a

similar elevation though usually restricted to the sea-border, and that

in the same way in Tahiti and in Fiji it may leave the coast-region and

extend into the heart of the islands, we are not inclined to look for

any marked differentiation in its character. This indeed appears to be

the case. Numerous varieties in different regions are referred to by Dr.

Warburg; but the only important one in the Pacific islands here

mentioned is a cultivated form from the Marshall Group. A variety from

Hawaii is distinguished chiefly by the smaller size of its drupes.

Assuming, therefore, that the inland species are as a rule not derived

from littoral species originally brought by the currents, and that no

birds of our own time are in the habit of carrying the drupes of

Pandanus to oceanic islands, in order to explain the distribution of

such species we have to choose between the possibility of the agency of

extinct Columbæ and birds similar in their habits and the alternative of

a continental connection. Dr. Warburg, who says but little of the mode

of dispersal of Pandanus drupes, regards the genus as having now two

centres, one in the East African islands (Madagascar, the Mascarenes,

and the Seychelles), and the other in Papuasia (New Guinea, extending

doubtless to New Caledonia). My readers will recall to their minds that

zoologists have at times felt bound to postulate a continent in both the

centres of the genus Pandanus. There is the well-known Lemuria of the

Indian Ocean, and then we have in the Western Pacific Forbes’ Antipodea

and Hedley’s Melanesian Plateau.

Before, however, we accept the indications of the distribution of

Pandanus as favouring a continental hypothesis for either area it is

essential to exclude the agency of the extinct Aves. In this connection

it is of prime importance to notice that the Mascarene Islands are

remarkable, when contrasted with all other oceanic islands, not only for

the predominance of peculiar species of Pandanus, but also as having

been the home of extinct Columbæ like the dodo and the solitaire. The

dodo’s habit of swallowing pebbles of the size of a nutmeg

(_Encyclopædia Britannica_, vii., 322), and the solitaire’s inclination

for swallowing stones as large as a hen’s egg (_Birds_, by A. H. Evans,

p. 331), doubtless represent, as explained below, a capacity for the

dispersal of large fruits and seeds that would be regarded as

“impossible” for distribution by birds now. It is quite possible that at

some time the ancestors of these birds possessed the powers of flight

now owned by the Nicobar pigeon, in the gizzard of which, in the Solomon

Islands, I found quartz pebbles half an inch across (_Solomon Islands_,

p. 324). In the work just quoted I refer on page 325 to the observation

of Messrs. Chalmers and Gill that the Goura pigeon of New Guinea usually

carries a good-sized pebble in its gizzard. We do not, however, seem to

possess any record of extinct Columbæ in the tropical islands of the

Western Pacific. The nearly extinct Didunculus of Samoa apparently

prefers berries and soft fruits. Dr. Reinecke says that it especially

favours the berries of Cananga odorata, the seeds of which are not over

a third of an inch (8 mm.) in length.

It would appear from Mr. Hamilton’s note in the _Transactions and

Proceedings of the New Zealand Institute_ (vol. 24) that the extinct

Struthious birds of New Zealand, as in the case of the moa, carried

crop-stones sometimes as large as a pigeon’s egg. These pebbles are, of

course, swallowed by birds to enable them to crush the hard seeds, and

“stones” of fleshy fruits, on which they feed. In the Solomon Islands I

noticed that the Nicobar pigeon was able in this way to crack the seeds

of Adenanthera pavonina, which for their fracture require a blow with a

hammer. The implication is that the extinct Columbæ were able to

transport to oceanic groups seeds and “stones” which no existing pigeon

could now carry over a tract of ocean. I am inclined to extend this view

also to extinct Struthious birds, and to suppose that they were able,

like the cassowary (see page 152), to fly across tracts of sea in ages

gone by. Though such an agency would come under discussion in connection

with the floras of New Zealand and Madagascar, we have no evidence to

show that birds of this family ever reached the tropical islands of the

open Pacific.

The Megapodidæ of the Western Pacific are a family of birds that suggest

themselves in this connection. Their distribution corresponds with that

of Pandanus in the Western Pacific, excepting the littoral species; and

like Pandanus the Megapodes have “differentiated” in every group. The

limited powers of flight possessed by existing species would unfit them

for crossing wide tracts of sea; but the parent form or forms of all

these species must have been able to traverse broad tracts of ocean.

These birds subsist on fallen fruits, seeds, &c.; but I have no data

relating to them as seed-dispersers.

It is evident from the endemic character of most of the species of

Pandanus in oceanic islands that, except with a few widely-spread

littoral species, the dispersal of the genus has been for ages

suspended. Whether the explanation is to be found in the isolation and

differentiation of the extinct Columbæ of the Mascarene Islands, where

the endemic species of Pandanus are most numerous, has yet to be

established. It seems to offer the only way out of the difficulty,

unless we accept the old view concerned with the continent of Lemuria.

BARRINGTONIA.

There are two littoral species of this genus in the Pacific, B. speciosa

and B. racemosa, both widely spread over the Old World, but only the

first is generally distributed over the Polynesian region reaching east

to Ducie Island, whilst the second does not extend east of Fiji and

Samoa. With the exception of one or two inland species in Fiji and Samoa

no inland species have been recorded from the groups of the open

Pacific, and the genus is not represented at all in Hawaii. If it were

not for a suspicion that the aborigines may have aided in the

distribution of the inland species, the advocate of the previous

continental connections of the islands of the Western Pacific would

receive from their occurrence in these islands considerable support for

his views. The fruits of the inland Fijian species are large, the

smallest being three inches in length; and the agency of birds seems to

be out of the question.

The fruits of the littoral species possess dry buoyant husks that enable

them to be carried by the currents over wide tracts of ocean. Those of

the Fijian inland species display only a trace of these buoyant

coverings and the floating power is much diminished or absent

altogether. These inland species are two or three in number. One of

them, described as a new species by Seemann under the name of B. edulis,

has edible kernels and is sometimes cultivated. A species that I found

growing in the plantations of the Solomon Islanders in Bougainville

Straits may be near the Fijian tree just named (_Solomon Islands_, pp.

85, 297). Its kernels are edible; and I may add that the Solomon

Islanders cultivate other species with edible fruits. We cannot,

therefore, exclude the agency of the aborigines in the distribution of

the inland species of this genus. Horne found an undescribed species in

Fiji, which may be that which I found on the slopes of Mount Seatura in

Vanua Levu, as described in Note 50; and it is quite possible that it

was originally a cultivated tree, though not necessarily within the

memory of the later generations of the aborigines.

This retrocession to the wild state of cultivated plants and the

resulting production of apparently new species is a point on which Dr.

Beccari lays considerable stress in the English edition of his book on

the Great Forests of Borneo. He takes the case of Nephelium and other

fruit-trees and shows how in old clearings, long since abandoned, they

have undergone singular alteration in characters. For these reasons,

therefore, Barringtonia can scarcely be regarded as offering in its

inland species unequivocal evidence of a previous continental condition

of the islands of the Western Pacific. Nor, as shown in Note 50, should

we be justified in establishing a genetic connection between the inland

and coast species; but a great deal of research is needed before we can

handle the numerous interesting problems connected with the genus; and

indeed it cannot be said that the specific limits of the inland

Polynesian trees have been definitely determined, or the species

themselves diagnosed.

SECTION V.

In this section are included those genera where within the same genus

some inland species have been derived from the coast species whilst

others have been originally brought by birds. Guettarda alone belongs

here. In this genus we find, as is so frequently the case, a littoral

tree (G. speciosa) widely spread in the Old World and ranging over the

whole tropical Pacific as far east as Pitcairn and Elizabeth islands,

but absent from Hawaii. Here also as with Pandanus it is only in the

Western Pacific that we find inland endemic species so distinct in

character from the littoral tree that they may be regarded as of

independent origin.

Since, however, there is an inland form of the coast species in Tahiti

(Guettarda speciosa, var. tahitensis) which, according to Drake del

Castillo, is distinguished only by its more rounded leaves and by the

more marked pubescence of the under leaf-surfaces, we evidently have

there an inland species in process of development from the littoral

species. This inland tree is found at elevations as great as 600 metres

or almost 2,000 feet above the sea; and indeed if we follow Nadeaud the

specific differentiation is complete. However, there is no doubt raised

as to its close affinity to the beach tree; and we are almost compelled

for another reason to regard it as a derivative of the shore species,

because, as pointed out in Chapter XXVII., there are very few inland

plants in the Tahitian flora possessing fruits as large as those of

Guettarda that owe their presence in those islands to frugivorous birds.

Of the two inland species of the genus found in Fiji, G. inconspicua and

G. vitiensis, it may at once be said that, as indicated in Dr. Seemann’s

work, their characters are far from suggesting any connection in origin

with G. speciosa, the shore-species, the inland and littoral plants

belonging to different sections of the genus. In their case we can only

look to the frugivorous bird for the explanation of their existence in

the group. The fruits would be probably small; and in this connection it

is to be noted that Mr. H. N. Ridley in his paper on the flora of

Fernando Noronha evidently looks to birds to account for the presence of

a species of Guettarda on the island, a species not found elsewhere.

But another inland Fijian form of Guettarda found by me in Vanua Levu at

elevations of 1,000 to 1,400 feet above the sea, and dubbed by the

natives with the name of the littoral tree (Mbua-mbua), corresponds in

its close relation to G. speciosa with the inland Tahitian form of that

tree, and is to all appearance a derivative of it. It is chiefly

distinguished by its thinner, more hairy leaves, which taper at each end

and are not subcordate at the base as is often the case with the leaves

of G. speciosa. The coverings of the fruit are less fibrous and the

putamen is not so deeply notched or grooved. The difference also extends

to the buoyancy of the fruits in accordance with the principle laid down

in Chapter II. Whilst those of G. speciosa float for many months and are

of common occurrence amongst the stranded drift of tropical beaches, as

for instance in the Keeling Islands, in the Solomon Group, and in Fiji,

those of the inland species float only for a few weeks, their softer

coverings decaying more rapidly in sea-water.

We seem therefore to have had two principles at work in Fiji in

determining the origin of the inland species of Guettarda. Whilst in one

case the inland species is so sharply distinguished from the coast

species as to require the independent agency of frugivorous birds to

explain its presence, in the other the inland form, as in the instance

also of the Tahitian variety, is so much akin to it that the probability

of derivation from it is very great.

SECTION VI.

In this section are contained genera possessing littoral species

restricted to the Western Pacific islands, and dispersed by birds, but

having little or no capacity for dispersal by the currents. They are

regarded as derived from the inland species of the genus in the western

part of the Pacific, and as distributed from thence over the islands in

that part of the ocean. We are here only concerned with Fiji, Tonga, and

Samoa and the neighbouring islands. The genera Eugenia, Drymispermum,

and Acacia are here comprised.

The genus Eugenia, though essentially inland in its station, is apt to

lend species to the beach-flora in different parts of the tropics. Such

species, being dispersed by frugivorous birds and other animals, and

possessing but slight capacity for distribution by the currents, are

usually restricted in their areas. Thus, Schimper (p. 118) names two or

three species, including E. javanica, as amongst the Indo-Malayan

strand-flora. Ridley notices that E. grandis is a common sea-shore tree

in the Malay peninsula; and the author observed two littoral trees of

the genus in the islands of Bougainville Straits in the Solomon Group,

the fruits of one of them that flourished in the interior of the coral

islets being found in the crops of fruit-pigeons. So also in Fiji, some

of the inland species, as E. rariflora, appear at times amongst the

strand vegetation and in the coral islets. There is, however, one Fijian

species found also in Samoa and Tonga that is a characteristic beach

tree, namely E. richii (Gray), and it is more or less confined to that

station. The fruits will float a fortnight in sea-water, which is nearly

twice as long as most other Eugenia fruits will float; and it is quite

possible that the currents may assist the pigeons in distributing the

species. This genus is dealt with more in detail in Chapter XXVI.

The genus Drymispermum (Thymeleaceæ) comprises in the Western Pacific a

number of species, of which two range over the groups of Fiji, Tonga,

and Samoa, whilst some four or more are peculiar to Fiji. All are inland

plants with the exception of D. Burnettianum, a characteristic littoral

shrub of these three groups. Its bright red drupes float only from five

to ten days, even after some weeks of drying; and like those of the

inland species they are well suited for dispersal by fruit-pigeons. This

beach-plant may be regarded as probably an intruder in the strand-flora

from the interior of one of the islands of the Western Pacific, whence

birds, perhaps assisted a little by currents, have carried it to the

neighbouring groups.

The very remarkable coast tree, Acacia laurifolia, alone represents its

genus in the littoral flora of the Pacific islands. It is confined to

the Western Pacific, having been found in New Caledonia, the New

Hebrides, Fiji, Tonga, and Samoa; but it is doubtful whether it is truly

indigenous in all these localities. Thus, in Samoa, though restricted to

the coast districts, as we learn from Reinecke it seldom flowers, and

according to that botanist it was probably introduced through

cultivation. It is, however, evidently regarded by the Samoans as a tree

of their group, as is shown in a curious legend, given by Dr. George

Turner in his latest book on those islands, which I have quoted in my

book on the Solomon Islands, p. 287. Both in Fiji and Samoa it bears the

name “tatangia” or “tatania,” whilst its hard wood was employed for

various purposes, the leaves being used as spoons. The tree flowers and

seeds freely on the Fijian beaches. The pods dry up on the plant, and do

not dehisce, but are apt to break across between the seeds into

article-like portions, the seeds being ultimately liberated by the decay

of the pod or its fragments. The seeds either sink at once or in the

course of a day or two; whilst the pods or their fragments float at

first in sea-water, but all are at the bottom in a week or less. With

its absence of any apparent means of dispersal this small tree presents

quite an anomaly in the strand-floras of the Western Pacific, and can

only be regarded as a loan from the inland flora, though probably of a

very ancient date, and perhaps going back like Acacia koa, the

forest-tree of Hawaii to some early epoch in the history of these

islands.

_The conclusions to be drawn from the discussion of the relations

between the littoral and inland species of the same genus in the Pacific

islands._ (Chapters XIV., XV., XVI.)

In ten of the twenty-two genera here dealt with (Calophyllum, Hibiscus,

Colubrina, Morinda, Scævola, Cordia, Ipomœa, Vitex, Tacca, Casuarina)

the shore and inland species have their own independent modes of

dispersal, usually by currents in the case of coast plants, and by birds

in that of inland plants; and the relations between the two are not such

as to suggest a derivation of one from the other.

In six genera the inland species are regarded as derived from the

littoral species. In two of them, as in Vigna and Premna, where the

coast and inland species occur in the same group of islands and are

connected by intermediate forms, there is direct evidence in favour of

this conclusion; but such a development of inland species need not have

taken place in every group, since in the instance of Premna it has

apparently occurred only in the Western Pacific, and the inland and

coast species have extended independently to the eastern groups through

the agencies of birds and currents.... In the other four genera

(Canavalia, Erythrina, Sophora, Ochrosia) we have presented the

so-called “Hawaiian difficulty,” that group being alone concerned.

Although these genera have no littoral species in Hawaii, they have

inland species in those islands, which are in three genera endemic.

Since these inland species have non-buoyant seeds or seedvessels, the

transport of which by birds half-way across the Pacific Ocean is in the

case of the first three genera unlikely and in the last impossible, it

is assumed that they are all derived from original coast species with

buoyant seeds or fruits, such as are widely distributed over the Pacific

but are not now existing in Hawaii. This assumption, in the instance of

the Leguminosæ, to which the first three genera belong, derives support

from the singular fact in the distribution of the order pointed out by

Mr. Hemsley, that it is wanting in many oceanic islands where there is

no littoral flora.

In one genus, Guettarda, the inland species are regarded as having been

sometimes developed independently of the coast species, and as at other

times derived from it, both principles having been at work in Fiji and

only the last in Tahiti.

In two genera, Pandanus and Barringtonia, which represent the “Fijian

difficulty,” there is no reason on grounds of affinity to connect the

inland with the coast species; and since the agency of existing birds is

improbable in the first genus and out of the question in the second,

whilst the operation of the currents is excluded for the inland species

of both genera, it is assumed that we must either appeal to the agency

of extinct birds, such as those of the Mascarene Islands, or we must

fall back on the hypothesis of a continental connection. In the instance

of Barringtonia it is also possible that some of the inland species may

have been derived from species spread through cultivation.

Lastly, in three genera (Eugenia, Drymispermum, Acacia) the coast

species are viewed as derivatives of the inland flora in the Western

Pacific, not necessarily in Fiji, but it may be in New Caledonia or in

one of the other large groups. In this case the coast species of all

three genera are either unfitted for dispersal by currents, or display

the capacity only in a small degree.

We thus see that in only seven of these twenty-two genera, containing

both littoral and inland species in the Pacific islands, can it be

argued from the standpoint of dispersal that the inland species are or

may have been derived from the shore species; and in most instances the

evidence is largely presumptive in its character. In three genera the

reverse has been the case, and here the coast has borrowed from the

inland flora. In twelve, or more than half of the genera, the shore and

inland species have been evidently independent in their origin. It is

accordingly apparent that in the Pacific the strand flora has lent more

to the inland flora than it has borrowed from it; but with a large

proportion of these coast genera no interchange has taken place.

Two-thirds of the genera of the beach-plants have no inland species, and

in their case the question of such a connection cannot be raised. With

the remaining genera such a relation can be suggested in only two-fifths

of the cases, or in about one-seventh of the total number of beach

genera. Where a connection can be traced, it points more frequently to

the derivation of the inland from the shore plant. Taking all the

evidence together, the beach flora presents itself in the Pacific as

practically independent of the inland flora as regards its origin. It

has received in these regions but few recruits from inland. It has

yielded, except in Hawaii, but few recruits to the inland flora. In this

ocean it bears the stamp of a high antiquity, though in the mass no

doubt of more recent origin than the mangrove flora.

Yet, as I have remarked in different parts of this work, even with the

beach genera possessing no inland species, considerable variety is

displayed in the behaviour of the strand species. Thus, whilst some,

like Pemphis acidula, Tournefortia argentea, and Triumfetta procumbens,

rarely if ever leave the beach, others, like Heritiera littoralis and

Excæcaria agallocha, find a home on the borders of the mangrove swamps,

and one or two extend inland and take their place in the forests, either

as trees (Afzelia bijuga) or as giant climbers (Entada scandens). Others

again, like Cassytha filiformis, Cerbera Odollam, and Cycas circinalis,

with a number of other beach-plants, may invade the interior of the

island wherever arid plains or exposed scantily wooded districts offer

conditions conformable to the xerophytic habit of the beach-plants.

It will thus be perceived that although the inland and coast floras of

an island are in the mass distinct, the line of separation is by no

means always well defined. Beach-plants are something more than

salt-lovers in their ways. They are in the first place xerophilous, or,

in other words, they will be equally at home in exposed situations away

from the coast where the soil is dry and the rainfall scanty. Whenever

these conditions are presented by the districts backing the coast, as we

find for instance in the plains on the lee or dry sides of many a

Pacific island, the shore-plants will often leave the beach and travel

far inland.

_Summary of Chapters XIV., XV., XVI._

(1) Though littoral floras are as a rule chiefly made up of two sets of

plants, one brought through the agency of the currents from regions

outside, and the other derived from the inland flora of the region

concerned, the proportion of the two varies much amongst temperate and

tropical strand-floras, the current-borne plants forming the majority in

the tropics, and those from the inland flora of the region prevailing in

the temperate zone.

(2) There is, therefore, far greater uniformity as a rule amongst

tropical strand-floras than in the temperate zone, since in temperate

latitudes the prevailing constituents of the strand flora vary with the

inland flora of every region, whilst in the tropics the predominant

plants are those ranging far and wide on the shores of the warm regions

of the globe.

(3) Regarding the tropical strand-flora as comprising two formations,

that of the beach and that of the mangrove swamp, the last, which is the

older of the two, may, it is suggested, be viewed as the remnant of an

ancient flora widely spread over the lower levels and coastal regions of

the globe, during an age when, in a warm atmosphere charged with watery

vapour and heavy with mist and cloud, vivipary or germination on the

plant was not the exception but the rule.

(4) But it is contended that even in the beach formation some of the

plants may date back to this age of vivipary, as is indicated by the

anomalous seed-structures of some of the genera, such as Barringtonia,

which seem to indicate a lost viviparous habit.

(5) Since the beach formation of the islands of the tropical Pacific is

largely formed of plants ranging over great areas in the tropics, there

is no reason to expect that it owes much to recruits from the inland

floras of this region. The discussion, therefore, of the relation

between the littoral and inland floras is mainly concerned with the

possible origin of inland from coast plants in these islands.

(6) Yet there are numerous cases of genera possessing both coast and

inland species that are of peculiar interest in determining the true

relation between the beach and inland floras.

(7) As the result of a detailed discussion of these genera, the

conclusion is formed that the beach and inland floras have been in the

main developed on independent lines, the beach flora receiving from the

inland flora but few recruits, and except in Hawaii yielding but few

plants to the inland flora. Only a third of the genera of the beach

flora have also inland species, and in only a few of these genera, or

about a seventh of the whole beach flora, can any question of a

connection between coast and inland species of the same genus be raised.

(8) Two special difficulties arise in this discussion. The first is the

“Hawaiian difficulty,” which is more particularly concerned with genera

of the orders Leguminosæ and Apocynaceæ. Here are genera which possess

both inland and littoral species, but only the first occur in Hawaii. In

the absence of any likely means of dispersal, whether by currents or by

birds, it is assumed that the inland species are derived from shore

plants, originally brought by the currents, that have since disappeared,

a view supported by the fact that Leguminosæ are wanting in oceanic

islands where there is no littoral flora. The second is the “Fijian

difficulty” which is best represented by Pandanus. From our inability to

regard the inland species as derivatives of the coast species, or to

supply them with a means of dispersal, we are compelled to regard them

either as having been a part of the original continental flora of Fiji

or as owing their existence there to the agency of extinct birds having

the habits of the Nicobar pigeon and of the extinct Columbæ of the

Mascarene Islands. Since the Mascarene Islands are noted not only for

their extinct Columbæ but also for their number of peculiar species of

Pandanus, the implication seems to lie against the continental view. The

subject, however, awaits further investigation. In the Western Pacific

the possible agency of the parent forms of the existing species of

Megapodidæ is worthy of attention. Like the Columbæ and Pandanus in the

Mascarene Islands, the Megapodes and Pandanus have “differentiated”

together in the Western Pacific.

(9) The general view of the independent origin of the beach and inland

floras of the Pacific islands is supported by the large number of genera

in the strand flora that only possess littoral species.

(10) Such shore species, together with other strand plants, sometimes

extend into the interior of an island, but only as a rule where the

requisite conditions for a plant of xerophilous habit exist.

(11) Shore plants, it is pointed out, are xerophytes first and

halophytes afterwards; and under certain conditions the purely

xerophilous inclination prevails and the plants travel far inland.

CHAPTER XVII

THE STORIES OF AFZELIA BIJUGA, ENTADA SCANDENS, AND CÆSALPINIA

BONDUCELLA

Afzelia bijuga.—The African home of the genus.—The double station of

Afzelia bijuga, inland and at the coast.—The nature of the buoyancy

of its seeds.—Summary relating to Afzelia bijuga.—Entada

scandens.—Its station and distribution.—Darwin’s opinion of the

plant.—The dispersal of its seeds by the currents.—Summary relating

to the plant.—Cæsalpinia bonducella and C. bonduc.—Their station and

distribution.—Their characters in various Pacific groups.—The

parents of inland species.—Their dispersal by the currents.—The

germination of their seeds.—A dream of vivipary.—The causes of the

seed-buoyancy.—Summary of results.

IN this chapter we have a study of Leguminous strand plants that are of

great interest. It can be safely said that the student of

plant-dispersal in the Pacific will be brought into contact with the

problems here involved wherever he goes.

AFZELIA BIJUGA (Gray).

This Old World tree, which belongs to the sub-family Cæsalpiniæ, is of

great interest to the student of plant-dispersal. It is one of that

large group of Indo-Malayan plants that extend into the Western Pacific,

and give the prevailing character to the floras of such archipelagoes as

that of Fiji. It is a large tree yielding a valuable timber used by the

Fijians and Samoans for many purposes, such as for canoes, house-posts,

clubs, kava bowls, &c., but it has not been recorded from the Tahitian

region, and is unknown from Hawaii. In the fact of its being a littoral

as well as an inland tree, it possesses a peculiar interest from the

standpoint of plant-dispersal, and especially since this difference in

station is associated with a difference in buoyancy, the seeds of the

inland trees usually sinking, whilst those of the coast trees usually

float, and often for a period of months.

A glance at the distribution of the genus will enable us to appreciate

some of the points that will be touched upon in the following

discussion; and it may be here remarked that the explanation of the

distribution of these Leguminous trees will go far to make clear some of

the most difficult points in plant-geography. Of the eleven species

enumerated in the _Index Kewensis_, five belong to tropical Africa,

occurring on both the east and west coasts as well as in the interior,

three are confined to the mainland of tropical Asia, and two are

peculiar to Malaya. In the last place we have the wide-ranging Afzelia

bijuga, which, if it does not actually occur on the east coast of

Africa, is found at all events in Madagascar and in the Seychelles, and

is to be followed by the way of the Chagos Archipelago to the Malayan

Islands and Queensland, and eastward to Fiji and Samoa.

The most suggestive feature in the distribution of the genus is to be

seen in the frequent station of the species by rivers. We learn from

Oliver’s _Flora of Tropical Africa_ that these trees find a home along

river-courses on both sides of the continent, as on the banks of the

Congo, the Niger, the rivers of Senegambia, and the Zambesi, the Zambesi

species being found also on the shores of Lake Nyassa. Since tropical

Africa possesses about half of the species, it would seem highly

probable that it is the home of the genus, and that from the

rain-forests in the heart of the continent rivers flowing east and west

have borne the buoyant seeds of the wandering species to the coasts of

the Atlantic and Pacific Oceans. The operation that I witnessed on a

miniature scale in the case of a _species_ of Entada (E. scandens) in

the Isthmus of Panama, as described in a later page of this chapter, has

been in progress through the ages with the _genus_ of Afzelia in the

breadth of the African continent. According to the principle illustrated

by Afzelia bijuga in the forests of Fiji, the seeds of the African

forest-trees would, as a rule, possess no floating power; but now and

then in the lapse of long periods of time buoyancy in some species would

be developed, and such species would ultimately, through their buoyant

seeds, find their station along the lower courses of the rivers.

To sustain this view it is not necessary that continuous rain-forests

should now clothe the elevated regions in the interior of tropical

Africa; but it is requisite that there should be sometimes a generic

similarity between the plants of the East African and West African

rain-forests; and it is evident that this is the case. Pechuel-Lösche,

as quoted by Schimper (_Plant-Geography_, p. 299), describes the

rain-forest on the Loango coast as covering the mountain ranges and as

extending to the river-plains. In such a locality the operation would be

rapid. In advancing this hypothesis I am referring to the possibility,

however, of such an operation having effected the distribution of

Afzelia in tropical Africa in the past rather than in the present. I

would suggest that botanists in other habitats of the genus, as for

instance in Queensland, might put it to the test of observation and

experiment.

The interest that attaches itself to the story of the genus in its

African home may be extended to the species that forms its outpost in

the Pacific, and we shall see there a littoral species that doubtless

had its home in the interior of a continent endeavouring, with a

considerable measure of success, to become again an inland plant. Horne

(p. 112), who was familiar with Afzelia bijuga at the two extremes of

its range, namely, in the Mascarene Islands and in Fiji, speaks of it as

characteristic of the shores of tropical regions; and Schimper, who

includes it in the Indo-Malayan strand-flora, implies that it is more or

less exclusively confined to the coast and its immediate vicinity (pages

121, 191-2). In the Seychelles, according to Mr. Button, this tree

attains gigantic dimensions on the sandy flats. Still larger trees occur

in the coral islands of the Chagos Archipelago; but in the atoll of

Diego Garcia, as we learn from Mr. Bourne, it is almost extinct only

some four or five trees existing there about twenty years ago, the

increase of the tree being prevented through the destruction of the

fallen seeds by the rats (_Journ. Linn. Soc. Bot._, vol. 22, 1887).

Afzelia bijuga may, therefore, be safely regarded as a littoral tree. We

shall now see the importance of this conclusion when we come to consider

its station in the Pacific islands, where it grows both inland and at

the coast, and we have to decide to which station we must assign the

priority. Speaking of its occurrence in Fiji, Dr. Seemann says it is

“common in the forests all over Viti,” but makes no allusion to it as a

littoral tree either in Fiji or elsewhere. On the other hand, Mr. Horne

(p. 112) describes it as “generally growing on the shore or sandy

beaches, and in rocky clefts, and by the sides of streams in the

interior of Viti Levu and Vanua Levu.” It was on or near the coast in

Fiji that the present writer was most familiar with this tree, sometimes

bordering the sandy beach, at other times growing behind the

mangrove-belt, or again thriving in the half sandy and half swampy soil

of some low islet off the mouth of the Rewa. Especially is it to be

found on those parts of the coast where the hill-slopes descend rapidly

to the beach, or where some lofty spur from the mountains of the

interior reaches the shore. It is also not uncommon on the banks of

rivers both in their lower and upper courses. But it is as a forest-tree

of the interior that it is most valued by both the white men and the

natives on account of the superior quality of its timber in that

station. There, far removed from stream or river, the Vesi, as the

Fijians name Afzelia bijuga, takes its place amongst the lofty

forest-trees, such as the Ndamanu (Calophyllum), the Ndakua (Dammara),

and the Wathi-wathi (Sterculia). It is not often that one finds a tree

in these islands that, like the Vesi, is able to make its home in almost

any station, excepting, however, the “talasinga” or “sun-burnt” regions

of the plains. Wherever tall trees grow gregariously in Vanua Levu, one

will probably find Afzelia bijuga, whether beside a sandy beach, or

bordering a swamp, or on a river’s bank, or on some rocky declivity, or

on the great forest-clad mountain-slopes and plateaux of the interior.

No doubt the same diversity of station is displayed in Samoa, where,

according to Dr. Reinecke, the tree is most frequent in the

“coast-bush.”

From the variety in station it might be expected that corresponding

variations in character would be found. There are differences, such as

in the quality of the timber and in the size of the seeds between coast

and inland trees; but the most important distinction in connection with

the study of the dispersal of the species is to be found in the

circumstance that whilst the seeds of the coast trees are, as a rule,

buoyant, and often float for months, those of the inland trees usually

sink, even after being kept for three or four years. I made a

considerable number of experiments on the buoyancy of the seeds of this

tree in Vanua Levu, and found that with the coast trees, as a rule,

either all the seeds or the majority of them floated in sea-water,

whilst with the inland trees either all of them or the majority of them

sank. The buoyant seeds are able in most cases to float for a long time.

Thus, in one experiment half were afloat after two months, and in

another half were afloat after five months. It is probable that several

of the exceptions, where inland seeds float, will prove to be connected

with an inland station by a river. (I experimented on eight sets of

seeds of coast trees from eight different localities, and found 70 to be

the mean percentage of buoyant seeds. In the same way, four sets of

seeds from four different inland localities gave 13 as the mean

percentage of buoyant seeds.)

As in the case of Entada scandens, there is a rather fine adjustment

between the mean specific weight of seeds and the density of water. If

we place a number of the buoyant seeds in sea-water and begin to lower

the density, some of the seeds will at once commence to float heavily

and afterwards sink; and when the density has been lowered to

approximately that of fresh water, usually about a third will be found

at the bottom of the vessel. Out of 100 coast seeds, 70 will, as a rule,

float in the sea and about 47 in the river; whilst of the same number of

inland seeds, 13 on the average will float in sea-water and 8 or 9 in

fresh water. The bearing of facts of this kind is especially discussed

in Chapter X.

Coming to the causes of the floating-power of the seeds, we find that

with the buoyant seeds the kernel floats, whilst with the non-buoyant

seeds it sinks, the seed-tests in neither case possessing any

floating-power. In this respect, therefore, the seeds of Afzelia bijuga

belong, with the seeds of some other Leguminous littoral plants of the

Pacific islands, such as Canavalia obtusifolia, Erythrina indica, and

Sophora tomentosa, to the second section of the second non-adaptive

group of buoyant seeds (page 107). But though we can in a measure

explain the cause of the buoyancy, we are still ignorant of the manner

in which the difference in the buoyant behaviour of coast and inland

seeds has been brought about. It is possible that this may be connected

with another difference between the coast and inland seeds, the latter

being markedly smaller, and it is noteworthy that in my experiments the

smaller seeds were generally those that sank. (Whilst the inland seeds

averaged between 8/10 and 1 inch, or 20 to 25 mm., in greatest diameter,

12 to 16 being required to make an ounce, the coast seeds measured 1 to

1-2/10 inch, or 25 to 30 mm., and only 10 or 11 were needed to weigh an

ounce.)

There can be no question that the seeds are at times transported by the

currents over wide tracts of sea, and this no doubt explains the

occurrence of Afzelia bijuga in oceanic islands. They may be usually

seen lying free in numbers on the ground beneath the tree or else still

inclosed in the fallen dehiscing and decaying pods; and they might be

swept sometimes into the sea or washed down into an adjacent stream.

They thus came under my notice amongst the stranded beach drift at the

mouths of estuaries in Fiji. But it is remarkable that the seeds have

not apparently been recorded from the beach drift of other tropical

regions. Penzig does not note them amongst the seeds stranded on the

shores of Krakatoa. They did not occur amongst my collections from the

beaches of Keeling Atoll or of the south coast of Java; nor does

Schimper mention them amongst the drift of the Java Sea. In the _Botany

of the “Challenger” Expedition_ the species is not even referred to in

any connection. Although, however, the capacity of these seeds for

dispersal by currents is for the first time established by me, their

fitness in this respect was surmised by Schimper (p. 191), when he

placed the species in his list of tropical shore plants evidently

distributed by the currents.

It will thus be gathered that we have yet much to learn in this matter;

and I would recommend any resident in the tropics to take up this

subject. When indeed we remember the fine adjustment existing between

the specific weight of the seeds and the density of water, and recall

the unknown factor determining the difference in buoyancy between the

kernels of coast and inland seeds, we can understand how under

particular conditions in certain portions of its range the seeds of

Afzelia bijuga may perhaps never possess any floating power. It would

seem, in fact, that the seeds are much more buoyant in the Western

Pacific than they are in the Java Sea; or it may be that the tree is

much less frequent; or that the stranded seeds are soon destroyed by

crabs, such as is the fate of much seed-drift on the Keeling beaches; or

lastly that, as in Diego Garcia, rats in destroying the fallen seeds are

bringing about the extermination of the species.

_Summary relating to Afzelia bijuga._

(1) Assuming that the genus has its home in the African continent, and

that the species have frequently a riverside station, it is argued that

the distribution of the genus on both sides of that continent can only

be explained by its dispersal by rivers from a centre in the interior.

(2) Afzelia bijuga, a widely distributed shore tree of tropical Asia,

occurs in Fiji both at the coast and in the inland forests.

(3) This double station is associated _inter alia_ with a different

buoyant behaviour of the seeds, those of the coast trees usually

floating for long periods, whilst those from inland generally sink.

(4) There can be no doubt that this widely ranging littoral tree has

been dispersed by the currents; but the specific weight of the coast

seeds is on the average but slightly less than that of sea-water; and it

is to this fine adjustment, always liable to be disturbed by variations

in the environment, that the irregularities in the distribution of the

species are to be attributed.

ENTADA SCANDENS (Benth.).

The story of Entada scandens, a plant familiar to many of my readers

under the name of the Queensland Bean, is a story of three continents,

Africa, Asia, and America. From the point of view of its dispersal two

features at once attract attention in the case of this giant-climber; in

the first place its wide distribution over the tropics of the Old and

New Worlds, and in the second place the great capacity of its large

seeds, often two inches across, for dispersal by the currents. But

before discussing these matters it will be necessary to glance at the

distribution of the genus, since much light will thereby be thrown on

some of the numerous difficult points affecting this extremely

interesting tropical plant. Of the thirteen species enumerated in the

_Index Kewensis_, seven are African, three are American, one is Burmese,

one hails from Madagascar, and, lastly, there is the world-ranging

Entada scandens, concerning whose home botanists are not agreed. Most of

the species would seem to be inland plants, whilst Entada scandens

thrives both inland and at the coast. Africa would thus appear to be, as

with Afzelia, the principal home of the genus, but with America as a

subsidiary centre.

In many points Entada scandens presents a parallel to Cæsalpinia

bonducella, another Leguminous tropical plant which occurs also at the

coast and inland. But since they both owe their wide distribution to

their littoral station, it will be as coast plants that they will be

most properly considered in this and the following chapter. Yet if the

student were to regard the distribution of these two plants in a

continental region as in India, where they extend inland to the

Himalayas, he might fail to discern their true station. To accurately

gauge the matter of their station, it is necessary for him to look at

the plants as they occur in the islands of the Pacific. There he will

first see the stranding of the seeds on a shore by the currents, then

their germination and their development into giant-climbers over the

littoral trees or into straggling bushes on the beach; and afterwards he

will observe the plants of both species extending inland, and in these

three stages he will learn their history in the Pacific; but a history,

it may be observed, that in this region represents their efforts to

return to an inland station, such as they once possessed in their

birthplace in some distant region of the globe.

Dealing first with the station of Entada scandens, it may be remarked,

as Dr. Seemann points out, that in Fiji it is most characteristic of the

mangrove-formation. But it also occurs amongst the trees at the back of

the mangrove swamp, on the beaches, on the banks of the estuaries, and

at the edge of the inland forests where they border on the plains.

Sometimes in the company of Derris uliginosa it grows not as a climber,

but as a prostrate plant on the sandy beaches; and here, not being able

to assume its normal habit of a climber, it does not seed. It is to be

found at times far inland in open-wooded districts. Thus in Vanua Levu I

found it growing in the Mbua district four miles inland, and 1,400 feet

above the sea. Reinecke speaks of it in Samoa only in connection with

the “urwald,” or primeval forest. Cheeseman describes it as most

abundant in the interior of Rarotonga, covering the trees with a

wide-spreading canopy of green. In the Malayan region Schimper refers to

it as a plant of the beach-tree formation. In Ecuador and on the Panama

Isthmus it grows not only at the coast, but also on the hill-slopes in

the rear of the mangrove-belt.

With reference to the distribution of the plant, it may be remarked

that, although it is found all round the tropics and possesses great

capacity for dispersal by currents, there are certain difficulties in

explaining its wide area and in accounting for its very peculiar

distribution in the Pacific islands. It was doubtless in allusion to

some of these difficulties that Mr. Darwin, in a letter to Sir Joseph

Hooker, remarked: “Entada is a beast” (_More Letters_, &c., i, 93).

There is at first the question of the identity of the species in the Old

and New Worlds. It is here assumed that it is the same in both

hemispheres; but it must not be forgotten that the identity is “not

beyond doubt” (_Bot. Chall. Exped._ iv, 147).

Then there is the difficulty connected with its occurrence on both

coasts of tropical America. In this respect it is at one with some other

littoral plants, like Ipomœa pes capræ, as well as with the plants of

the mangrove formation, as is pointed out in Chapter VIII. Whilst with

the mangroves it is necessary to assume that they antedate the land

connection between North and South America, this is not requisite in the

case of Entada scandens, since it grows in the interior of the Panama

Isthmus, and rivers on the north and south sides now carry its seeds

seaward from the same “divide” to the Atlantic and Pacific Oceans, as

described in Chapter XXXII.

But, as I have also shown in Chapter VIII, America forms with the West

Coast of Africa a region characterised by the same tropical littoral

flora. This region, on account of the arrangement of the currents,

stands in a very peculiar relation with the Asiatic region, which

comprises the rest of the tropics, and to a great extent possesses its

own peculiar strand-flora. There are a few littoral plants, like Entada

scandens, Canavalia obtusifolia, Sophora tomentosa, and Ipomœa pes capræ

that occur in both areas; but the large majority are confined to one or

other of them, either to the American region, including the African West

Coast, or to the Old World region, which includes the African East

Coast. The American region gives to the Old World, but it can receive

nothing in return. For this reason, it is argued, we are compelled to

regard most, if not all, of the cosmopolitan tropical shore plants that

are dispersed by the currents, such as those above named, as having

their home in the American region. Entada scandens would, therefore,

from this standpoint have its home in America.

Then, again, there is the difficulty connected with the distribution of

this plant on both sides of tropical Africa. Though Oliver in his _Flora

of Tropical Africa_ mentions this species only in connection with the

West Coast, he says it is probably widely spread in that continent, and

he refers to a pod in the Kew Museum indistinctly labelled “Lake Ngami.”

I have not come upon any reference to its being a littoral plant on the

East Coast, but since numerous littoral plants of tropical Asia are

found on that coast its occurrence there or in the East African islands

would be expected. However, as the genus has a centre in America, and as

this species is regarded as of American birth, we are not called upon to

employ the argument used in assigning to a non-American genus like

Afzelia an African home. Since the African West Coast belongs to the

American region of tropical shore plants dispersed by the currents, the

presence of Entada scandens on that coast of Africa can be readily

explained, whilst if it has reached the Malayan Archipelago from America

by way of the Pacific, it would, by extending like many other Malayan

coast-plants along the shores of the Indian Ocean, almost complete its

circuit of the globe. It is in this fashion, I believe, that the other

littoral plants,, like Cæsalpinia bonducella, Canavalia obtusifolia, and

Ipomœa pes capræ, that are found all round the tropics, have performed

the circuit of the globe with America as their home.

One may remark in passing that the double home of the genus in America

and the Old World, though offering a serious difficulty in plant

geography, has no immediate bearing on the present mode of distribution

of Entada scandens. Questions relating to the distribution of tropical

shore-plants that are dispersed by the currents at first resolve

themselves into considerations of the arrangement of the currents.

Entada is not alone amongst the genera containing littoral species in

having a home both in the Old and in the New World. Carapa is another

instance, and additional cases might be cited.

The next peculiarity in the geographical range of this species is

concerned with its irregular distribution in the archipelagoes of the

tropical Pacific. Notwithstanding its great capacity for dispersal by

the currents, although it occurs in all the groups of the Western

Pacific as well as in the Cook Islands, it has not been recorded from

the Society Islands, the Paumotus, the Marquesas, and Hawaii. Since,

however, its seeds have been gathered by Mr. Arundel on the beaches of

Flint Island, lying about six degrees north of Tahiti (_Bot. Chall._ iv,

302), it is not unlikely that it will be found growing in other parts of

Eastern Polynesia south of the equator. One might have looked for an

explanation of its rarity in Eastern Polynesia to the absence of

mangrove swamps, in which, as in Fiji, it is sometimes most at home; but

this is negatived by its abundance in Rarotonga, where mangrove swamps

do not exist.

_The dispersal of Entada scandens by the currents._—This plant offers

one of the most conspicuous examples of the transport of seeds across

oceans through the agency of the currents. In the pages of many

botanical works, from the close of the 17th century onward, reference is

made to the transport of its beans (often in association with those of

Mucuna urens and Cæsalpinia bonducella) by the Gulf Stream or other

currents across the Atlantic to St. Helena, the Azores, the west coast

of Ireland, the Hebrides, the Orkney Islands, the coasts of Scandinavia,

and even as far north as Nova Zembla (see Hemsley’s _Bot. Chall.

Exped._; Sernander’s _Skand. Veg. Spridningsbiologi_, &c.). That the

seeds of Entada scandens retain their germinating capacity after this

ocean-transport has been demonstrated not only by the germination of

stranded seeds on the shores of St. Helena, but also by the germination

when sown at Kew of seeds drifted to the Azores, as well as by the

results obtained by Lindman, who procured the germination of the seeds

of this plant and of Mucuna urens that had been washed up on the

Scandinavian beaches (see Sernander, pp. 7, 390).

One of the most interesting references to the conveyance by currents of

these seeds to the coasts of Europe is to be found in Dr. Sernander’s

recent work on the modes of dispersal of the Scandinavian flora, where

he sums up the results of Lindman’s investigations respecting the Gulf

Stream drift. The stranded seeds of Entada scandens, it appears, have

been found all along the Norwegian coast, but occur most frequently

north of the Söndmöre district. They have even been found in a

sub-fossil condition in the peat-bogs of Tjörn on the Bohuslän coast in

Sweden, having been originally stranded on a beach in that locality at

some distant, but post-glacial, epoch. Few phenomena in

plant-distribution are more suggestive than this ineffectual transport

through the ages of these large tropical beans to coasts within the

Arctic Circle. The seed, no longer under the care of the mother-plant,

becomes a waif, exposed to the pitiless laws of the physical world which

here prevail. It was not thus that the plant was reared, but it is in

this haphazard fashion that its seeds are spread. The philosopher could

unravel most of the tangled problems connected with present and past

plant-dispersal, if he could follow the clue supplied by this stranded

tropical seed on a Scandinavian beach.

It is a far jump from the North Cape to the coral islands of the Pacific

and Indian Oceans; yet it is within the area covered by the drifting

Entada bean. The stranded seeds occur commonly on the Fijian beaches and

on other islands of the South Pacific; but I never found them in Hawaii.

They were gathered by me on the shores of Keeling Atoll in the Indian

Ocean, and on the south coast of Java. Penzig found on the Krakatoa

beaches, in 1897, not only the stranded seed but the established plant.

They came under my notice in numbers on the beaches of Ecuador and on

the Pacific and Atlantic coasts of the Panama Isthmus; and, as I

learned, they are equally common on the other parts of the coasts of

Central America. Not uncommonly these stranded seeds in various parts of

the world are to be found incrusted with polyzoa and tubicular annelids,

which afford proof of prolonged flotation in the sea. These seeds are

also to be frequently noticed floating in the drift of the tropical

estuaries. Thus they came under my observation afloat in numbers in the

Fijian estuaries, in the Guayaquil river, in the estuary of the Chagres

at Colon, and in the mouth of a river on the Panama side of the isthmus.

The mode of liberation of the seeds is worthy of a passing remark. The

huge pods, often several feet in length, ultimately break up into

separate joints bearing the seeds. The joints may decay on the ground,

and the seeds are thus freed; or not infrequently in a mangrove-swamp

they fall at once into the water, and there they float, as may often be

observed in Fijian rivers, until their decay sets free the seed.

The seeds of Entada scandens are often quoted, and justly so, as

striking examples of the dispersal of seeds by currents. Yet in few

plants could the nature or the structural cause of the buoyancy have so

little claim to be considered as adaptive in its character. Quite half,

and sometimes even the majority, of the seeds freshly liberated from the

plant have no buoyancy at all. The mean specific weight of the seed is

about that of sea-water, but markedly higher than that of fresh water;

whilst the principal determining cause of the buoyancy is, as shown

below, purely mechanical, and one that, whilst favouring the wide

distribution of the species, could not be improved by or come within the

scope of Natural Selection.

From experiments made in Fiji and Ecuador, it appears that at least 50

per cent., and often more than half, of the seeds when first liberated

from the pod have no buoyancy in sea-water. Of those that float in

sea-water, a proportion varying between one-third and one-half sink in

fresh water, so that in the case of plants growing on the banks of a

river only about one-fourth or one-third would be carried down to the

sea. So fine is the adjustment of the specific weight of these seeds to

the density of water, a subject discussed in its general bearings in

Chapter X, that if one gathers a number of drift seeds on a beach, let

it be in Fiji or in Ecuador, although, of course, all will float in the

sea, only one-half or two-thirds will float in the neighbouring

fresh-water stream. Those that float appear to be able to float almost

indefinitely. This is sufficiently established by the transport of the

seeds in a sound condition by the currents across the Atlantic, and by

such evidence as the stranding of seeds incrusted with polyzoa and

serpulæ on the beaches of Keeling Atoll. It has been also proved by the

following experiment. Several years since, I placed a seed in a vessel

of sea-water, where it still floated buoyantly in a perfectly sound

condition twelve months afterwards.

With regard to the cause of the buoyancy, investigation shows that

neither the seed tests nor the seed contents have any floating power,

the buoyancy arising from a large central cavity produced by the

shrinking and bending outward of the cotyledons during the drying and

hardening of the maturing seed (see figure in Chapter XII). With the

seeds that sink, this cavity is, as a rule, reduced to small dimensions,

and may be represented only by a narrow slit. In some cases, however,

where the cotyledons are unusually thick and heavy, even a large central

cavity will not give floating power to the seed. There is an indication

in my experiments that seeds from inland plants that have matured their

pods in the forests sink in a much greater proportion than seeds of

coast plants, or of those growing on the banks of estuaries. This we

might expect, since in the shade of the forests the drying process that

accompanies the setting and final maturation of the seed would be less

complete and the intercotyledonary cavity smaller than with the seeds

matured in more exposed situations. This is a point, however, that

requires further investigation.

It will thus be seen that in respect of buoyancy the seeds of Entada

scandens are to be referred to the mechanical or non-adaptive group of

buoyant seeds, described in Chapter XII, which comprises several other

Leguminous strand-plants, including Cæsalpinia bonducella. I especially

studied the various stages in the development of the buoyancy of seeds

in this mechanical group in the case of the species of Cæsalpinia just

named, and the description of the process as given under that plant will

apply to all.

_Summary relating to Entada scandens_

(1) This plant, which has been distributed by the currents over the

tropics of the globe, has its station in the mangrove swamp, on the

beach, by the estuary, and in the inland forest.

(2) It is regarded as an American plant that has reached the shores of

the Indian Ocean by crossing the Pacific, and the coast of West Africa

by crossing the Atlantic.

(3) Its occurrence on both coasts of America is attributed to its having

a focus of dispersal in the forests of Central America, from which its

seeds have been transported by the rivers to the shores of the Atlantic

and Pacific Oceans.

(4) Its irregular distribution in the Pacific islands, to wit, its

absence from Hawaii and its rarity in the Tahitian region, is not to be

easily explained, but it is more than likely that it will be

subsequently recorded from other localities in Eastern Polynesia.

(5) Although the seeds offer a striking example of dispersal by

currents, since they are to be found stranded on beaches over much of

the globe, from within the Arctic Circle to the Coral Sea, in few plants

could the character of the buoyancy and the structure connected with it

have so little claim to be considered as adaptive in their nature. At

least 50 per cent. of the seeds sink in sea-water, and the cause of the

buoyancy of the other seeds is only to be connected with the large size

of a cavity produced by the shrinking of the embryo within the seed

tests during maturation.

CÆSALPINIA

This genus is represented in the tropics of both the Old and the New

World by some eighty species of trees, shrubs, and climbers, some of

which are noted for their dye-woods, and others for the beauty of their

flowers. In the Pacific islands the botanist is only concerned with

three widely distributed species, all more or less littoral in their

station, and in great part dispersed by the currents, namely, Cæsalpinia

nuga (Ait.), C. bonducella (Flem.), and C. bonduc (Roxb.).

With Cæsalpinia nuga we have little to do, since, although widely

distributed in tropical Asia and the Malayan region, and reaching to

both New Guinea and North Australia, it has not apparently penetrated

into the Pacific further east than the Solomon and New Hebrides groups.

I found it growing on the coasts of the larger islands of the Solomon

group, but no observations were made on its mode of dispersal. However,

as its seeds were identified at Kew (_Bot. Chall. Exped._ iv, 311)

amongst my collections of stranded drift from those islands, it would

appear to be to some degree dispersed by the currents, though since it

does not extend far into the Pacific, its capacity for dispersal by this

agency would seem to be limited. Schimper includes it among the

strand-plants of the Indo-Malayan region.

It is with the other two species, Cæsalpinia bonducella and C. bonduc

that we are especially interested. Their extremely hard, marble-like

seeds at once attract attention, and when pale in colour they look not

unlike quartz pebbles as they lie stranded on a beach. The prickly pods

and the recurved prickles of the leaf-branches often make these plants

provokingly evident to a stranger. Though usually to be characterised

when growing on a beach as straggling shrubs, they will often climb

trees when opportunities occur, and they then display themselves as

stout-stemmed climbers. I have seen one or other of them in the mangrove

swamps of Fiji ascending the Bruguiera trees to a height of 30 feet and

more, the stem quite bare below, but leafing and flowering in the

tree-branches above.

From the standpoint of dispersal there are few more interesting plants

in the Pacific islands; but their discussion raises several difficult

questions, and it will be, therefore, requisite to treat them somewhat

in detail. With regard first to the diagnostic characters between the

species, it may be observed that, as a rule, they are sufficiently

evident, such, for instance, as the number, size, and form of the

leaflets, the presence or absence of foliaceous stipules, and the colour

of the seeds, though, as shown below, the seed-colour in the case of

Fijian plants does not always present a constant distinction. Yet as I

found in Fiji the difference between the two species is not in all cases

well pronounced, and intermediate forms occur, about which it is

sometimes difficult to decide to which of the two species they should be

assigned.

Mr. Hemsley remarks (_Bot. Chall. Exped._ iii, 114, 145, 300) that the

two species have been often confused. I venture to think that this has

been in some cases due to the occurrence of these intermediate forms.

One has only to look at the different “distributions” given by botanists

for C. bonduc, as indicated below, in order to suspect that the cause of

confusion has been at times with the plants themselves. When in Fiji I

paid a good deal of attention to this subject, and the results of the

comparison of the foliage and seeds of the plants obtained from fourteen

different localities in Vanua Levu are given below.

It will be seen in this table that I distinguish in Fiji three littoral

forms and one inland or mountain variety, which may perhaps be a

distinct species. Those of the strand include Cæsalpinia bonducella, C.

bonduc, and an intermediate form. C. bonduc is typically distinguished

by its large leaflets, by the absence of foliaceous stipules, and by its

pale yellow seeds; whilst C. bonducella is similarly characterised by

its small leaflets, its foliaceous stipules, and its lead-coloured or

darkish grey seeds. But in the first species the colour of the seeds may

often be yellow mixed with pale-grey, or almost white; whilst in the

second species the seeds may be stained with brownish-yellow patches.

It seemed to me when examining fresh specimens in Hawaii and Fiji that

the ultimate colour of the seed is a good deal determined by the degree

of alteration of the original olive-green colour of the immature seed.

All gradations may be noticed from the olive-green of immaturity to the

yellow, pale grey, and dirty white hues of the mature seeds of

Cæsalpinia bonduc and to the lead or slate-colour of those of C.

bonducella. It almost appeared as if the changes might be compared to

the bleaching which a dark volcanic rock undergoes in the weathering

process through the hydration and removal of the iron oxides.

CÆSALPINIA IN FIJI, TAHITI, AND HAWAII.

A = Locality.

B = Species.

C = Foliaceous stipules.

D = Pairs of pinnæ.

E = Leaflets—Pairs.

F - Leaflets—Length in inches.

G = Leaflets—Form.

H = Seeds—Size in tenths of inch.

I = Seeds—Colour

+--------+---------------+--------+------+------+-------------+-------------------+-------------+----------------------+

| A | B | C | D | E | F | G | 8 | 9 |

+--------+---------------+--------+------+------+-------------+-------------------+-------------+----------------------+

| {| Bonducella |Present | 8-9 | 9-11 |1-1/4 - 1-1/2| Oblong, obtuse |6-1/2 - 7-1/2| Usually lead- |

| {| | | | | | mucronate: base | | colour with at |

| {| | | | | | rounded and in— | | times brownish- |

| {| | | | | | equilateral | | yellow patches. |

| {| | | | | | | | |

|Coast, {| Bonduc |Absent | 5-6 | 4-6 | 2-1/2 - 5 | Oblong, acuminate | 5-1/2 - 6 | Pale yellow |

|Fiji {| | | | | | mucronate, base | | |

| {| | | | | | rounded or |6-1/2 - 7-1/2| Pale grey, sometimes |

| {| | | | | | subcordate | | mixed with |

| {| | | | | | | | yellow. |

| {| | | | | | | | |

| {| Intermediate |Present | 7-8 | 7-9 | 2-3 | Oblong, obtuse | 6-7 | Lead-colour or |

| | | | | | | mucronate, rounded| | pale grey with |

| | | | | | | at base; upper | | brownish-yellow |

| | | | | | | leaflets may be | | patches |

| | | | | | | elliptical | | |

| | | | | | | | | |

|Inland, | Mountain |Present | 5-6 | 9-10 |1-1/2 - 2-3/4| Lanceolate with | 6 | Yellowish or pale |

|Fiji | species | | | | | long tapering | | grey or mixed. |

| | | | | | | aristate apex | | |

| | | | | | | and rounded | | |

| | | | | | | base | | |

| | | | | | | | | |

|Coast, | Bonducella |Present | | | 1/2 - 1-4/5| Oblong | | |

|Tahiti | | | | | | | | |

| | | | | | | | | |

|Inland, | Bonduc |Absent | | 5-6 | | Oblong | | |

|Tahiti | | | | | | | | |

| | | | | | | | | |

|Inland, | Bonducella | | 4-6 | 6-8 | 1-4/5 - 2 | Oblong, obtuse, | 6-7 | Lead-colour. |

|Hawaii | | | | | | not cordate at | | |

| | | | | | | base | | |

+--------+ --------------+--------+------+------+-------------+-------------------+-------------+----------------------+

_Note._—The characters of the Fijian plants are from my own

observations. Drake del Castillo is quoted for Tahiti, and Hillebrand

for Hawaii. Reinecke observes that the pods of C. bonducella in the

inland forests have no prickles.

In Fiji all three coast forms may be found on the same beach, or they

may exist apart. The large-leaved species (C. bonduc) appears to be much

the most frequent in Vanua Levu; and the intermediate form is common

enough to disturb the serenity of the observer’s mind when he is anxious

to diagnose rather than to collect cumbersome specimens. The mountain

form, which came under my notice as a climber in the forest at an

elevation of 1,700 feet on the slopes of Koro-mbasanga in Vanua Levu,

acquires from the lanceolate shape of its leaflets quite a character of

its own, though it comes nearest to Cæsalpinia bonducella. Mountain

forms also occur, as indicated in a later page, in the forests of Samoa

and in Tahiti; but in the first-named group they are referred by

Reinecke to C. bonducella, and in Tahiti by Drake del Castillo to C.

bonduc. In the Samoan forests the inland plants possess pods deprived of

the prickles that are so characteristic of the beach plants. Before one

can pronounce definitely on the relation between the coast and inland

forms in any of the groups, a thorough investigation of the connections

between the two shore-species is needed. I am inclined to think that

they will prove to belong to a single dimorphic (or perhaps polymorphic)

species.

_The distribution of Cæsalpinia bonducella and C. bonduc._—Botanists

agree in giving C. bonducella a distribution around the tropics of the

globe; but they are not at all unanimous with respect to the other

species. According to Mr. Hemsley this species is by no means so

universally dispersed as C. bonducella. It is unknown from Africa and

Australia; but it is generally characteristic of tropical Asia and the

Malay Archipelago. The same authority alludes to specimens in the Kew

Herbarium from Florida and the West Indies (_Bot. Chall._ iv, 300).

Drake del Castillo gives both species a range through the tropics,

whilst Schimper seems in doubt about the occurrence of C. bonduc in the

New World, and Mr. Burkill makes no allusion to its American habitat in

his paper on the Tongan flora. The cause of this confusion is doubtless

to be mainly attributed to the variation in characters of the plants,

and to the occurrence of intermediate forms.

We should be scarcely consistent if we assumed that of two kindred

shore-species dispersed by the currents one had its home in America and

the other in the Old World. The same home must belong to both. According

to the principle laid down in Chapter VIII, and referred to under Entada

scandens, it is held that a strand-plant, with its home in Asia, on

account of the arrangement of the currents could never reach the

American continent, and that American shore-plants are for the most part

native-born except those hailing from the African West Coast, which,

however, lies within the American province of tropical strand-plants.

From this standpoint Cæsalpinia bonducella would be regarded as now

having its home in the New World, and since it is found on both the

Pacific and Atlantic coasts of that continent (as well as on both coasts

of Africa), it is assumed, as with Entada scandens, that it has reached

the African West Coast by crossing the Atlantic, and the African East

Coast by way of the Pacific and Indian Oceans. The genus, I may remark,

is distributed over the tropics of the eastern and western hemispheres.

As regards the general distribution of the two species in the Pacific

islands, it would appear from the writings of Seemann, Hillebrand,

Hemsley, Drake del Castillo, Reinecke, Cheeseman, and Burkill that with

the exception of Hawaii and Samoa, where Cæsalpinia bonducella alone

occurs, and of Rarotonga where C. bonduc alone is found, they are

generally associated in the larger groups, as in Fiji, Tonga, Tahiti,

and the Marquesas.

_The station of Cæsalpinia bonducella and C. bonduc._—Both the species

are to be regarded as littoral plants likely to stray inland. The

first-named is described in the _Botany of the “Challenger” Expedition_

as essentially a sea-side plant, though flourishing equally well inland,

and in India extending to the Himalayas as far as Kumaon, and up to

elevations of 2,500 feet. Schimper speaks of both species as

characteristic of the Indo-Malayan strand-flora, and he quotes Kurz when

referring to C. bonduc as a constituent of the beach-jungle of Pegu.

In the Pacific islands they are typically littoral in their station; but

they may extend inland, and in one or two groups they are only known in

their inland station. Dr. Seemann speaks of both species only in

connection with the beaches in Fiji, and alludes to Cæsalpinia

bonducella (p. 72) as sometimes climbing over the mangroves. In Vanua

Levu both came under my notice on the beaches, and in their immediate

vicinity, usually as straggling bushes, whilst at times they were to be

observed climbing the mangroves at the borders of the adjacent swamp. In

this island of the Fijis they do not, as a rule, stray far from the

beach, and strange to say are not to be ranked amongst those seashore

plants that frequent the “talasinga” regions or inland plains. Judging

from the mountain form found in the forests of Koro-mbasanga, if they

extend inland in Fiji they prefer the forests and become differentiated

in character. In Tahiti, as we learn from Nadeaud and Drake del

Castillo, C. bonducella occurs on the beach and extends inland to the

mouths of the valleys; whilst C. bonduc is only recorded from the

mountains at elevations of 600 to 700 metres (2,000 to 2,300 feet).

Jouan is quoted by Mr. Hemsley as remarking that C. bonduc is as common

in the Marquesas as brambles are in Europe (_Bot. Chall. Exped._ iii,

145). In Rarotonga, according to Cheeseman, C. bonduc is restricted to

the interior. In Samoa, as we are informed by Reinecke, C. bonducella is

frequent both in the coast districts and in the mountain-forests. In the

Samoan mountains the pods lose their prickles, and from this

circumstance, as well as from the extremely widespread distribution of

the species over the islands, the German botanist concludes that the

plant has been for ages established in the group.

In Hawaii, Cæsalpinia bonducella, which alone occurs, rarely figures as

a beach plant; but it is found, as Hillebrand observes, in the lower

plains of all the islands. In the large island of Hawaii I found it not

on the scanty beaches of the coast, but on the partly vegetated surface

of the old lava-flows at distances varying usually between a hundred

yards and a mile from the sea, but extending at times a few miles

inland, and in one locality reaching an elevation of 2,000 feet above

the sea. It was mostly observed by me on the dry side of the island,

where, associated with Erythrina monosperma, the Cactus, and the

Castor-Oil plant, it thrives in very arid localities, where the rainfall

is only a few inches in the year. Farther inland, where the old

lava-surfaces were more vegetated, it was associated with such shrubs as

Osteomeles anthyllidifolia and Cyathodes tameiameiæ. Dr. Hillebrand,

writing of a generation and more ago, says that in his time the plant

was less common than formerly.

_The Methods of Dispersal of Cæsalpinia bonducella and C. bonduc._—We

come now to the modes of dispersal of these plants; and in so doing we

have to choose between the agencies of birds and of currents. The seeds

of C. bonducella are on the average 7/10 of an inch (18 mm.) in

diameter, whilst those of C. bonduc are rather smaller (6/10 of an inch

or 15 mm.). As far as their size and character go, it would seem

scarcely likely that birds could transport these seeds across an ocean;

but our knowledge of the agency of birds is of a very imperfect nature.

Yet their occasional dispersal by birds is not improbable. When I was in

the Keeling Islands the residents informed me that the seeds of C.

bonducella are sometimes found in the stomachs of sea-birds, such as

frigate-birds and boobies. (See Note 59.)

However, it has long been known that the seeds of one or both of these

species are carried great distances by the currents; but it is to be

gathered that the older botanists, in alluding to this fact, more

usually referred under the synonym of Guilandina bonduc to Cæsalpinia

bonducella. De Candolle, loth to attach much importance to the effective

transport of seeds by currents, was compelled to admit this species in

his scanty list of current-dispersed plants (see Note 33). For more than

two centuries it has been known that the seeds of C. bonducella are

carried in the Gulf Stream drift to the coast of Europe from the

American side of the Atlantic; and ever since they were recorded by

Sloane in 1696 as stranded in a fresh condition on the beaches of the

Orkney Islands, they have been found washed up on other localities, as

on the coasts of Ireland and of Scandinavia and on the shores of the

islands of the Western Atlantic. According to Robert Brown, a plant was

raised from a seed cast up on the west coast of Ireland; and with

respect to Scandinavia, Dr. Sernander informs us that the seeds of

Cæsalpinia bonducella, like those of Entada scandens and of Mucuna

urens, are of frequent occurrence amongst the “Gulf Stream products”

stranded on the Norwegian coasts. The seeds of this species are commonly

washed ashore at St. Helena, and there are specimens in the Kew Museum

that were stranded on Tristan da Cunha. (Those interested in the subject

will find it discussed by Mr. Hemsley in the _Botany of the “Challenger”

Expedition_, and also by Dr. Sernander in his recent work on

Scandinavia.)

The seeds of Cæsalpinia bonducella have been also found stranded on

beaches in other parts of the world. Thus Prof. Schimper found them in

the beach-drift of the south coast of Java. Prof. Penzig noticed them

amongst the stranded seeds of the Krakatoa beaches; but it does not

appear that the plant had established itself up to the date of his visit

in 1897, or fourteen years after the great eruption. They have been

picked up on the other side of the Indian Ocean on the east shores of

Africa (_Bot. Chall. Exped._ iv, 300). They came frequently under my

notice stranded on the beaches of Keeling Atoll in the same ocean; and

seedlings sprouting from the seeds were sometimes to be seen growing

amongst the drift just above the high-tide level. The seeds of both C.

bonducella and C. bonduc have been found also on the shores of Jamaica.

Those of both species are not uncommon amongst the stranded drift of the

Fijian beaches; but notwithstanding a careful search I found only a

solitary seed of C. bonducella in the Hawaiian beach-drift, a

circumstance explained below as arising from the usual non-buoyancy of

Hawaiian seeds.

That the seeds of Cæsalpinia bonducella stranded on the coasts of an

oceanic island are able to germinate and reproduce the plant is, of

course, established by the distribution of the species; and we have just

observed that the process was noticed by the author on Keeling Atoll

where the plant has found a home. It is to be noted that the plant

collected by Darwin in this atoll was identified by Prof. Henslow as C.

bonduc; but the plant observed by me was more like C. bonducella, and

the stranded seeds collected by me were referred at Kew to this species.

Some curious considerations arise from the fact that although, just as

in the Keeling Islands, the plants of C. bonducella have evidently

established themselves from drift seed in one locality in the Bermudas,

they do not seem to have done so either on the shores of Krakatoa, or of

St. Helena, where, although they are frequently washed ashore, Mr.

Melliss never met with an instance of germination (see _Bot. Chall.

Exped._ iv, 300, and Penzig). This is doubtless in part the result of

the destructive efforts of the crabs, which, as I have shown in my paper

on Keeling Atoll, nibble off the shoots of many germinating seeds in

beach drift.

The readiness or non-readiness of seeds to germinate on a beach, and the

nature of the conditions essential for the process, are matters that are

directly concerned with their effective dispersal by currents. On

account of the stony character of the seeds of these two species, it

might be expected that germination would only take place under

exceptional conditions. It should, however, be observed that the fine

transverse striæ on their outer surface represent original fissures or

cracks in the epidermis of the soft immature seed; and as such may be

regarded as lines of weakness in the seed-tests. If a pod is opened

before the seeds are mature, we find the seeds about twice the size of

maturity, and so soft that they can be indented by the nail. The

transverse striæ that mark the mature seed are displayed as indistinct

cracks in the epidermis; and if the immature seed is exposed to the sun,

in a few hours these cracks gape widely, and the seed has the grooved

appearance of a top. If a pod opens prematurely on a plant, as sometimes

happens, the immature seeds will be noticed with the epidermis scaling

off. It is evident that the “setting” or the induration of the

seed-coats and the final great contraction of the seed take place in the

pod before dehiscence. From these remarks it would seem probable that

seeds lying exposed to the fierce rays of the sun on a tropical beach

would be liable to develop cracks along the old fissures, and that such

cracks by permitting the entrance of moisture would favour germination.

My experiments show that high temperature under moist conditions will

not of itself induce germination or in any way affect the seed. Thus in

two sets of experiments, in 1890 and 1902, I failed to induce the

germination of seeds which, after floating a year in sea-water, were

kept in moist soil at a high temperature. In one case a temperature

varying from 80° to 110° F. was sustained for several weeks, and in the

other experiment a temperature of 80° to 90° was kept up for five

months. When, however, an incision was made into the epidermis, or the

seed-coats were partially penetrated with a file, the seeds swelled up

in a day or two, and in a few days began to germinate.

The rapid transformation of the stone-like seed into a softened,

swollen, germinating mass ranks amongst the numerous little wonders of

the plant world. The seed, in fact, assumes again the appearance of

immaturity, and in so doing it suggests to us that the rest-stage

exemplified in the hard, pebble-like seed is but an adaptation to

general climatic conditions, and that in a region of great heat and

humidity, where there are no seasons, and where the sun’s rays are for

ever screened off by mist and cloud, it could be dispensed with

altogether. One of my Hawaiian dreams was to establish vivipary in

Cæsalpinia bonducella by subjecting the maturing pod on the plant to

very warm and humid conditions, my expectation being that the soft,

swollen seed would at once proceed to germinate in the pod, and that the

final process of setting, as indicated by the induration and contraction

of the coats, or in other words the rest-stage, would be done away with.

The dream, however, bore some fruits in enlarging my standpoint in the

matter of vivipary, and I have referred to the subject in Chapter XXXI.

The seed-shell, about 1·5 mm. in thickness, consists of three coats: the

outer skin very tough and waterproof; the inner skin seemingly

permeable; and the intermediate layer of hard prismatic tissue, the

“prismenschicht” of Schimper (p. 164). This middle layer absorbs water

rapidly and in large quantity, so that if a fragment of the shell is

placed in water it will be found after a day’s soaking to be three times

as thick as it was in the dry state. If one files a seed, or makes a

small incision, so as to expose the middle layer without piercing the

inner coat, and then places it in water, it will be noticed that the

middle layer at once begins to absorb water; and within a couple of days

the whole seed will swell and attain the size it possessed in the

so-called immature condition. During the process the outer skin

stretches, usually without rupturing; and all three coats, previously so

hard that a heavy blow with a hammer is required to break the seed,

become in a day or two soft enough to be easily cut with a knife. The

seeds thus treated swell in two days to three times their original size

and increase their weight fourfold. Water finds its way to the nucleus

or embryo partly through the dilated inner opening of the micropylar

passage and partly through the inner skin. The nucleus then swells up

into a fleshy mass, filling the seed-cavity, and in two or three days

more germination begins.

I pass now to the discussion of the buoyancy of the seeds. Considering

that both species occur in oceanic islands, and that the currents are

active agents in transporting the seeds, their behaviour under

experiment appears at first sight to be full of anomalies. Thus, it was

ascertained at Kew (_Bot. Chall. Exped._ iv., 301), both with

comparatively fresh and with older seeds, that those of Cæsalpinia

bonducella floated in salt water, whilst those of C. bonduc sank; but in

the record given of the experiment no mention is made of the original

station of the parent plants; and it will be shown later on that the

station of the plant, whether at the coast or inland, has an important

determining influence on the buoyancy.

In Fiji I found that almost without exception the seeds of littoral

plants of Cæsalpinia bonducella floated both in sea-water and in fresh

water. On the other hand, in Hawaii the seeds of this species, obtained

from three typical localities removed inland from the beach, sank

without exception, even after drying for several months; and the only

buoyant seed noted in these islands was a solitary seed collected from

the beach drift. In Hawaii, however, as before remarked, the species is

not strictly a littoral plant, occurring as it does in the lower levels,

but not necessarily in the vicinity of the coast. In the case of seeds

of littoral plants of C. bonduc in Fiji, I found that sometimes all

floated in sea-water and sometimes only a portion of them, whilst their

specific weight was on the whole rather greater than that of the seeds

of the other species. Thus, in one experiment half the seeds floated in

sea-water and a quarter in fresh water, whilst with seeds from another

locality 90 per cent. of the seeds floated in sea-water and 80 per cent.

in fresh-water; and in a third set of seeds all floated in both waters.

The above experiments on Fijian seeds all relate to littoral plants. In

the instance, however, of the inland species from the mountains of Vanua

Levu, all the seeds sank in sea-water, even after being kept for five

years. If we follow the indications of these several experiments we

shall find that Cæsalpinia presents another illustration of the general

principle established in Chapter II that the seeds of inland plants sink

and those of coast plants float.

My data, therefore, show that with the seeds of Cæsalpinia buoyancy goes

with station and not necessarily with species. It is probable,

therefore, that with the two widespread species, C. bonducella and C.

bonduc, varying results will be obtained with seeds from different

localities, whether insular or continental, according to the original

station. The typically buoyant seeds of the former species may, as we

have seen in Hawaii, lose their floating powers when they grow inland;

and the seeds of an inland species from the mountains of Fiji sink at

once. It is essential in interpreting the results of experiments on the

seeds of these plants to be acquainted with the stations; and in this

respect those of the Tahitian plants may be regarded as probable test

cases. We have seen that in Tahiti, C. bonduc is an inland plant, and C.

bonducella usually a beach plant; and I have no doubt that experiments

in that island on the seeds of these two species from the particular

stations just referred to will give results in agreement with the

principle here laid down.

With reference to the duration of the floating powers of these seeds it

may be observed that a seed of Cæsalpinia bonducella, originally found

stranded on the beaches of Keeling Atoll, floated after a year in

sea-water as buoyantly as at the commencement of the experiment. Seeds

of Fijian littoral plants of both C. bonducella and C. bonduc floated in

my experiments after two and a half years’ immersion in sea-water,

showing no change whatever. Some of the seeds removed at the end of the

first year were filed and placed in soil, when they germinated

healthily. In Chapter IX it is pointed out that some buoyant seeds of

other Leguminous plants, such as Mucuna urens, would be apt to germinate

abortively and to sink in crossing the more heated areas of tropical

seas. The seeds of Cæsalpinia, judging from my experiments and

observations noted on page 84, seem to be quite proof against such

risks. This was well brought out in an experiment where seeds of the two

species of Cæsalpinia were kept afloat for two and a half years in a

vessel of sea-water together with seeds of Mucuna and Strongylodon. None

of the Cæsalpinia seeds attempted to germinate in the sea-water; but

with the other genera some of the seeds began to germinate, and sank in

the course of the first warm season, when the water-temperature ranged

from 75 to 90° Fahr.

The seeds develop their buoyancy during the great contraction that, as

before described, marks the final setting of the seed-coats and the

ultimate maturation, as it is termed, of the seed. During this shrinking

process the kernel also shrinks within the seed-tests, and cavities are

thus produced within the seed-shell, on the relative size of which

depends the buoyancy of the seed, neither the seed-shell nor the kernel

possessing independent floating-power. These cavities, as illustrated in

the figures given in Chapter XII, are of two kinds. That usually

produced, being the one that mainly determines the buoyancy, is a large

central hollow caused by the arching outwards of the cotyledons during

the shrinking process, such as is found also in the seeds of Entada

scandens, Mucuna urens, and some other Leguminous littoral plants. With

such seeds the kernel never rattles when the seed is shaken, since the

cotyledons lie in close contact with the seed-shell. The other kind of

cavity is produced between the seed-shell and the kernel by the general

or partial shrinking of the kernel away from the shell, the cotyledons

remaining in apposition, as shown in the figures. When the shrinking

away from the shell is general, the kernel lies loose within the shell,

and the seed rattles when shaken. When the shrinking is partial the

cavity is on one side of the seed and the kernel is fixed.

Professor Schimper (p. 164) remarks that the buoyant seeds of Cæsalpinia

bonducella all rattle when shaken, and that it is to the incomplete

filling of the seed-cavity, thus indicated by the loose kernels, that

the buoyancy of the seed is due. The rattling of the kernel was,

however, quite exceptional in the seeds handled by me, even in the case

of originally buoyant seeds kept for five years. Seeds with loose

kernels were, in fact, more frequent with non-buoyant seeds than with

those that floated. Thus in Fiji I found that whilst with the buoyant

seeds 17 to 20 per cent. had loose kernels, with non-buoyant seeds the

proportion was as much as 60 per cent.

The normal cause of buoyancy is, therefore, a large intercotyledonary

cavity with the cotyledons lying in close contact with the seed-shell;

but the two kinds of cavity may sometimes be combined. Out of a number

of buoyant seeds of Cæsalpinia bonducella examined by me, 80 per cent.

owed their buoyancy solely to a large central cavity (4 to 5 mm.

across). In 6 per cent. it was due solely to the shrinking of the kernel

away from the seed-shell; whilst in 14 per cent. it was to be attributed

partly to a reduced central cavity (2 to 3 mm. wide), and partly to a

space outside the kernel. The only difference noted in the structure of

the buoyant seeds of C. bonduc was that the two kinds of cavities were

more often combined.

The reason of the absence of floating power was clearly indicated in the

non-buoyant Hawaiian seeds, where there was no central cavity, or it was

represented by a narrow slit. The solitary buoyant seed found in the

beach drift had a typical large central cavity. With the non-buoyant

seeds of the inland species of the mountains of Vanua Levu it was

ascertained that two-thirds had loose kernels with the cotyledons

closely appressed. In the others there was a lateral cavity outside the

kernel, the central cavity being only represented by a slit, a hair’s

width in breadth. In the non-buoyant seeds of C. bonduc, the central

cavity was only 2 to 3 mm. wide, and the lateral cavities were small.

Respecting the influence of “station” in producing the differences in

buoyancy, it cannot be said to be connected with the maturation of the

seeds of inland plants under more humid conditions than those which

prevail at the coast. In Fiji some of the littoral plants with buoyant

seeds grow on the mangrove-trees in the shade and humidity of the

swamps; whilst in Hawaii the inland plants of Cæsalpinia bonducella with

their non-buoyant seeds thrive in exposed arid situations in districts

of little rainfall, such as on scantily vegetated lava-flows. With

non-buoyant seeds, where there is little or no cavity, the cotyledons

are always thicker and moister than in the case of the seeds that float.

Though associated with differences in station, as implied in the terms

“coast” and “inland,” the cause of the difference in buoyancy is not

connected with different degrees of humidity, but with some other cause

or causes acting on the spot which, while they favour the drying of the

kernel in coast plants before the seed-coats finally set, impede it in

the inland plants. That the seed does not subsequently acquire floating

power, even after years of drying, was shown in several of my

experiments.

The light, unopened prickly pods of both species float buoyantly, even

when the inclosed seeds have no floating power. In an experiment on

Cæsalpinia bonduc in Fiji the pods remained afloat after a month in

sea-water. With those of C. bonducella in Hawaii I found that they

floated for several weeks, and in one case a pod was afloat after three

months. The pods dehisce on the plant; but they sometimes do not open

sufficiently to allow the seeds to fall out. The pods, however, have to

be torn off from the plant, and are not likely to occur in the drift.

Indeed, they never came under my notice in any locality in the drift,

and as an effective aid to dispersal they must be disregarded. The

buoyancy of the seeds and their well established distribution by

currents render unnecessary an appeal to the floating pod.

The following is a summary of the foregoing remarks on Cæsalpinia

bonducella and C. bonduc.

(1) The two species in Fiji are not always sharply distinguished, since

intermediate forms occur, and here probably lies the explanation of the

confusion that has sometimes occurred in diagnosing the species.

(2) Both are typical littoral plants, distributed over most of the

tropical zone, and occurring in company in most of the Pacific

archipelagoes; but they at times extend far inland.

(3) Though it is not unlikely that sea-birds may have aided in their

dispersal, the oceanic currents have been the great agencies in their

dispersal, as is indicated by the frequent transport of seeds in the

Gulf Stream drift across the Atlantic, and by their occurrence in beach

drift in various parts of the world.

(4) Having regard to the present arrangement of the currents and the

distribution of the two species, reasons are given for the belief that

their original birthplace was in the interior of the American continent.

(5) Notwithstanding the stony hardness of the seeds, when a notch is

made in the outer skin a seed rapidly takes up water, and in a few days

it becomes a soft and much swollen germinating mass. The author is

inclined to think that this was the original condition of the seed, and

that the rest-stage is an adaptation to secular differentiation of

climate in later epochs.

(6) Unlike the seeds of other Leguminous littoral plants, those of

Cæsalpinia are not likely to germinate abortively when floating in warm

tropical seas, a risk that restricts the distribution of several

littoral species.

(7) As tested by experiment, the seeds of both species are often able to

float unharmed for years; but on the other hand seeds not infrequently

have no floating power.

(8) Observation, however, shows that buoyancy goes with station, and

that the general rule here applies that the seeds of coast plants float

and those of inland plants sink.

(9) The nature of the influence of “station” on the seed-buoyancy is

obscure; but it is evidently not connected with the usual differences

between coast and inland localities, such as those concerned with

exposure or shade, dryness of soil, relative humidity, and similar

contrasts.

(10) The buoyancy of the seed is developed during the final shrinking

process associated with its maturation, a large cavity between the

cotyledons being usually produced.

--------------

_Note._—Since most of the principal conclusions of this work are

involved in my especial study of the littoral species of Afzelia,

Cæsalpinia, and Entada, the reader is advised, if he wishes to form an

opinion of the author’s method of investigation, to read this chapter

carefully through. With most other shore-plants, though in not a few

cases studied with the same detail, the exigencies of space have often

limited me to the employment of the general results in the appropriate

chapters without entering into details. Should he desire to test any

view of his own relating to plant-dispersal, he could not do better than

begin with the materials here provided.

CHAPTER XVIII

THE ENIGMAS OF THE LEGUMINOSÆ OF THE PACIFIC ISLANDS

Leguminosæ predominate in tropical littoral floras.—The anomalies of

their distribution in the Pacific islands.—They conform to no one rule

of dispersal or of distribution.—Strangers to their stations.—The

American home of most of the Leguminous littoral plants.—Summary.

IT is my intention here to gather up some of the “ends” of the great

tangle presented by the Leguminosæ in the Pacific. When we look at the

indigenous phanerogamic floras of Fiji, Samoa, Tahiti, and Hawaii we

find that the Leguminosæ form 5 or 6 per cent. of the total in each of

the three first-named groups, and only about 2·5 per cent. in Hawaii.

The paucity of Leguminosæ in oceanic floras was long ago pointed out by

Sir Joseph Hooker, whose work forms the foundation of much of our

knowledge of insular plant-life. This is emphasised by Mr. Hemsley in

his volume on the _Botany of the “Challenger” Expedition_ (Introd. p.

25), where he makes the very significant remark that the Leguminosæ are

wanting in a large number of oceanic islands where there is no truly

littoral flora. The islands, however, here more especially referred to,

are those of the southern Atlantic and Indian oceans, such as St.

Helena, Tristan da Cunha, and Amsterdam. It is especially true of New

Zealand, where the Leguminosæ barely make 2 per cent. of the total. Of

the Polynesian islands, as he points out, it is not so correct; and, in

fact, the proportion found in the Fijian, Samoan, and Tahitian floras,

respectively, is much the same as that which characterises the British

flora, namely, 5 to 6 per cent.

When we come to explain the paucity of the Leguminosæ in the Hawaiian

flora we bring to light the singular principle that _Leguminosæ are far

more characteristic of the littoral flora than of the inland flora of a

Pacific island._ About half of the Leguminosæ of Fiji and Tahiti are

coast plants; and about 30 per cent. of the littoral plants of the

islands of the tropical Pacific belong to this order. Since, therefore,

Hawaii possesses much fewer shore-plants (30) than does Tahiti (55) or

Fiji (80), the paucity of its Leguminous plants is readily accounted

for.

We have next to notice a principle, which is, in fact, deducible from

the first, namely, that _buoyant seeds are much more characteristic of

the Pacific Leguminosæ than of any other order_. Three-fourths of the

species have buoyant seeds, and, in fact, about a third of the littoral

Polynesian plants with buoyant seeds or fruits belong to this order.

It may, therefore, be inferred that _the Leguminosæ owe their presence

in the islands of the tropical Pacific mainly to the currents_.

From Mr. Hemsley’s conclusion that the Leguminosæ are wanting in a large

number of islands where there is no truly littoral flora, the

presumptions arise that _when inland species exist that possess no

capacity for dispersal by currents they are to be regarded as

derivatives from the littoral flora, and that they owe their origin to a

strand-plant possessing buoyant seeds originally brought by the

currents_. It has been shown in the case of Afzelia bijuga and of

Cæsalpinia that when Leguminous shore-plants extend inland the seeds

often lose their buoyancy, and it is probable that divergence in other

characters may occur, leading, as in the mountains of Fiji, to the

development of a new species of Cæsalpinia. It is urged that by a

continuation of the same process the inland species, Erythrina

monosperma, has been developed in Tahiti and Hawaii, and the inland

species, Canavalia galeata and Sophora chrysophylla, have been produced

in the last-named group. All these species have non-buoyant seeds, and

in all three cases there is no littoral species in Hawaii, it being

assumed that the parent strand-plant has been driven inland from the

beach. _It is not necessary that the littoral species should be now

represented in the flora._

It is remarkable that _in almost all cases the cause of buoyancy is of

the non-adaptive or mechanical kind, due either to cavities formed by

the shrinking of the seed-nucleus during the setting of the seed or to

the light specific weight of the kernel_. There is but little to show

that the buoyancy of the seeds of Leguminosæ is anything but an

adventitious character of the seed, as far as its relation to dispersal

by currents is concerned. Although this capacity has been the great

factor in the wide distribution of the species, yet it is evident that

Nature here takes advantage of a quality that could never by its aid

become a specific distinction. The upshot of the selecting process would

be the dispersal by the currents of nearly empty seeds or seeds that

have lost their germinating capacity.

The distribution of the Leguminosæ in the Pacific islands, and indeed of

tropical islands generally, is often full of inconsistencies. This is

the only order that sets at nought most of the principles established

for the other plants of the sea-coast, and that defies the application

of the laws of plant-dispersal now most in evidence. Take, for instance,

the inexplicable affinity of Acacia koa, the well-known Koa tree of the

Hawaiian forests, to Acacia heterophylla, a tree restricted to the

Mascarene islands of Mauritius and Bourbon. Mr. Bentham, who placed them

in the same group with three or four Australian species, even doubted

whether the difference between the Hawaiian and Mascarene species

amounted to specific rank. These two closely related Acacia trees of

far-separated islands of the Indian and Pacific Oceans represent

outliers of the great formation of phyllodineous Acacias that have their

home in Australia (_Introd. Chall. Bot._ p. 26). As far as I can gather

Acacia seeds have no known means of dispersal. Not even when the tree

has a littoral station, as in the case of Acacia laurifolia in Fiji,

have the seeds or pods any capacity worth speaking of for dispersal by

currents. We must appeal to the birds; but to what birds we may ask,

unless it be to the extinct Columbæ and their kin, or to the Megapodes.

Some of the other Hawaiian difficulties connected with the inland

Leguminosæ are repeated in the Mascarene Islands. Thus, Bourbon, like

Hawaii, has its inland species of Sophora of the section Edwardsia.

In their irregular distribution the Leguminosæ of the Pacific islands

are often a source of perplexity to the student of plant-dispersal.

Take, for example, the inland Erythrina, E. monosperma, of Hawaii,

Tahiti, and perhaps New Caledonia. Then look at the singular

distribution of the Sophoras of the Edwardsia section in Chile and Peru,

Hawaii, New Zealand, Further India, and Bourbon. The botanist, again,

finds a climber like Strongylodon in the forests of Fiji, Tahiti, and

Hawaii, and he picks up the seeds on the beaches of those islands and

notices that they float unharmed for many months in the sea, yet when he

pays heed to the distribution of the genus he finds that it only

comprises four or five species, and that it occurs outside the Pacific

only in the Philippines, Ceylon, and Madagascar. The extraordinary

distribution of Entada scandens in the Pacific islands has been before

alluded to in these pages. Here we have a plant, the seeds of which are

known to be transported unharmed by currents all round the tropics. Yet

it is absent from Hawaii and from almost all of the islands of Eastern

Polynesia. In many cases an endeavour has been made in this work to

explain these difficulties. But the order in the Pacific teems with such

difficulties. We may ask with astonishment why it is that the genera,

and sometimes even the separate species, of the Leguminosæ seem so often

to follow in each case a principle of their own.

Plants of this order in the Pacific conform to no one rule of dispersal

or distribution, whether we regard a species, a genus, or the whole

order. Take, for instance, the presence in Hawaii of Canavalia galeata,

a plant that, as we know it now, could not possibly have reached there

through the agency of the currents, and the absence from the same group

of Entada scandens that could have been readily transported there by the

currents from America. Or, if we take the whole order and look at the

structures connected with the buoyancy of the seeds, we find two types

of structure and the elements of a third. Then, again, whilst most

littoral plants with buoyant seeds retain the buoyancy of their seeds

when they extend inland, Leguminous shore-plants, like Afzelia bijuga

and Cæsalpinia bonducella, when they extend inland in Fiji and Hawaii,

lose in great part or entirely the floating power of their seeds.

Furthermore, most strand-plants, being typically xerophilous in

character, when they extend inland shun the forests and prefer the dry

soil and sparsely vegetated surface of the open plain; but the

Leguminous genera and species (Mucuna, Afzelia, Entada, &c.) when they

leave the coast take to the forests, growing usually as stout lianes,

but sometimes as tall trees. Here again the Leguminosæ seem to follow a

principle of their own. As far as I know, this is the only order in the

Pacific possessing forest-trees which, as in the case of Afzelia bijuga

in Fiji, are equally at home in the woods of the interior and of the

coast.

Indeed, judging from Professor Schimper’s observations, the littoral

Leguminosæ of the tropics often display a physiological constitution

that seems in some respects out of touch with their surroundings. They

may, as in Sophora tomentosa and in Canavalia, present the xerophytic

character of strand-plants, but frequently they are not halophilous or

“salt-loving,” like other plants associated with them on the same

shore-station. They are often shy of salt in their tissues, though able

to thrive in salt-rich localities. That capacity which strand-plants

usually possess of storing up chlorides in their tissues, and especially

in their leaves, without injury to themselves, is but slightly possessed

by such characteristic shore-plants as Canavalia, Pongamia glabra, and

Sophora tomentosa. This capacity, which, as Professor Schimper

indicates, goes to determine whether or not plants are capable of living

in salt-rich localities, has often no determining influence with the

Leguminosæ. (See Note 60.)

Though the plants of this order form such a large element in the

strand-flora of the Pacific islands and of the tropics generally, they

seem in other respects, besides those just referred to, to act as if

they were strangers to the station. Look, for instance, at the readiness

of the floating beans of Mucuna, Strongylodon, &c., to germinate, as

shown in Chapter IX, in the tepid waters of the warmer areas of the

tropical oceans. This is a great deal more than a disturbing factor of

distribution. It is significant also of the plants being out of touch

with their dispersing agencies.

One may notice in conclusion the fact brought out in Chapter VIII that

nearly all the littoral plants dispersed by the currents that are common

to the Old and the New Worlds belong to the Leguminosæ. This is held to

indicate that their home is in America, since that continent distributes

but does not receive tropical littoral plants dispersed by currents.

_Summary._

The Leguminosæ are far more characteristic of the littoral flora than of

the inland flora of the Pacific islands; and since the greater number of

them have buoyant seeds, it follows that this order mainly owes its

presence in this region to the currents.

As it has been shown that in a large number of islands where there is no

littoral flora the Leguminosæ are wanting, the presumption arises that

when, as in Hawaii, inland species occur which at present have no

capacity for dispersal by currents, they have been derived from

strand-plants originally brought by the currents, even though such shore

species no longer belong to the flora.

As far as its relation to dispersal by currents is concerned, the

buoyancy of the seeds of Leguminosæ is merely an adventitious character,

and the structure connected with it has no specific value.

Plants of this order in the Pacific are a source of much perplexity and

conform to no one rule of dispersal, whether as regards their

disconnected distribution, their means of dispersal, the structural

cause of buoyancy, the loss of buoyancy of inland species, and in other

particulars. Even in their physiological constitution they are often at

variance with the bulk of littoral plants when they grow on the

sea-shore, since typical beach-plants of the order, though thriving in

salt-rich localities, are shy of salt in their tissues.

It is probable that whilst the Pacific islands have derived most of

their littoral plants that are dispersed by currents from the tropics of

the Old World, they have received most of their strand Leguminosæ from

America.

CHAPTER XIX

THE INLAND PLANTS OF THE PACIFIC ISLANDS

PRELIMINARY COMPARISON OF THE PHYSICAL CONDITIONS OF HAWAII, FIJI, AND

TAHITI

Introductory remarks.—The tranquil working of the winds and currents

contrasted with the revolutionary influence of the bird.—The Hawaiian,

Fijian, and Tahitian groups.—Their surface-areas and elevations.—Their

climates.—The mountain climate of Hawaii.—The rainfall of the three

groups.—Summary.

INTRODUCTORY REMARKS.

I WILL carry my readers back to that moment when we began to investigate

together the composition of the floras of the islands of the tropical

Pacific from the standpoint of dispersal. It will be remembered that

after collecting all the fruits and seeds of a particular island we

placed them in sea-water, and that some nine-tenths of them went to the

bottom at once or in a few days. We found, speaking generally, that the

buoyant seeds and fruits belonged to coast plants, whilst those at the

bottom of the vessel proved to be obtained from inland plants. Since

that period we have been occupied in following up the clue supplied by

the floating seeds and fruits. In their company we have travelled far

beyond the Pacific islands. We have not only seen their fellows in other

parts of the tropics, both on the coral atoll and on the continental

coast, but we have met their representatives on the beaches of Europe

and of temperate South America. We have followed them in their ocean

traverses round most of the tropical zone, and on the way we have

naturally interested ourselves in the question of the currents. We have

weighed these seeds and fruits and have compared their specific weight

with that of sea-water. We have cut them up and carefully examined them,

and under their guidance we have explored the mangrove-swamps both of

Polynesia and of Ecuador, and have penetrated the mysterious _cul de

sac_ of vivipary. Having formed our opinion of them, we now bid the

subject farewell, and stand once more on the same Pacific beach where,

it seems so long ago, our investigations began.

For the seed and fruits lying at the bottom of the sea-water we have to

appeal to other agencies than to that of the currents if we wish to

inquire into their means of arriving at this island. In imagination we

leave the reef-lined shores for the interior, and exchange the

exhilarating surroundings of a coral beach, where “the sky is always

blue and the wind is always true,” for the arid conditions of an inland

plain, or for the humid conditions of the forest, where the rain is

incessant and the cloud-cap and mist seemingly eternal. When we look at

the motley collection of fruits and seeds obtained in such localities,

we are at a loss to know where to take up the clue. After vainly

endeavouring to obtain some inspiration as to the manner of commencing

the inquiry, we do what all good naturalists in the Pacific islands do

from force of habit when they meet with difficulties of any kind—we sit

down and light our pipes. Then come a flood of old memories and old

trains of thought that came to us years before on some mountain-top or

in a shady gorge or on some river-bank, in regions Pacific and

non-Pacific, and by degrees our ideas shape themselves and we begin to

think the matter over in an orderly fashion.

When the winds first brought the spores of ferns to this Pacific island,

the ocean currents brought the seeds and fruits of littoral plants, and

the birds transported the seeds and “stones” of various inland species.

All three agencies have been working side by side since the earliest

stage in its history. Yet it is only in the work of the wind and the

current that we find any indication of stability in the floral history

of the island. With the work of the bird it has been very different.

Since the first bird carried seeds to this locality all else has been

turmoil and change. Wave after wave of migrant plants has overrun the

interior of the island, and all have left their mark; but the great

distributing factor and disturbing agent has always been the bird.

Genera have been born and have disappeared, and in their place new

genera have arisen. Whole families even have participated in the

revolutions of the plant-world, and species have grown rankly in the

great confusion. Last of all came man with his cultivated plants and his

weeds, introducing new elements of change and discord into the island,

and often upsetting the floral economy altogether. The history of man’s

most troubled epoch would not be more full of catastrophes and great

events than the history of the plants of this Pacific island. Yet

through all these changes the winds and currents have been quietly

carrying on their work, bringing the same plants to beach and hillside

that they did before the age of unrest began.

The monotonous character of an island flora that has been supplied by

the winds and currents can be readily imagined. For their variety the

floras of the Pacific islands are mainly indebted to the bird, the great

disturber of the peace of the plant world. We cannot attach too much

importance to the contrast in the results produced by these several

agencies in stocking a Pacific island with its plants. On the one hand

we have the tranquil working through the ages of the winds and currents.

On the other hand there has been the revolutionary influence of the

bird. One cannot doubt that many of the species of flowering plants now

growing on the beach and many of the ferns on the upper mountain-slopes

have witnessed changes within the forest-zone of the island, such as an

antediluvian might record if he had lived through the ages to the

present time.