From not having a personal acquaintance with this region it is not

possible for me to discuss the extension of the shore-plants inland

except in a general way. From the pages of the work of Drake del

Castillo we can, however, infer that several plants such as Cassytha

filiformis, Cerbera Odollam, Colubrina asiatica, Hernandia peltata,

Morinda citrifolia, and Pandanus odoratissimus have extended inland to

the mouths of the Tahitian valleys, and have ascended the lower slopes

of the hills that lie near the coast. Others, like Cæsalpinia Bonduc,

Gyrocarpus Jacquini, and Ochrosia parviflora, have climbed far up the

mountain-sides to elevations of from 2,000 to 2,400 feet above the sea.

It is also evident from Mr. Cheeseman’s memoir on the Rarotongan flora

that coast plants also stray inland in that island. In an island like

Rarotonga, where a sorry substitute for a mangrove-swamp exists in the

form of a few coastal muddy places occupied by Vitex trifolia and

Sesuvium Portulacastrum, Entada scandens takes to the hills; and thus it

is that in this island it is most abundant in the interior, climbing to

the tops of the highest trees and “covering acres of the forest with a

dense canopy of green.”

_Summary of the Chapter._

(1) The Tahitian region possesses most of the plants that frequent the

sandy beaches of the Pacific islands.

(2) But it lacks the mangroves and the associated plants of the

mangrove-swamp.

(3) It also wants many of the plants that grow in the vicinity of such

swamps.

(4) But since the plants last-mentioned often possess fruits or seeds

capable of being carried great distances by the currents, their absence

is to be attributed to the necessary conditions being lacking on account

of the failure of the mangroves.

(5) Most of the beach plants, however, owe their existence in this

region to the transport of their buoyant fruits or seeds by the

currents.

(6) The negative features of the Tahitian strand-flora are mostly to be

connected with the absence of Rhizophora and Bruguiera, the pioneers of

the mangrove-swamp; and their absence is, in turn, to be attributed to

the inability of their floating seedlings to reach this region in a fit

condition for establishing themselves.

CHAPTER VII

THE HAWAIIAN STRAND-FLORA

Its poverty.—Its negative features.—Their explanation.—The subordinate

part taken by the currents.—The Oregon drift.—The inland extension of

the beach plants.—Summary.

COMPARED with the rich strand-flora of Fiji, that of Hawaii presents but

a sorry aspect. In the number of species (30) it does not amount to

half; whilst it lacks the great mangrove-formation and the luxuriant

vegetation accompanying it that gives so much character to the shores

and estuaries of Fiji. Strangely enough, it is also deprived of most of

the familiar trees that, whether in foliage, in flower, or in fruit,

form the chief attraction of the sandy beaches of the Pacific islands.

Neither the mangroves, therefore, nor the plants of the intermediate

formation, are to be found in Hawaii; and when we reflect that the

absentees from the beach formation include most of the trees, under the

shade of which the visitor to the Pacific islands can nearly always find

protection from the fierce rays of a tropical sun, it cannot be a matter

of surprise that this littoral flora has such a poverty-stricken

appearance. We look in vain for such shady beach trees as Barringtonia

speciosa, Terminalia Katappa, and Hernandia peltata; and we are lucky if

we find some small trees under which we can obtain a scanty shade.

I have been speaking, of course, of the indigenous shore-plants, those

that have arrived at these islands without the assistance of man. Yet it

must be added that the existing littoral flora does include some of the

missing indigenous trees, though rarely in any number. There is,

however, scarcely one of them that is regarded by Dr. Hillebrand as

having formed part of the original flora. That botanist would indeed rob

the present beach flora, scanty as it is, of most of its conspicuous

plants, as far as their claims to be considered indigenous are

concerned. Dr. Hillebrand indeed includes Calophyllum Inophyllum,

Hibiscus tiliaceus, Thespesia populnea, Morinda citrifolia, Cordia

subcordata, and Pandanus odoratissimus in the present Hawaiian flora,

and nearly all of them are to be found at times at the coast as well as

inland; but he regards all, excepting the last-named, as having been

introduced by the aborigines. I was not inclined at first to go quite so

far as Dr. Hillebrand in this direction; but he carefully considered the

case of each individual plant, and, remembering his sojourn of twenty

years in the islands, his authority cannot be lightly put aside. In the

list of Hawaiian strand-plants given in Note 28 there are several

species not always littoral in the group, but typically littoral in

other tropical regions. One species, Ipomœa glaberrima, Boj., has not

been recorded before from these islands.

A strong reason in favour of the contention of this botanist is that all

the trees above-named are useful in some way to the natives; and,

indeed, when we look at the works dealing with the floras of the islands

of the South Pacific, we observe that in almost all the groups one or

other of these six trees bears the reputation of having been introduced

by the aborigines. All of them in their turn lose their fame as truly

indigenous plants in some group or other. The occurrence of two or three

useless South Pacific beach trees, that are known to be dispersed by the

currents, in the indigenous strand-flora of Hawaii, would go far to

invalidate Dr. Hillebrand’s argument, since the six trees in dispute are

also known to be dispersed by the currents. But such trees are not to be

found; and we look in vain for trees like Cerbera Odollam, Guettarda

speciosa, Gyrocarpus Jacquini, and Hernandia peltata, that are spread

far and wide over the beaches of the South Pacific.

It is also of interest to notice how trees like Morinda citrifolia and

Terminalia Katappa, concerning the non-indigenous character of which

there can be but little doubt, are in our own day acquiring a littoral

station. The second is not even regarded by Dr. Hillebrand as having

been introduced by the natives, but is referred by him to the European

epoch. After having been extensively planted, it is now, as I found,

becoming a littoral tree on the coast of Oahu, and supplies its buoyant

fruits in a regular way to the beach drift. Its native name of Kamani is

merely that of Calophyllum Inophyllum. All the six trees in dispute are

known in Hawaii by the names by which they are distinguished far and

wide over the South Pacific, a fact of which the reader may satisfy

himself by referring to my paper on Polynesian plant-names. The

Hawaiians, when their ancestors abode in the South Pacific, must have

been well acquainted with one or other of the prevailing names of

Terminalia Katappa (Talie, Tara, &c.); but it had lapsed in the memory

of the race when the Europeans introduced the tree into Hawaii.

It may be added in this connection that Dr. Hillebrand weakens his

argument by regarding Pandanus odoratissimus as of pre-aboriginal origin

or as truly indigenous. Like the other six trees in question, its fruits

are known to be capable of dispersal far and wide by the currents; and

if this species of Pandanus is indigenous, we are obliged to assume that

its fruits were first brought by the currents. That being so, we cannot

exclude the probability of the currents having been also effective with

several of the other plants regarded by Hillebrand as of aboriginal

introduction, more especially those with large fruits like Calophyllum

Inophyllum, and Cordia subcordata, where the alternative agency of

frugivorous birds would be impracticable, at least over a wide extent of

ocean. Pandanus odoratissimus is, as I venture to think, a tree that was

introduced ages since by the aborigines. Next to the Coco palm, few

trees have been more utilised by island-peoples, more particularly

perhaps in the ruder stages of their history.

This point has been discussed at some length, because on the correctness

of Dr. Hillebrand’s view depends the explanation to be subsequently

given of the origin of the shore-flora of Hawaii. Though differing in

some details, my observations on the Hawaiian coast plants, which are

given in Note 29, tend to strengthen his contention.

I now return to the consideration of some of the negative features of

the Hawaiian strand-flora, and will allude first to the absence of the

mangroves and of the numerous other plants that live in and around a

mangrove-swamp. This cannot be connected with a total absence of

suitable stations. Although it is true that there are but few large

rivers and but few suitable localities, yet such localities exist. The

shores of Hilo Bay might readily have been the home of a mangrove-swamp;

and one can point to different places on the coast of Oahu, such, for

instance, as Pearl Harbour, which in Fiji would have been occupied by a

luxuriant growth of mangroves. The same argument applies to the missing

beach trees, such as Barringtonia speciosa, Hernandia peltata, Guettarda

speciosa, &c., that adorn the beaches of many a coral island or of many

a coral-bound coast in the South Pacific. Although in a large island

like Hawaii with its lava-bound coasts but few white calcareous beaches

exist where we might expect to find such a flora, yet such beaches occur

wherever the scanty coral reefs are found off the coast; and it is just

in those localities, as is pointed out in the account of my observations

in Note 29, that the “plantes madréporiques” of the French botanists,

the plants of the coral atoll and of the reef-girt coast, make their

best endeavours to establish themselves. In other islands like Oahu,

where coral reefs are more developed, calcareous beaches are more

frequent, and there the few “madreporic” plants of Hawaii make a home.

Nor can the deficiencies in the Hawaiian strand-flora be connected with

climatic conditions. That its meagre character cannot be so explained is

indicated by the manner in which the Indo-Malayan shore-plants have

pushed their way northward on the western side of the Pacific to the

Liukiu and Bonin Islands. Here in latitude 26-27° N. we find several

Fijian littoral trees and shrubs, such as Hernandia peltata, Pemphis

acidula, Pongamia glabra, Sophora tomentosa, Terminalia Katappa,

Tournefortia argentea, &c., that do not occur in Hawaii, although this

group is some degrees nearer the equator, namely, in latitude 19-22° N.

They are accompanied by the mangroves (Rhizophora, Bruguiera, &c.) in

strength as far as South Liukiu in latitude 25° N.; but we learn from

Dr. Warburg that the mangroves thin off further north, though they reach

to South Japan, where Döderlein found in latitude 32° N. solitary

examples of Rhizophora mucronata. These interesting facts of

distribution, which are taken from Schimper’s work on the Indo-Malayan

shore-plants (pp. 85, 90), show us that we can scarcely look to climatic

conditions for the explanation of the absence of mangroves and of many

other tropical littoral plants from Hawaii. We form the same opinion

when we regard the extension northward of the mangrove-formation on the

American coasts of the North Pacific Ocean. According to the account of

Dr. Seemann given in the “Botany of the Voyage of H.M.S. _Herald_,” the

mangroves with the coco-nut palm, and many other littoral plants common

on the western shores of tropical America, reach their northern limit a

little north of Mazatlan within the mouth of the Gulf of California in

latitude 24° 38ʹ N. The parallel of 25° N. latitude, as indicated in

Drude’s Atlas, probably represents the extreme northern limit, which is

thus five or six degrees north of the latitude of the large island of

Hawaii.

Neither can the explanation be found in the deficient floating powers of

the seeds or seedvessels of many of the “absentees.” Those of

Barringtonia speciosa, Guettarda speciosa, Heritiera littoralis, the two

species of Terminalia, &c., possess great buoyant powers equal to, and

probably often exceeding, those of the plants that, like Ipomœa pes

capræ, have succeeded in establishing themselves in Hawaii. One has only

to look at the lists giving the results of flotation experiments in

Notes 2 and 3, in order to realise that there are very few of the

“absentee” littoral plants, the non-existence of which in Hawaii could

be attributed to deficient floating powers of the fruit or seed. Being

able to float unharmed for months, and in several cases even for years,

the seeds or fruits of the shore-plants unrepresented on the Hawaiian

beaches have been carried far and wide by the currents over the tropical

Pacific even to Ducie and Easter Islands, that is, as far as the islands

extend.

The only plants about which one could express a doubt concerning their

ability to reach Hawaii through the agency of the currents, and to

establish themselves there, are the true mangroves of the genera

Rhizophora and Bruguiera. Since germination takes place on the tree, it

is only through the floating seedlings that they could reach these

islands; but, as shown in Chapter XXX., it is doubtful whether the

seedlings would be in a fit condition for reproducing the plant after

such a long oceanic voyage. If they had been as successful in

establishing themselves in Hawaii as they have been in the Liukiu

Islands, which lie in latitude a few degrees farther north, these two

species through their reclaiming agency would alone have prepared the

way for the whole mangrove formation. We have seen in the preceding

chapter that the absence of the mangrove formation from Tahiti appears

to be mainly due to the failure of the pioneer species of Rhizophora and

Bruguiera to establish themselves there. This evidently also applies to

Hawaii, the cause of their exclusion being connected neither with

climate nor with station, but as in Tahiti with the general unfitness of

the floating mangrove seedlings for crossing broad tracts of ocean

without injury to the growing plantlet.

With regard, however, to the bulk of the “absentee” littoral plants,

those of the beach-formation, no such incapacity on the part of the

buoyant seed or fruit can be accepted. These plants, which have reached

Tahiti in numbers, have in the mass failed to reach Hawaii. It will,

therefore, be of interest to glance at the character of the fruits of

the “absentee” trees, which a traveller fresh from a visit to the coral

islands and reef-girt coasts of the South Pacific sadly misses on the

Hawaiian beaches. We notice in the first place that the absent trees,

such as Barringtonia speciosa, Cerbera Odollam, Guettarda speciosa,

Heritiera littoralis, Terminalia Katappa, &c., have large fruits which

could only have been carried to Hawaii by the currents, the agency of

birds being quite out of the question. On the other hand, almost all the

littoral plants of Hawaii, whether trees, shrubs, or herbs, which are

regarded as truly indigenous by Mann, Hillebrand, and other Hawaiian

botanists, have only small fruits or seeds available for dispersal, from

which the agency of birds cannot, on the point of size, be excluded.

Amongst these shore plants possessing buoyant seeds or fruits are

Cassytha filiformis, Colubrina asiatica, Ipomœa pes capræ, Scævola

Kœnigii, Vigna lutea, and Vitex trifolia; whilst amongst the plants with

non-buoyant fruits or seeds are to be reckoned Heliotropium anomalum, H.

curassavicum, Tephrosia piscatoria, Tribulus cistoides, &c. The seeds or

seedvessels of the plants of the buoyant group possess great floating

powers; and it seems at first sight scarcely credible that the currents

which have failed to establish Barringtonia speciosa, Guettarda

speciosa, and the other trees that through this agency have often found

a home on the remotest islands of the Pacific, should have succeeded in

the instances of plants like Scævola Kœnigii and Vitex trifolia.

It would indeed almost seem that in nearly all cases where it would be

impossible in point of size for a bird to transport the fruit or seed of

a shore-plant to Hawaii, such a plant is not to be found in the

strand-flora of that group, even though it is well adapted for dispersal

by the currents. Many of the littoral trees missing from the Hawaiian

coast-flora, having large buoyant fruits, come into this category; and

grave suspicion is thus apparently cast on the agency of the currents in

the case of the plants with small fruits and seeds that really compose

the strand-flora, even when their capacity for sea-transport has been

well established by observation and experiment. The efficacy of the

currents would thus seem to be called into question for the whole

littoral flora of Hawaii.

If, however, we were to adopt such a sweeping conclusion we should be

led into an error. It is pointed out in the following chapter that

nearly all these large-fruited beach trees that are found far and wide

over the South Pacific, but are absent from Hawaii, do not occur as

indigenous plants in America. If, therefore, the fruits of such Old

World littoral trees as Barringtonia speciosa, Cerbera Odollam,

Guettarda speciosa, Ochrosia parviflora, Terminalia Katappa, &c., that

could be dispersed only by the currents, have failed to reach Hawaii, it

is essential to remember that they have also failed to reach America.

This suggests that Hawaii may have received some of its littoral plants

from America through the agency of the currents; and it is shown in the

following chapter that, as a rule, when a South Pacific plant with

buoyant fruits or seeds is not found in America, it is equally absent

from Hawaii. The question thus acquires quite a different aspect, and we

shall accordingly have to regard tropical America in the next chapter as

a possible centre of diffusion of littoral plants over the globe, a

centre possibly as important as that connected with the tropics of the

Old World.

Although, however, the currents have played a part in stocking the

Hawaiian beaches with their plants, their share in the work has been

unimportant, and the number of plants concerned is limited. If we take

away the seven or eight littoral plants introduced by the aborigines, as

well as the three endemic species as indicated in the list in Note 28,

and then remove from the residue the plants with small fruits or seeds

possessing little or no buoyancy, there remain only the following eight

species, the presence of which in Hawaii might be attributed to the

currents, namely, Cæsalpinia Bonducella, Cassytha filiformis, Colubrina

asiatica, Ipomœa glaberrima, Ipomœa pes capræ, Scævola Kœnigii, Vigna

lutea, and Vitex trifolia. Of these plants, three species, those of

Cassytha, Scævola, and Vitex, possess fruits that would be likely to

attract frugivorous birds, and are in some cases known to be dispersed

by them (see Chapter XIII.); so that we are not in these instances

restricted to the agency of the currents. With the other five the

currents offer the readiest explanation, but, as is indicated in the

cases of Cæsalpinia Bonducella and Ipomœa glaberrima (Chapter XVII.), it

is quite possible that birds have occasionally intervened. Altogether we

may infer that in stocking the Hawaiian beaches with their littoral

plants the currents have taken a subordinate part.

Coming to the Hawaiian littoral plants having seeds or fruits that have

no floating power, we find that they present a motley group. It has been

already remarked that this is the group of shore plants that derives

most recruits from the inland flora, and that it is in this group that

the differences between the shore-floras of tropical regions find their

expression. Yet a very odd collection of plants is here exhibited.

Sometimes the beach-flora is composed in great part of these plants; and

a sorry spectacle is presented by a beach possessing such plants as

Gossypium tomentosum, Heliotropium anomalum and H. curassavicum,

Lipochæta integrifolia, Tephrosia piscatoria, Tribulus cistoides, &c.

Yet to the student of plant-distribution such a motley collection would

be full of suggestiveness. From the circumstance that species of

Cuscuta, Jacquemontia, and Lipochæta, that are peculiar to the Hawaiian

Islands, have made their homes on the beach, he would infer that since

Nature has been compelled to borrow from the endemic inland flora, there

has been some difficulty in stocking the beaches with their plants. The

occurrence of endemic species amongst the strand-plants would be viewed

by him as especially indicating incapacity on the part of the ocean

currents.

Yet in the quantities of drift timber, showing evidence of many months

and probably even of years of ocean-transport, to be seen stranded on

the weather coasts of these islands, the observer discerns undoubted

evidence of the efficacy of the ocean currents. But what he finds are

huge stranded pine logs of “red-cedar” and “white-cedar” from the

north-west coasts of America. He may search the drift for days together,

as I have done, and discover no tropical fruits or seeds except such as

could be supplied by the present Hawaiian flora. The subject of this

drift is especially discussed in Note 30; and it need only be mentioned

here that it is not improbable that, as shown in the next chapter, some

drift may reach Hawaii from tropical America under exceptional

conditions, and that its presence is masked by the Oregon drift.

The agency of the drifting log in carrying small seeds in its crevices

would be effectual in the instance of plants from the temperate coasts

of North America. For example, the nutlets of Heliotropium curassavicum,

which have no buoyancy, might easily be washed, together with sand, into

the cracks of a pine log stranded temporarily on the Oregon coast where

this plant occurs. The _modus operandi_ was brought home to me when

examining the drift brought down by the Chancay River on the coast of

Peru. Here I found this species of Heliotropium growing on the margin of

a swamp near some stranded logs, that would probably be carried out to

sea when the river was next in flood.

It is probable, I may add, that the seeds or fruits of some of the

plants of the non-buoyant group of the Hawaiian littoral flora may be

dispersed in birds’ plumage. For instance, the spiny fruits of Tribulus

cistoides sink in sea-water; but they are well suited for entangling

themselves in birds’ feathers.

It is possible that the hairy seeds of Gossypium tomentosum may have

been thus distributed; but there is much that is enigmatical about this

plant (see Chapter XXVI).

THE INLAND EXTENSION OF THE BEACH PLANTS OF HAWAII.—When we regard the

inland extension of littoral plants in Hawaii, we get fresh indications

of the meagreness of the strand-flora. Several of the species, as

Cæsalpinia Bonducella, Cassytha filiformis, Tephrosia piscatoria, &c.,

show themselves only occasionally on the sandy beaches, though they are

common enough on the old scantily vegetated lava-flows near the coast

and are often found miles inland. Indeed, Dr. Hillebrand not

infrequently in describing the station only gives prominence to the

situation of the plants away from the beaches, and places most of them

on the old lava plains that extend inland from the coast. It is only by

a detailed examination of extensive coast lines in these islands that I

have succeeded in preserving to a small degree their reputation as beach

plants. A few of them behave somewhat strangely in their inland station.

Thus, the seeds of Cæsalpinia Bonducella obtained from various

localities showed no buoyancy in my experiments; and had I not found a

solitary buoyant seed in the stranded drift I should have inferred that

this was a rule without exception.

It is to be remarked that whilst some plants like Scævola Koenigii

occasionally stray a few hundred yards inland on the surface of the old

lava-flows, others like Ipomœa pes capræ and Vitex trifolia, that are

spread far and wide over the inland plains of Fiji, are confined in

Hawaii to the beaches and their immediate vicinity. Some of the plants

like Hibiscus tiliaceus, Morinda citrifolia, and Pandanus odoratissimus,

that are regarded as having been introduced by the aborigines, behave

exactly like indigenous plants in the inland plains; but this is not

necessarily an indication of an indigenous plant in this group, since

the Cactus (Opuntia Tuna) and the Castor-Oil Plant (Ricinus communis)

have spread all over the drier lower regions of the islands, whilst

Aleurites moluccana, the Candle-Nut Tree, which has no means of reaching

these islands without man’s agency, now forms entire woods on the

mountain slopes, usurping the place often of the original forests....

Further details relating to this subject are given in Note 31.

The principal points in the foregoing discussion of the strand-flora of

Hawaii may be thus summed up:—

(1) The indigenous, that is, the pre-aboriginal, strand-flora of this

group lacks not only the mangroves and their associated plants, but also

most of the characteristic beach-trees of the South Pacific, which are

known to owe their wide distribution in tropical regions to the

currents.

(2) The meagreness of the littoral flora is intensified by the tendency

of some of the plants to extend inland and to desert the coasts, and by

the occurrence on the beaches of peculiar species not found outside the

Hawaiian Islands.

(3) The absence of the mangrove formation and of so many of the typical

beach trees of the Pacific cannot be attributed either to the lack of

suitable stations, or to climatic conditions, or to deficient floating

power of the seed or fruit.

(4) As in the case of Tahiti, the mangroves and their associated plants

are lacking because the floating seedlings of Rhizophora and Bruguiera,

the pioneer plants of a mangrove-swamp, have failed to reach Hawaii in a

fit condition for establishing themselves. The numerous plants that

accompany a mangrove-swamp have thus been unable to find a home, though

the buoyant powers of their fruits or seeds are often great.

(5) With the missing beach-trees, however, which possess fruits that can

float for years unharmed in sea-water, no such incapacity is suggested.

Most of them have large fruits, which could only reach Hawaii through

the currents. This absence from the Hawaiian indigenous strand-plants of

most, if not all, of the large-fruited species, where on account of size

the agency of birds is absolutely excluded, is very remarkable; and it

at first seems to throw grave suspicion on the efficacy of the currents

for the whole strand-flora.

(6) It is, however, to be noticed that these large-fruited beach trees

have not only failed to reach Hawaii but have also failed to reach

America. The question thus acquires quite a different aspect, and

America becomes the possible source of most of the Hawaiian plants with

buoyant seeds or fruits.

(7) This subject is discussed in the next chapter; but it is here shown

that at their best the currents have taken but a secondary part in

stocking the Hawaiian beaches with their plants, since many of the

plants have non-buoyant seeds or fruits.

(8) The drift stranded on the shores of the Hawaiian Islands is composed

of logs from the north-west coast of North America. No drift from the

south has been discovered; but it is not unlikely that future

investigators will find some seed-drift from tropical America.

[Illustration:

THE WORLD

SHOWING

OCEAN CURRENTS

John Bartholomew & Co., Edin^r.

]

CHAPTER VIII

THE LITTORAL PLANTS AND THE CURRENTS OF THE PACIFIC

The working value of the currents as plant-dispersers.—The relation

between the currents and the distribution of shore-plants.—The clue

afforded by the American plants.—Two regions of tropical shore-plants,

the American and the Asiatic.—America, the home of the cosmopolitan

tropical shore-plants that are dispersed by the currents.—Hawaii and

the currents.—Summary.

ACTIVE as the currents are in dispersing seeds and fruits over the

Pacific, it should be remembered that those plants that owe their

distribution to this agency are only shore-plants, and not, indeed, all

the shore-plants, but only those with buoyant seeds or fruits. Even the

coral atoll owes a great deal to the agency of the fruit-pigeon and of

other birds; for instance, their species of Ficus, Eugenia, and Pisonia.

In order, therefore, not to form an exaggerated notion of the efficacy

of the currents, it will be necessary to obtain some numerical idea of

what they have really accomplished in transporting seeds and seedvessels

over the oceans in a state fit for successful germination on the shores

upon which they are stranded. It is requisite to make this proviso,

because in some cases the currents work to no purpose. Thus, the empty

nuts of Aleurites moluccana are carried far and wide over the Indian and

Pacific Oceans, and are stranded on the beaches of the various islands,

as I have found myself in the cases of Keeling Atoll, Java, and Fiji.

The Coco-de-Mer, or the Double Coco-nut Palm, is another apt instance.

Though its fruits have been carried far and wide over the Indian Ocean,

the species is restricted to the Seychelles. So also the acorns of

various species of Quercus are widely but ineffectually distributed by

the currents both in temperate and tropical regions. (This subject of

useless dispersal is dealt with in Chapter XIII.)

It is essential to bear in mind at the outset that for their inland

plants the Pacific islands can draw on the floras of a relatively large

portion of the globe. Such plants, having as a rule fruits or seeds that

sink in sea-water, or are incapable of floating for long periods, could

only have arrived at these islands, where man’s interference is

excluded, through the agencies of winds and birds, assisted by other

lesser agencies, as those of bats, insects, &c. On the other hand, for

their littoral plants, which are for the most part dispersed by the

currents, the source of supply is very restricted. The shore-plants with

buoyant seeds or fruits of the islands of the tropical Pacific, that are

here dealt with, number only about seventy, and it is not likely that

this number will be greatly increased, since, whatever may be the

deficiencies in our acquaintance with the inland floras of these

islands, we have a fairly complete knowledge of the strictly littoral

plants.

I do not suppose, indeed, that the number of such plants with seeds or

fruits capable of being transported unharmed over wide tracts of sea

would much exceed 100 for the whole Indo-Pacific region from India to

Tahiti. Professor Schimper gives a list containing 117 tropical plants

distributed far and wide over the shores of this region, and made up of

species dispersed by currents, birds, and man. Taking a liberal

estimate, not over two-thirds of the plants mentioned in this list are

dispersed by currents. Then, again, if the flora of a coral atoll, like

that of Diego Garcia or of the Keeling Islands, is taken as affording an

index of the work of the currents, the number of plants dispersed by the

currents would appear to be indeed restricted, since in either case

their indigenous flowering plants, including those of both the buoyant

and non-buoyant groups, do not exceed fifty.

About twenty years ago, Mr. Hemsley, who, in his work on the botany of

the _Challenger_ Expedition, prepared the way for the investigation of

this subject, made a list of not less than 120 plants, almost all

tropical, that are “certainly or probably dispersed” by the currents

(Introd. _Chall._ Bot., p. 42). This is admittedly only a preliminary

list, and as the result of recent investigations some plants have to be

omitted and others to be added; but I doubt whether, numerically, it is

far below the mark. The relative efficacy of the currents seems to have

been first systematically discussed by De Candolle in his _Géographie

Botanique_, which was published in 1855. Data were then very scanty, and

out of a list of nearly 100 inter-tropical species (Old World plants

found in the New World and New World plants found in the Old World) he

designates nine only as exclusively dispersed by the currents. Even this

list, in one respect, needs correction (see Note 33); but it is of

interest to note that this eminent botanist from the first never looked

upon the agency of the currents as a very important factor in

plant-dispersal; and, finding in the specially directed and carefully

performed experiments of Thuret confirmation of his views, he reiterated

his opinion in a note to that author’s paper in 1873 (cited in Chapter

III.).

However, De Candolle was quite right in minimising the effect of

currents on the distribution of plants. His extensive survey of the

plant-world from the standpoint of dispersal gave him that sense of

proportion in assigning values to dispersing agents which enabled him to

feel his way almost intuitively, even where exact data were often

lacking. It is, however, a little disappointing to find such a slight

treatment of the subject in Kerner’s great work on the Natural History

of Plants, though one can scarcely controvert his opinion that the

dispersion of plants, as a whole, is not appreciably affected by this

process. Numerically speaking, this is in the main correct; yet it is

here that the genius of Schimper led him to recognise and to mark out a

line of investigation, fruitful in important results, in connection with

the weighty question of “Adaptation.” If the author of this work has

been able to add a little to our acquaintance with this subject, he owes

much to the inspiration he received from Schimper’s memoir on the

Indo-Malayan Strand-Flora.

Still, it must be admitted that the effectual operations of the currents

as plant-dispersers are limited to the shore-plants with buoyant seeds

or fruits. If we were to include in our list the shore-plants of

temperate regions that possess seeds or fruits capable of floating in

sea-water for long periods, and of afterwards germinating, the total for

the whole world would not, I imagine, reach 200. We cannot here concern

ourselves with those purely river-side plants that contribute their

buoyant seeds and seed-vessels to river-drift, since there is no

evidence indicating that river-side plants are effectively dispersed by

the currents unless they also frequent the estuary and the coast-swamp;

and in that case they come under the head of littoral plants. The total

for the whole British flora would probably not far exceed a dozen, and

nearly all of them are very widely dispersed.

The working value of the currents as plant-dispersers in the Pacific can

be rudely estimated by the number of littoral plants with buoyant seeds

or fruits that occur in the various groups. Most of these plants hail

from the Indo-Malayan region. Speaking generally of the extension

eastward of the Indo-Malayan strand-plants over the Pacific, Prof.

Schimper (page 195) remarks that they become fewer and fewer in number

as they extend farther from their original home, their number shrinking

to a very few in the most remote groups of the Marquesas and the

Hawaiian Islands. This is well illustrated in the following numerical

results that I have prepared. Of the whole number, some seventy in all,

of the littoral plants of the tropical Pacific with buoyant seeds or

fruits, Fiji possesses about sixty-five, Tahiti about forty, and Hawaii

only about sixteen. As shown, however, in Chapter VII., some of the

Hawaiian littoral trees that are useful to the aborigines were probably

introduced by them. The number actually introduced through the currents

into Hawaii in all likelihood therefore does not exceed ten. There is a

method in this diminution in numbers, as the plants migrate eastward and

northward over the Pacific, which has been described in detail in the

preceding chapter. The efficacy of the currents as plant-dispersers in

the tropical Pacific therefore diminishes as we proceed eastward.

In the South Pacific the littoral plants preserve their Old World origin

as far as the Polynesian archipelagoes extend eastward across to

Pitcairn, Elizabeth, and Ducie Islands, where we find in one or other of

them such characteristic Indo-Malayan beach trees as Barringtonia

speciosa, Cerbera Odollam, Guettarda speciosa, Hernandia peltata, and

Tournefortia argentea (see Note 34). In the more distant Easter Island

there is a suspicion, for the first time, of immigration from South

America in the presence of Sophora tetraptera. In the islands relatively

close to the American continent, as in Juan Fernandez and in the

Galapagos group, the Indo-Malayan strand-plants are no longer

represented.

We come now to consider the relation between the distribution of the

shore-plants and the currents. It is quite legitimate to discuss the

currents of the Pacific from the botanist’s point of view, that is to

say, from the standpoint of the distribution of littoral plants with

buoyant seeds or fruits. For ages the buoyant seeds and fruits of the

strand-plants of the tropical Pacific have been drifting over that

ocean, and we have the results now before us in the dispersal of the

species to which they belong. There is no necessity to endeavour to make

the distribution of such littoral plants square with the arrangement of

the currents as shown in a chart. The usual result of such a comparison

has been to lead the investigator, whether an anthropologist, a

zoologist, or a botanist, to find his facts at variance with the course

of the prevailing currents. Man, animals, and plants have entered the

Pacific from the west, whilst the most available currents are from the

east; and one may be perhaps permitted the solecism that the Pacific

islands have apparently been stocked with their shore-plants, with their

aborigines, and with much of their fauna by currents running in the

wrong direction. These Pacific islands could only have had a direct

communication with the Old World, from which they have mainly derived

their shore-plants, by the currents; but since both the aborigines and

the plants have forced their way across the ocean to the Tahitian region

in the teeth of the regular currents, indicated as such in the chart, we

are compelled to assume that they have availed themselves either of the

Equatorial Counter-Current or of the occasional easterly drift currents

that mark the prevalence of westerly winds during the short season of

the year when the easterly trade-winds do not prevail.

The Equatorial Counter-Current hypothesis would involve a preliminary

crossing of the whole breadth of the Pacific Ocean, that is to say, a

voyage of some 8,000 miles, before the drifting seed doubled back to the

Polynesian Islands. The other view is a much more probable one, as is

sufficiently indicated by the following extract from the “Admiralty

Sailing Directions for the Pacific Islands” (II., p. 25, 1900).... “In

the western part of the Pacific these trades ... are frequently

interrupted by winds which blow from west or north-west, especially

during the months of January, February, and March, when the north-west

monsoon of the Indian Ocean extends out in the Pacific as far as the

Samoa Islands.” In various works on this region one may find reference

to canoes blown off the shore during this season and carried some

hundreds of miles to the eastward. A ship can then sometimes sail with a

fair wind from the southern end of the Solomon Group to the Fijis; and

as we learn from Mariner, the crocodile may be at such times carried

away from the Solomon Islands and stranded in Fiji. Mr. Hedley, in his

exceedingly interesting paper on a zoogeographic scheme for the

mid-Pacific (_Proc. Linn. Soc. N.S.W._, 1899), gives many details of

this nature; but there is no space to deal further with the matter here.

After all, the botanist must take his cue from the drifting seed and the

distribution of the plant. He finds the seed floating in the open sea as

well as stranded on the beach. He then discovers the plant growing on

the beaches, and by experiment he tests the floating capacity of the

fruit or seed. Finally he ascertains the home of the plant. He does this

for all the littoral plants with buoyant seeds or fruits, and he forms

his own conclusions of the efficacy of the currents independently of the

current-chart, remembering that he has in Time an important factor that

the geographer does not possess in dealing with the currents. The effect

of time has often been to obscure the differential results of the

operations of the currents in the case of those species that, like

Barringtonia speciosa, are almost universally distributed in the islands

of the Pacific. It is obvious that such plants cannot aid us much in the

matter of ascertaining the track followed by the drifting seed in

entering this ocean. But if we find a littoral plant with buoyant seed

or fruit that has only partially performed the traverse we shall possess

in the interrupted operation an important piece of evidence.

Several years ago, in my paper on Polynesian plant-names, read before

the Victoria Institute, I developed this argument when endeavouring to

find in the floating seed a clue to the route pursued by the Polynesians

in entering the Pacific. Since that time my acquaintance with these

islands and their plants has been considerably extended; but no

important modification of the principal argument is now needed. It was

then pointed out that in Nipa fruticans, the swamp-palm of the Malayan

Islands and of tropical south-eastern Asia, we have a plant well fitted

for the purpose and one well known to be dispersed by the currents over

small tracts of ocean. The Nipa Palm has attempted to enter Polynesia

from the Malayan region by two routes, namely, by Melanesia and by

Micronesia. Along the first route it has in the course of ages reached

the Solomon Islands, where I found it in 1884. Along the second route it

has extended its range to Ualan at the eastern end of the Caroline

Group, where it was observed by Kittlitz many years ago, as indicated in

the narrative of his voyage (_Reise nach russische America, nach

Mikronesien_, etc., 1858, ii. 35), and in Dr. Seemann’s English edition

of the same author’s _Vierundzwanzig Vegetationsansichten ... des

stillen Oceans_.

The question now arises as to which of these two routes was taken by the

drifting seed. In my paper I adopted the view that the shore plants

reached Fiji and Samoa by Micronesia, that is to say, by the Caroline,

Marshall, and Gilbert Groups. This is the route which, as mentioned by

Mr. Hedley in the paper above quoted, Mr. Woodford prefers for some of

the Lepidoptera; and it is the one that is favoured by Mr. Wiglesworth

for the birds, since in his memoir entitled _Aves Polynesiæ_ he remarks

that certain indications tend to show that the Pelew Islands have served

as a sort of bridge for the spread of species from Indo-Austro-Malaya

right across the Pacific. Though I still think that the beach trees,

most of which would find a home on the numerous coral atolls of the

Marshall, Gilbert, and Ellice Groups, often followed that track, yet I

am now inclined to consider that the mangroves and their associates,

plants which find their most suitable home in the estuaries of large

elevated islands, like those of the Solomon Group, in all probability

reached Fiji in the mass by the Melanesian route.

[Illustration:

TRADE ROUTES

OF THE

PACIFIC OCEAN

(_On Mercator's projection_)

John Bartholomew & Co., Edin^r.

]

Although the Old World has supplied to the Pacific islands most of their

littoral plants that are dispersed by the currents, that is to say, the

plants with buoyant seeds or seed vessels, yet there is an appreciable

American element, and it is with the plants occurring in the New World

that we are now concerned. The total number of the littoral plants of

these islands that possess buoyant seeds or fruits is, according to the

lists given under Note 35, about seventy. Of these about forty-five are

exclusively Old World species, sixteen occur in both the Old and New

Worlds, three are exclusively American, and six are Polynesian.

The question we have now to ask ourselves is whether the shore plants

common to both the Old World and America have their homes in America, or

whether they have been derived from the other hemisphere. With one or

two exceptions, as in the cases of the Australian genera Dodonæa,

Scævola, and Cassytha, which, as shown in a later page in this chapter,

present no great difficulty, there does not seem to be any serious

objection, as far as the numerical distribution of the species is

concerned, in regarding America as a possible home of the genus. It is

not often we shall come upon such a striking instance of the principle

that where the species are most numerous there is the home of the genus,

as in the instance of Cocos. The Coco-nut palm has been carried around

the world through the agencies of man and the currents, whilst the home

of the genus is in America.

Now assuming that in having to choose between the Old World and the New

World as the home of most of the genera in the list we selected the

latter, we have to ask ourselves in what degree this would be consistent

with the place America holds with regard to the distribution of tropical

shore-plants dispersed by the currents and with reference to the

arrangement of the currents. If we except the African continent, there

is no part of the world that bears such a definite relation to the

currents as America, and with an ordinary chart of these regions their

arrangement is to be understood at a glance. Yet strange to say, as far

as the distribution of tropical littoral plants is concerned, America

holds a position that the present system of the currents on its coasts

will not altogether explain. Within the lifetime of the species of

mangroves and other plants of the coast swamps that are found on both

the Pacific and Atlantic coasts of tropical America the two continents

of this name have been united by the emergence of the Isthmus of Panama.

Few things are more significant in plant-distribution than the

arrangement of the tropical littoral plants with buoyant seeds or

fruits, a subject that is discussed with some detail by Professor

Schimper in his work on the Indo-Malayan strand-flora (page 190). These

plants group themselves into four sections:—

(_a_) Those of the Pacific and Atlantic coasts of tropical America

(including the West Indies) and of the West Coast of Africa. They

include mostly plants of the mangrove-swamps and their vicinity, such as

Anona paludosa, Avicennia tomentosa, A. nitida, Conocarpus erectus,

Laguncularia racemosa, Rhizophora mangle, etc.

(_b_) Those of the Old World excluding the African West Coast and

extending from the East Coast of Africa eastward to the Pacific islands.

This is much the largest group and comprises many of the plants named in

the list given in Note 35 under Old World species. One may cite as

examples of plants ranging almost all over this area, Barringtonia

speciosa, B. racemosa, Bruguiera gymnorhiza (in its most comprehensive

sense), Carapa moluccensis, Derris uliginosa, Guettarda speciosa,

Hernandia peltata, Heritiera littoralis, Pemphis acidula, Rhizophora

mucronata, etc. Plants of the mangrove-swamp and of the beach are,

therefore, here included.

(_c_) Those occurring all around the tropics and including many of the

plants mentioned under Note 35 as Pacific island shore-plants found also

in America. Most of them belong to the Leguminosæ, and there may here be

mentioned Canavalia obtusifolia, Cæsalpinia Bonducella, Entada scandens,

Gyrocarpus jacquini, Ipomœa pes capræ, Sophora tomentosa, and Vigna

lutea.

(_d_) Those confined to a portion of the two great regions, such as Nipa

fruticans in the Old World, and the Manchineel (Hippomane mancinella) to

tropical America.

It is to be noted that the ubiquitous species do not include any of the

mangroves. Each of the two regions has its own species, none being

common to both the American and Asiatic regions, although, as is shown

in Chapter XXX., the American species of Rhizophora is now seemingly

breaking its bounds and intruding into the Pacific islands. On the other

hand, some of the mangrove genera, Avicennia, Carapa, and Rhizophora,

are found all round the globe, whilst others are restricted to one or

other of the two regions, Bruguiera, Lumnitzera, and Sonneratia, for

instance, to the Old World region, and Laguncularia to the American and

West African region.

For convenience we may designate the two great regions of tropical

strand-plants, with buoyant seeds or fruits, the American and the

Asiatic regions, remembering that the first includes both coasts of

America as well as the African West Coast, whilst the second extends

from the East Coast of Africa to Polynesia. Excluding the ubiquitous

species, these two regions are well distinguished from each other. If we

look at the chart of the currents we perceive the reason of the American

region including the West African Coast, and we see why none of the

indigenous plants of this region occur on the African East Coast. So

also with the Asiatic region, a glance at the chart will show that all

the portions of its area are in connection with each other directly or

indirectly through the currents, and that only time is required for the

transport of buoyant seeds over most of the region.

Hitherto I have mainly followed Professor Schimper in this matter; but

since my visit to Ecuador and the Panama Isthmus some further

considerations have presented themselves to me. If the reader will look

again at the map of the currents, he will observe that there is little

reason for supposing that the Asiatic region can lend its littoral

plants to the American region. On the other hand there are greater

facilities, as far as currents are concerned, for America supplying the

Asiatic region, namely by means of the great equatorial currents that

course westward across the Pacific to the tropics of the Old World.

It would therefore seem that the American region can receive nothing by

the currents from the Asiatic region. If accordingly it gives but gets

nothing back, we are compelled to assign an origin in the American

region to all littoral plants dispersed by the currents that are found

in the tropics around the globe. This is what we have already regarded

on other grounds as possible for nearly all the littoral plants of the

tropical Pacific with buoyant seeds or seedvessels that are found in

America. These plants are practically the same as those distributed

around the tropical zone which are enumerated in the list given under

Note 35, _b_. With their home in America, by crossing the Pacific they

would ultimately arrive at the East African coast, where their course

westward would terminate; whilst commencing their journey from the east

side of the American continent they would reach the West African coast;

and their distribution around the tropics of the world would be

explained. There follow from these considerations the corollaries that a

tropical strand-plant dispersed by the currents which has its birthplace

in Asia could never reach the American region, and that American

strand-plants are for the most part native-born, excepting those, if

there are any, that hail originally from the African West Coast.

It is necessary in passing to explain the similarity of shore plants on

the Pacific and Atlantic coasts of Tropical America. For the mangroves

and their accompanying plants inter-communication between the two coasts

is now impossible; and a communication between the two oceans must be

postulated within the lives of the existing species. For the plants like

Entada scandens and Ipomœa pes capræ, which occur inland as well as at

the coast, it is easy to show that in the case of the Panama Isthmus,

their seeds could be readily carried into the Atlantic and Pacific

Oceans by rivers draining the opposite slopes of the same “divide,” so

that the dispersal of the same species from a common centre into two

oceans may be seen in operation in our own day. My observations on this

subject are given in Chapter XXXII., to which the reader is referred.

I have now gone far enough to indicate the place that America holds with

regard to the distribution of tropical shore-plants dispersed by the

currents and with regard to the currents. There is every probability, as

I venture to think I have shown, that the Pacific islands have derived

most of their ubiquitous shore-plants with buoyant seeds or fruits from

America. But one of the results of our discussion of America in this

double aspect was that excepting in the case of the African West Coast

it gives but does not receive plants from the Old World. We apply this

test, with perhaps a little hesitation, to the shore-plants of the

Pacific islands that are dispersed by the currents; and we find, as will

be seen below, that it is responded to in a remarkable manner.

It has been observed in the previous chapter that scarcely any of the

large-fruited beach-plants of the South Pacific islands, that could only

have been dispersed by the currents, have reached Hawaii. We do not find

amongst the truly indigenous coast flora of this group any of the

following trees: Barringtonia speciosa, Calophyllum Inophyllum, Cerbera

Odollam, Guettarda speciosa, Hernandia peltata, Ochrosia parviflora,

Pongamia glabra, Terminalia Katappa, Terminalia littoralis, &c. It was

also noted that the currents had not only failed to establish these

plants in Hawaii, but that they had also failed to establish them in

America, the suggestion being that the Hawaiian Islands had been, in

part at least, stocked by the currents from America. That the

Indo-Malayan strand-plants in their extension eastward over the Pacific

should have failed to reach America, is a result we might have expected

from the arrangement of the currents. Yet mingled with them we have

plants like Ipomœa pes capræ, Canavalia obtusifolia, and Sophora

tomentosa, that also occur in America. Since, however, their seeds are

not better adapted for accomplishing the passage across the Pacific from

the Old World to America than the equally buoyant fruits of the

above-named littoral trees that have failed, the presumption arises that

their home is in America, and that they have performed the easier

passage across the Pacific westward from America to the Old World.

The exclusion of so many characteristic shore-trees from America that

range often over the whole tropical region from the African East Coast

to the islands of the Central Pacific, is not a matter of seed or

fruit-buoyancy, but a matter concerned with the home of the species, and

with the arrangement of the currents. Those shore-plants of this region

that occur also in America have their home in that continent, and have

subsequently been carried across the Pacific by the currents westward to

the Asiatic shores.

The only exceptions, that I can recall, to the rule that America does

not receive shore-plants dispersed by the currents from the Old World,

are presented by the three Australian genera, Dodonæa, Scævola, and

Cassytha, of which widely spread littoral species occur in America,

namely, Scævola Lobelia, Dodonæa viscosa, and Cassytha filiformis. They

offer, however, but little difficulty, since, as pointed out in other

parts of this work, Dodonæa viscosa has probably been in part dispersed

by man, whilst the other two species are as well fitted for dispersal by

birds as by currents. The occurrence therefore of these species in

America does not necessarily raise the question of the currents.

The same exclusive principle is illustrated in the scanty littoral flora

of Hawaii. Deprived, like America, of the characteristic large-fruited

beach-trees of the South Pacific, species that could only have reached

it through the agency of the currents, it is scarcely to be expected

that it would have received its few littoral plants with buoyant seeds

from the source which has failed it in the cases of the numerous

absentees. It is to America therefore that we look for the source of its

littoral plants as far as the agency of the currents is concerned.

The Hawaiian Islands contain about twelve plants, named in the list

given in Note 36, that possess seeds or fruits known to be dispersed by

the currents, and capable, as experiments indicate, of floating in

sea-water for prolonged periods. Not all of them are at present littoral

in their station in this group; but their claim to be considered such in

other regions is established in the Note above mentioned. Of these

plants, seven at least are found in America, five in the Old World also,

and two exclusively in America. This proportion of American plants is

far greater than that characterising the whole littoral flora of the

Pacific islands dispersed by currents, where out of some seventy species

only nineteen are found in America (see Note 35). As far as the

distribution of the plants is concerned, it is therefore quite possible

that Hawaii has received most of its plants that are dispersed by the

currents from tropical America.

We will now consider how such a possibility is in accordance with the

arrangement of the currents in the North Pacific. If we look at the

Quarterly Current Charts for this ocean published by the British

Admiralty we notice that all through the year the Hawaiian Group lies

more or less within the area of currents flowing from the West Coast of

America, the Northern Equatorial Currents as they are collectively

named. Except in the winter months these currents come from the N.E. and

E.N.E., and bring drift from the coasts of British Columbia, Oregon, and

Northern California. It is then that they pile up huge pine logs on the

shores of the Hawaiian Islands, as I have described in Chapter VII. and

in Note 30; and, according to Dr. Hillebrand, they transport this drift

timber much farther south to the shores of the Marshall and Caroline

Groups. One might cite other facts illustrative of the working of these

currents, such as one finds in the pages of Fornander and other authors;

but this would scarcely come within the province of this work. I may

here remark that when in Honolulu I was informed that a bell-buoy which

had got adrift on the Californian coast was subsequently washed up on

the coasts of Kauai. It is stated in Findlay’s “North Pacific Directory”

(1886, p. 1068), that a junk carrying nine hands that had been blown off

the south coast of Japan in a typhoon, anchored, after ten or eleven

months at sea, in December, 1832, near Waialea in Oahu, the view taken

of its course being that after drifting along in the Japan Current it

came within the range of the south-west current that carries pine timber

to Hawaii from the West Coast of America.

The portion of the Northern Equatorial Current that strikes the Hawaiian

Group during the greater part of the year is no doubt a south-westerly

deflection of the Japan Current from the American West Coast; and it

would be impossible to find any tropical drift mingled with the pine

logs stranded on the islands during that period. However, in the winter

months, centering in January, the Japan Current flows down the West

Coast of America to about the latitude of Cape Corrientes on the coast

of Mexico, before being deflected westward. Here it meets with a portion

of the Peruvian Current, and both flow westward, the united stream

striking probably only the southernmost islands of the Hawaiian Group.

It is at this season alone that there would be any likelihood of drift

from tropical America being stranded on the Hawaiian beaches, and it is

quite possible that at such a time the Northern Equatorial Current may

carry intermingled in its stream pine logs from Oregon and seed-drift

from Panama.

I am not inclined to attach any value except in the Western Pacific to

the agency of the Equatorial Counter-Current in transporting seeds and

fruits over the Pacific. It presents seemingly the only opportunity of

the transportal of the seeds and fruits of Asiatic littoral plants to

America; but if at all effective in this way, it would have endowed the

littoral flora of the western shores of tropical America with many of

the trees so characteristic of the coral islands of the Pacific. In this

sense, it has failed completely as an effective agency in

plant-dispersal; and judging by results we may, I think, dismiss it from

our consideration. However, Dr. Hillebrand (p. xv.) assumes that during

the prevalence of south-westerly gales in winter in the Hawaiian

Islands, the Equatorial Counter-Current would be pushed northward so as

to mingle to the east of the group with the North Equatorial Current. In

this manner it is supposed that seed-drift brought direct from the

Asiatic side of the Pacific would be stranded on these islands. This

appears to me to be most improbable, since some ten or twelve degrees of

latitude usually intervene between the Hawaiian Group and the Equatorial

Counter-Current (_see_ Admiralty Sailing Directions, Pacific Islands,

1900, II., 31, and the Quarterly Current Charts; also Encyclopædia

Britannica, vol. 18, p. 118).

The most serious objection from the botanist’s standpoint against such a

view as that of Dr. Hillebrand is the absence from Hawaii of most of the

shore-plants that we should expect the currents to have brought from the

Old World. It is also evident that as far as the currents are concerned

the Hawaiian Islands are far more likely to receive littoral plants from

America than from the Old World. Though no tropical drift has yet been

found stranded on the coasts of these islands, yet it is not unlikely

that future investigators may find some seed-drift from Central America

on the most southerly coasts of the group, as on the south-east shores

of the large island of Hawaii. It would only be stranded in the winter

months and then probably in small quantities.

_Summary of the Chapter._

(_a_) Since the effective operations of the currents are limited to the

shore-plants with buoyant seeds or fruits, such plants forming but a

small proportion of any flora, it must be acknowledged that, numerically

speaking, the results of the dispersing-agency of the currents on

plant-distribution in general are but slight.

(_b_) Yet the importance of the subject is by no means to be measured by

a numerical scale of results, a line of inquiry being here opened up

leading to fields of investigation full of promise for the student of

plant-distribution.

(_c_) Whilst dealing with the relation between the distribution of

shore-plants and the arrangement of the currents, it is quite legitimate

to discuss the currents of the Pacific from the point of view of the

botanist, who, after all, must take his cue from the drifting seed and

the resulting distribution of the plant.

(_d_) The shore-plants of the Pacific islands that are dispersed by the

currents being mainly Indo-Malayan in origin, it follows that they have

extended eastward over the Pacific to the Tahitian islands against the

stream of the South Equatorial Current and against the trade-wind. It

is, however, shown that they could have availed themselves of the

interval between January and March when the North-west Monsoon reaches

the Pacific.

(_e_) It is claimed that whilst the mangroves and their associated

plants have for the most part entered the Pacific by the Melanesian

route through the Solomon Islands, the beach-plants have also followed

the route through Micronesia by the Caroline, Marshall, and Ellice

Groups.

(_f_) A small number of the strand-plants of the Pacific islands that

are dispersed by currents occur in America as well as in the Old World;

and questions of prime importance arise when we have to decide whether

their home is in the Old World or in the New World.

(_g_) Good reasons are given for regarding them as chiefly of American

origin; and it is shown that America with regard to the arrangement of

the currents stands in the singular relation of being a disperser but

not a recipient of shore-plants.

(_h_) It is pointed out that the tropical shore-plants that are

distributed by currents belong to two great regions which are the effect

of the present arrangement of the currents, viz., the American including

the West Coast of Africa, and the Asiatic comprising the remainder of

the tropical zone. Each region has its own plants, and those that occur

in both, being in fact distributed all round the tropics, are regarded,

according to the principle above stated, as having their home in the

American region.

(_i_) The occurrence of the same strand species on the Pacific and

Atlantic coasts of tropical America is regarded as indicating that the

arrangement of the existing species of its shore-plants, more

particularly of the mangroves, antedates the emergence of the Panama

Isthmus. This hypothesis is not needed for the coast plants like Entada

scandens that occur inland, since we can now observe their seeds being

carried down into the Atlantic and Pacific Oceans by rivers draining the

opposite slopes of the same “divide” in the Panama Isthmus.

(_j_) It is shown that the currents of the Pacific have failed to

establish the numerous beach-trees (possessing buoyant fruits) of the

Pacific islands, not only in the Hawaiian Group, but also on the coast

of America; and it is therefore argued that we should expect the

Hawaiian Group to have received through the currents its shore-plants

with buoyant seeds or fruits from the tropical west coasts of America.

(_k_) In support of this contention it is pointed out that most of the

Hawaiian strand-plants that are dispersed by the currents are found in

America, and some indeed in America to the exclusion of the Old World.

(_l_) The arrangement of the currents in the North Pacific also favours

the view that the Hawaiian Islands are more likely to receive plants by

the agency of the currents from America than from the Asiatic side of

the Pacific.

CHAPTER IX

THE GERMINATION OF FLOATING SEEDS

Germination in the floating seed-drift of tropical estuaries.—A strain

of vivipary.—Abortive germination of seeds in warm seas.—A barrier to

plant dispersal.—The borderland of vivipary.— Summary.

THE tendency of the floating seed or fruit to germinate in the estuaries

of tropical rivers is especially characteristic of the plants of the

mangrove-swamps and of their borders. In the Fijian rivers, and

particularly in the estuary of the Rewa, where the river-water is

usually mixed with that of the sea, there are frequently to be found in

a state of germination floating fruits of Barringtonia racemosa, Carapa

obovata, Clerodendron inerme, Derris uliginosa, Smythea pacifica, &c.;

whilst the floating fruits of more characteristic beach-trees like

Barringtonia speciosa and Cerbera Odollam, that grow also on the sides

of the estuaries, were never noticed in this condition. That this

tendency should be restricted to the plants of the mangrove-formation

and is not to be observed in the beach-trees is a singular fact. There

is, however, an intermediate group of littoral plants mostly belonging

to genera of the Leguminosæ and Convolvulaceæ, such as Mucuna and

Ipomœa, where germination of the floating seed is apt to begin but ends

abortively, and results in the sinking and death of the seed. The

subject of the germination of seeds in the floating drift of tropical

estuaries presents itself, therefore, in three aspects:—

(1) As concerning the plants of the mangrove-formation, where, excluding

the viviparous species (when germination takes place on the plant),

germination is frequent in the water:

(2) As concerning the beach-trees where it is rare or absent altogether:

(3) As concerning certain Leguminous and Convolvulaceous littoral plants

where germination is not infrequent but always abortive.

Dealing first with the plants of the mangrove-formation, it may be

remarked that the same tendency of the floating fruits or seeds to

germinate, which is above noticed in the case of the estuaries of Fiji,

came under my observation in the floating drift of the estuary of the

Guayaquil River in Ecuador, the germinating fruits and seeds being

carried far out to sea. The seeds of Anona paludosa, which float in

quantities in the river-drift, were often found germinating; and the

same may be said of the fruits of Laguncularia racemosa and of the

“joints” of Salicornia peruviana which abound in the creeks of the

mangrove-delta and are carried out to sea in the germinating condition.

It might be expected that this readiness to germinate in the brackish

water of estuaries would prove to be a formidable obstacle to the

dispersal of these plants over wide tracts of ocean. The exposed

portions of the seedling might be deemed ill-suited to withstand,

without injury, the “wear-and-tear” of transport by currents over long

distances, even when not affected by the sea-water; and it might be

thought that they would be often nibbled off by fish or destroyed by

other aquatic animals. Only the specially organised seedlings produced

by a viviparous process on the tree, such as those of Rhizophora and

Bruguiera, might be regarded as able to survive the effects of prolonged

immersion in the oceanic currents.

Observation, indeed, shows that such seedlings are exposed to and suffer

from these perils; yet it is evident from the distribution of the

species that, whether in the germinating condition or not, the seeds and

fruits of Anona paludosa and Laguncularia racemosa have been carried by

the currents from America to the West Coast of Africa. The seedlings of

Avicennia and of Rhizophora mangle have also performed the same

trans-Atlantic voyage. Those of both these mangroves are to be observed

floating off the coasts and in the estuaries of both coasts of America.

The seedlings of Avicennia are particularly abundant in the

mangrove-creeks of the delta of the Guayaquil River; and I observed them

in a healthy condition, ten to twenty miles out at sea, floating

together with those of the Rhizophora. Since, as in the case of

Rhizophora, germination occurs normally on the plant, Avicennia can only

be dispersed by its floating seedlings. Yet it is noteworthy that

although Avicennia seedlings appear, to a marked degree, less fitted for

ocean transport than those of Rhizophora and Bruguiera, the species have

a much wider distribution. Avicennia officinalis has a cosmopolitan

distribution in the tropics and beyond, occurring as it does on the

Atlantic and Pacific coasts of America, on both coasts of Africa, over

Asia and Australia, as well as in New Caledonia and New Zealand, but not

in Polynesia (_Bot. Chall. Exped._, III., 178).... I have now gone far

enough to show that the tendency displayed by the seeds and fruits of

several of the plants of the mangrove-formation to germinate either on

the tree or in the floating drift of estuaries has not affected the

general distribution of the species in its main outlines. Few fruits are

found more often in a germinating condition in the floating drift of the

Rewa River in Fiji than those of Barringtonia racemosa, yet the species

ranges from the African East Coast eastward to Polynesia. Seedlings as

well as seeds or fruits, whether or not in a germinating condition, are,

therefore, able in such cases to disperse the species.

This readiness of the floating fruits of plants of the mangrove

formation (excluding the viviparous species) to germinate in the

estuaries is, I am inclined to think, due in the main to the strain of

vivipary that runs through nearly all the plants of the mangrove-swamp

and of its borders. It would, indeed, appear that the viviparous habit

(the capacity of germinating on the plant) which finds its extreme

development in Rhizophora and Bruguiera of the Fijian swamps is

represented in its earliest stage in the readiness of the floating

fruits of Barringtonia racemosa, Carapa obovata, &c., to germinate in

the Fijian estuaries, and as remarked in Note 37 there is a suspicion of

vivipary in the instances of both the species just named. Intermediate

cases, as that of Laguncularia in the Ecuador swamps, occur in other

regions with species where germination only takes place at times on the

plant. This subject is, however, generally discussed in Chapter XXX. and

need not be further dealt with here.

A predisposing cause of the germination of floating seeds and fruits in

tropical estuaries would seem to be afforded by the super-heating of the

water of the estuary. This came under my notice both in the Rewa River

in Fiji and in the Guayaquil River in Ecuador, where the water of the

estuary is often noticed to be some degrees warmer than that of the sea

outside, and of the water from the river above the estuary. (See Note

38.)

We come now to the subject of the non-germination in tropical estuaries

of the floating fruits of the beach-trees, such as Barringtonia speciosa

and Cerbera Odollam, that in the Pacific islands may contribute to

river-drift. Such trees may grow on the banks of the estuary, and their

fruits would thus readily fall into the water; but in the Rewa estuary

in Fiji it was evident that the fruits and seeds of beach-plants, such

as Scævola Koenigii, are also brought in by the tide. The seeds of

Morinda citrifolia were often noticed in the Rewa drift together with

the fruits of Heritiera littoralis, which is both a beach and a swamp

plant, but never in a germinating condition. The same remark applies

also to the fruits of beach trees found afloat in the sea between the

islands, such as Cordia subcordata, Guettarda speciosa, and Terminalia.

It is possible that a few of these plants, as in the case of

Barringtonia speciosa, display traces in the structure of their fruits

of a lost viviparous habit. (See Note 50.) It is pointed out in

discussing Guettarda that germination is much more easily induced than

one would expect in the case of fruits with such a hard ligneous

putamen.

An interesting subject is presented in the abortive germination of the

floating seeds of many plants of the Leguminosæ and Convolvulaceæ both

at sea and in a tropical estuary. My conclusions on this matter are

based partly on observations made in Fiji, but mainly on the results of

numbers of experiments, this being unavoidable, since the abortive

germination causes the sinking of the seed. The principal determining

cause of the germination in water of one of these floating seeds is

evidently to be sought in the temperature of the water, it being

immaterial for the earliest stage of germination, as many of my

experiments indicate, whether the seed or fruit is afloat in the sea or

in the river. In these flotation experiments, when conducted under warm

conditions with sea-water, the earliest signs of germination were

frequently displayed in the softening, swelling, and sinking of the

seed. If the swelling seed is taken out in time and planted after a

preliminary soaking in fresh water, the germinating process is at once

resumed and is often successfully and rapidly completed; but if the seed

is allowed to remain in the vessel after it has absorbed sea-water the

vitality of the embryo is destroyed and the seed decays.

That many seeds would fail from this cause to cross an ocean my

experiments repeatedly demonstrated. Nor does the appearance of a seed

afford any indication of its probable failure to cross an ocean. Some

seeds of Mucuna, as far as their hard coverings could guide one, would

seem to be quite secure from such a risk. The stony seeds, for instance,

of M. urens D.C. look as if they might safely be transported by the

currents round and round the globe; and De Candolle very rightly placed

this species in his scanty list of plants dispersed by currents. Yet few

seeds are more treacherous when their buoyancy in sea-water is tested in

a warm place, as in a hot-house. They may take up water, swell, and sink

in a week, or they may float unharmed for a year.

The seeds most exposed to this risk are those of the Leguminous giant

climbers, the lianes of the coast and inland forests of the islands of

the tropical Pacific. They belong to the genera Mucuna, Strongylodon,

&c.; and thus several of the plants that constitute for the student of

plant-dispersal the enigmas of the Pacific are here included. The seeds

of Mucuna are especially liable when afloat in sea-water under warm

conditions to display the early signs of germination, swelling up and

sinking to the bottom of the vessel, a process, however, soon arrested

and followed by the death of the embryo unless the seed is removed in

time. Yet the seeds of this genus are notably long “floaters.” Those of

an American species, variously designated as Mucuna pruriens D.C. and M.

urens D.C., have long been known to be washed ashore together with the

seeds of Entada scandens on the western shores of Europe, and

particularly on the Scandinavian coast, where they form regular

constituents of what the Scandinavian botanists correctly term the

Gulf-stream Drift.

Mucuna urens D.C. occurs with other American shore-plants that are

dispersed by the currents on the African West Coast; and there is no

reason to doubt that its seeds perform the trans-Atlantic voyage. It is

found in Polynesia, in Hawaii, in the Marquesas, and according to

Reinecke also in Samoa; and probably it occurs in other groups. The

specific determinations of the genus, however, need thorough

overhauling, so that it is not possible to deal more than in general

terms with the distribution of a species. The distribution of Mucuna

urens in the Pacific is, however, irregular, and no doubt this is to be

connected with the uncertain behaviour of its seeds when transported by

tropical currents. The seeds would, I venture to think, often sink

through abortive germination in the warm areas of equatorial seas.

When in Hawaii I kept ten of the seeds of this species (M. urens D.C.)

in sea-water for four and a half months, none of them sinking in that

period, the temperature of the water rarely reaching over 80°F., the

average daily temperature being 76-77°. However, when four years

afterwards in England I placed five of the seeds obtained at the same

time in sea-water under conditions where the water-temperature ranged

for the first few weeks between 75° and 90°, three of them began to

swell within ten days, and on removal at once germinated healthily. The

remaining two were afloat at the end of twelve months, and when planted

one of them germinated a month afterwards.

Having experimented on the seeds of about half a dozen different species

of Mucuna in sea-water, all with buoyant qualities, it is possible for

me to lay down the general rule for the buoyant seeds of the genus that

sinking is the result of an attempt at germination, which, as before

observed, proves abortive unless the seed is removed in time. It is

obvious that the gardener wishing to raise plants of this genus without

delay might profitably adopt the method of keeping them afloat in water

at a temperature of 80-90° F. until they begin to swell, which may

happen in some cases in a few days. Sea-water seems to produce the most

rapid results.

When on Keeling Atoll in the Indian Ocean I collected, amongst the

stranded seed-drift brought by the currents to those islands, the seeds

of five or six species of Mucuna, two of which were identified at Kew as

M. macrocarpa, Wall., and M. gigantea D.C. (see my paper on the

dispersal of plants at Keeling Atoll). No plant of this genus appears up

to that time to have been recorded from the Keeling Islands, so that at

all events most if not all of the seeds had been brought by the currents

from the Indian Archipelago, some 700 miles away. It may be added that

amongst the drift gathered by me on the south coast of Java the seeds of

three species of Mucuna were identified at Kew, including the two

above-named species from Keeling Atoll.

These current-borne seeds of the Keeling beaches had probably performed

an ocean journey of a thousand miles, since the route could scarcely

have been direct. Yet their behaviour when placed eighteen months after

in sea-water in a hothouse in England was most erratic. Of three seeds

of Mucuna gigantea all swelled and sank within eight days. Two seeds of

M. macrocarpa sank after floating from sixty to a hundred days; whilst

of two seeds of another species both remained afloat after a year. In a

sea-water experiment in England on five Hawaiian seeds of M. gigantea,

under the conditions referred to in the Mucuna urens experiment, one

sank within ten days, whilst three of them were afloat after twelve

months, one of them subsequently germinating. This species, it may be

remarked, is widely distributed as a coast plant over tropical Asia,

Australia, and in Polynesia. It seems to take the place in the Old World

which Mucuna urens takes in America, and it is curious that they meet in

Polynesia, being sometimes associated as in Hawaii. In the chapter on my

observations in Ecuador and in Panama it is remarked that Mucuna seeds

are frequent constituents of river, sea, and stranded drift. I,

therefore, have enjoyed the opportunity of observing the behaviour of

the seeds of this genus in a variety of localities, namely, in the

Keeling Islands, in West Java, in Fiji, Hawaii, and tropical America;

and this may be pleaded as an excuse for entering into so much detail

respecting them.

The large seeds of Strongylodon lucidum (S. ruber), a Leguminous liane

that ranks with the species of Mucuna amongst the huge climbers of the

forest of the Pacific islands, behaved in a similar way in my flotation

experiments in sea-water. Though, as shown in Note 3, these seeds can

float for a year and retain their germinating power, some of them

brought their buoyant capacity prematurely to an end by an abortive

attempt at germination. These black rounded seeds form a common object

amongst the river seed-drift stranded on some of the Fijian beaches in

the vicinity of estuaries. They are so hard and durable that they are

mounted in brooches in Honolulu. Yet these pebble-like seeds will

sometimes begin to swell in a few days in sea-water. Out of five seeds

placed in sea-water in England under warm conditions (the water

temperature for the first few weeks ranging between 75° and 90° F.), one

swelled and sank within ten days, another did so after two months,

whilst the other three were afloat after twelve months, and one of them

subsequently germinated. There is some disagreement amongst botanists as

to the limits of the specific characters of the plants of this genus

(see Note 39); but the plan seemingly most in accord with the

fundamental principles regulating plant-distribution in this region of

the Pacific is to regard the forms found in Hawaii, Tahiti, and Fiji, as

referable to one species. In addition to the Polynesian forms there are

only two or three species, found in the Philippines, Madagascar, and

Ceylon, and it is with the species from the last-named locality that the

Polynesian species is by some identified.

The seeds of several other Leguminous climbers would probably act in a

similar way, for instance, those of Entada scandens; but the seeds of

this plant experimented on by me were too few to enable an opinion to be

formed. Of four seeds of Dioclea violacea from Fiji that were subjected

to the same experiment as those of Strongylodon lucidum, all floated in

sea-water after a year, with the exception of one that did not swell and

sink until after ten months. On the other hand, in my experiment in Fiji

on the fresh seeds of Canavalia obtusifolia, a plant found on tropical

beaches all round the globe, seventy per cent. sank in the first six or

seven weeks, swelling and displaying the first signs of germination, but

quite ten per cent. were afloat after three months.

My experiments on the foregoing and other littoral species of the

Leguminosæ merely indicate that under the ordinary temperature of

tropical currents a portion of the seeds will probably sink owing to

abortive attempts at germination. It is likely that if in the

experiments in England a constant temperature of 85° to 90° F. had been

sustained throughout, most if not all of the seeds would have swelled

and sunk within a month or two. The temperature of the experiments in

Fiji and Hawaii did not exceed that of many tropical currents; but there

are areas of superheating in equatorial seas, which I think would prove

insurmountable barriers in the path of most drifting Leguminous seeds, a

subject to which further reference will be made.

Coming to the Convolvulaceæ, my experiments show that the buoyant seeds

often lose their floating powers from the same cause. Those of Ipomœa

pes capræ may be taken as an example. I was surprised to find when

experimenting on the buoyancy in sea-water of these seeds in Fiji and

Hawaii that a considerable proportion, about a third, sank in the first

two months, swelling and sinking to the bottom. That this swelling

represented the early stage of germination was well brought out in

parallel experiments in fresh water and sea-water made in England on the

buoyant seeds of the British littoral species, Convolvulus soldanella. A

good proportion of the seeds in the first part of the experiment

absorbed water, swelled, and sank, those in fresh water proceeding at

once to germinate healthily at the bottom, whilst those that sank in

sea-water merely decayed. Of the survivors about fifty per cent. in

either case floated after six months. It may be added that the seeds of

other tropical littoral species, such as those of Ipomœa glaberrima and

I. grandiflora, behaved in the same way.

It would appear from my experiments, and it is a result that we should

expect, that buoyant seeds of the Leguminosæ and Convolvulaceæ would

often float for much longer periods under cool than under warm

conditions. There must be areas of high temperature in mid-ocean that

would prove much more fatal to the chances of a drifting tropical seed

than the icy waters of a Polar current. In my paper on Keeling Atoll I

have described how I procured the germination of a seed of Ipomœa

grandiflora, Lam., after a year’s flotation in sea-water in London,

which included a period of three weeks when the water temperature was at

or about 32° F. These seeds from this point of view would be exposed to

much more risk of sinking through abortive attempts at germination when

drifting across some parts of the Pacific Ocean. It would appear from

the Admiralty Chart of Surface-Temperatures, published in 1884, that

such an area with a surface-temperature of 83° to 86° throughout the

year extends north and east of New Guinea well into the Pacific,

reaching in the first half of the year as far east as the Tahitian

region. It would seem highly probable that the immersion of Leguminous

or Convolvulaceous seeds for many months in these tepid waters would in

most if not in all cases induce incipient germination which would lead

to the sinking of the seed. There are, however, exceptional cases, as

that of Cæsalpinia bonducella, which, as my experiments recorded in

Chapter XVII. indicate, appear to be quite proof against any conditions

of temperature such as are likely to be found in tropical seas in the

present day.

There are a few general considerations arising out of the foregoing

observations to which reference may now be made. The study of the

behaviour of the floating seed or fruit often carries us, as I have

before implied, to the borderland of vivipary. When from a canoe on a

Fijian river we lift up the germinating fruit of Barringtonia racemosa

from amongst the drift floating past in the stream and pull down from

the branches overhead the seedling a foot in length of Rhizophora, we

hold in our hands the two extremes of the series of vivipary. With many

of the plants of the mangrove-formation there is a fine adjustment with

respect to the germinating capacity of the seed, or in other words a

delicate balancing of organisation on one side and of physical

conditions on the other. A slight disturbance of the equilibrium would

produce great results in plant distribution. Thus, an elevation of the

temperature of the sea-water in the tropics to 90° F. would, I

apprehend, produce the abortive germination of nearly every floating

seed and fruit in equatorial seas, even of those of the beach-trees like

Barringtonia speciosa and Terminalia littoralis that are regarded as

proof against such risks under existing conditions where the

surface-temperatures would average 78° to 80°.

There would thus be a barrier to the dispersal of plants by currents as

effective as that of a frozen ocean. In the warm, humid climates of the

early geological ages, seed-transport by currents may have been often

impossible, since the seeds that did not begin to germinate on the

plants of the swamps would probably do so in the tepid water of the sea.

Viviparous plants would, however, be placed at no greater disadvantage

than they are at present, since the genera Rhizophora, Avicennia, and

others are now only dispersed by the floating seedlings. But such an

increase of temperature at the present time would mean the death in the

current of the floating seeds and fruits of nearly all non-viviparous

shore-plants. As a rule every Leguminous and Convolvulaceous seed would

swell up and go to the bottom; whilst fruits like those of Barringtonia

racemosa and Carapa obovata, that often germinate afloat in tropical

estuaries, would invariably do so under the changed conditions, and the

seedlings not being adapted for ocean transport would perish.

Yet we know that with the seeds of many inland plants temperature has

seemingly very little to do with starting the process of germination. We

are familiar with the fact that the seeds of many plants that fail to

germinate in the summer of their production habitually germinate under

apparently less favourable conditions of temperature in the following

spring. This is attributed by botanists to the immaturity of the seed on

first falling from the plant, a further period of maturation being

necessary before, under any conditions, germination is possible.

We see this also well illustrated in the floating seeds and fruits of

the Thames drift. Most of them fail to germinate in the drift at the end

of the summer and the beginning of autumn, and defer the process until

the following spring, when they germinate freely in the water under much

cooler conditions than those which they experienced in the early part of

their flotation in the drift. There are, however, exceptions to this

rule. Plants like Caltha palustris, for instance, are rarely represented

in the spring seed-drift of ponds and rivers, because most of the fruits

or seeds germinated soon after falling into the water in the previous

summer.

In most of my sea-water experiments in England the immersion had a very

marked influence, not in causing premature germination and destroying

the germinating capacity, as often happens with the floating seeds of

Convolvulaceæ and Leguminosæ, especially in the tropics, but in

postponing without injury to the seed the process of reproducing the

plant. Such seeds or fruits when placed in fresh water after many months

of flotation in sea-water germinated very freely in a few days, whilst

those left in the sea-water under precisely the same conditions remained

unchanged. This is true of many of the seeds and fruits found in the

Thames drift, such as those of Ranunculus repens, Lycopus europæus,

Rumex, &c. A striking instance was also afforded by the seeds of

Arenaria (Honckeneya) peploides, where seeds transferred directly to

fresh water, after many months flotation in sea-water, germinated in a

few days; whilst those left in the sea-water remained unchanged. This

subject is discussed at length in Note 19, and needs no further mention

here.

If the seeds of many plants in Great Britain postpone through immaturity

their germination to the following or even to the second spring, it goes

without saying that this does not exclude temperature as the ultimate

determining factor in germination. The immaturity of seeds adds another

link to the series of the germination-range in plants. This range begins

with the plants where germination takes place on the tree and the

seedlings hang suspended from the branches, as in the typical mangroves

Rhizophora and Bruguiera. Here, as is shown in Chapter XXX., there is

evidently no period of repose between the completion of the maturation

of the seed and the commencement of germination. The range ends with the

detachment of immature seeds which ripen apart from the parent plant,

and may postpone the germinating process for months and often for years.

All intermediate stages exist between these two extremes. Thus the

seedling may at once detach itself from the parent as in Avicennia, or

the germinating process on the plant may be limited to the protrusion of

the radicle as in Laguncularia, or the seeds may be quite mature and

ready to germinate as soon as they fall to the ground, as we find with

many small seeded plants. All the stages, of which only a few are here

indicated, are full of suggestiveness for the student of plant-life.

This subject is dealt with from other standpoints in Chapter XXX., but

the reader will now see more clearly what was meant when I said that the

study of the behaviour of the floating seed leads us to the borderland

of vivipary. In this range of the germinating process we may possess an

epitome of the history of the climatic conditions of plant-life from an

early era in the world’s story, beginning with those ages when perhaps

under the uniform conditions that then prevailed, all plants were more

or less coast-plants and more or less viviparous, and coming down to the

present era when with an extensive and varied land-surface there is

great variety both in climate and in the range of germination. The

mangrove-swamp and its viviparous trees would thus represent from this

point of view a condition of things once more or less universal on the

globe.

_Summary of the Chapter._

(_a_) The tendency of the floating seed or fruit to germinate in the

brackish water of tropical estuaries is especially characteristic of the

plants of the mangrove-swamp and their vicinity; but with those of the

beach trees that occur in the river-drift it is rarely if at all to be

observed.

(_b_) From the wide distribution of plants of the mangrove-formation it

is evident that this readiness of the floating seed or fruit to

germinate is not prejudicial to the dispersal of the species.

(_c_) It may perhaps be in the main attributed to a strain of vivipary

running through all the plants of the mangrove-formation, which finds

its extreme development in the viviparous species, where germination

takes place on the tree. But it is probably favoured by the superheating

of the waters of tropical estuaries.

(_d_) In the case of the buoyant seeds of several climbers and creepers

of the Leguminosæ and Convolvulaceæ, more or less littoral in their

station, it is shown that in warm water, whether fresh or salt, a good

proportion are apt to sink through incipient germination, which results

when the experiment is made in sea-water in the death of the embryo.

(_e_) Though in tropical currents of ordinary temperature a good number

of such floating seeds would escape this risk, it is argued that there

are certain warm areas in the tropical seas that would prove much more

fatal to the chances of these drifting Leguminous and Convolvulaceous

seeds than the icy waters of a polar current. It is thus held that these

seeds often sink in mid-ocean in tropical latitudes through abortive

germination.

(_f_) The study of the behaviour of the floating seed or fruit leads us

to the borderland of vivipary. In the scale of the germinative capacity

of plants it is possible to arrange a continuous series that commencing

with the mangroves, where germination takes place on the tree, ends with

those numerous inland plants where seeds are liberated in an immature

condition.

(_g_) It is suggested that the viviparous habit may have been the rule

under the uniform climatic conditions of early geological periods and

that with the differentiation of climates that marked the emergence and

extension of the continental areas the viviparous habit has been lost,

except in those regions of the mangrove-swamps which to some extent

retain the climatic conditions once general over the globe. With

differentiation of climate the true seed-stage with its varying

rest-periods has been developed.

CHAPTER X

THE RELATION OF THE BUOYANCY OF SEEDS AND SEEDVESSELS TO THE DENSITY OF

SEA-WATER

The general principles concerned.—The subject assumes a statistical

character.—Seeds and seedvessels are as a rule either much heavier

than sea-water or much lighter than fresh water.—The present littoral

plants with buoyant seeds or seedvessels could be equally well

dispersed by currents in oceans of fresh water.—Seed-buoyancy has no

relation either in the present or in the past to the density of the

sea.—Though an accidental attribute, the specific weight of seeds has

had a profound influence on plant-distribution.—Summary.

TO find amongst the results of my numerous experiments examples

illustrating the influence of density on flotation has not been so easy

as I at first imagined. Excluding all adventitious causes of buoyancy, a

matter discussed in Note 40, it may be inferred that the great majority

of seeds and fruits sink both in fresh water and sea-water. Of those

that are buoyant many float indefinitely in both waters, whilst in a

very few cases, where the floating power is derived from an outer fleshy

covering, as with the fruits of Potamogeton natans, the fruits float a

much shorter time in sea-water than in fresh water, on account of the

injurious effect of the salt upon their coats.

Experiments have to be specially directed towards this subject. It would

be useless to experiment in fresh water at one time and in sea-water a

month later. Nor would it answer to employ seeds and fruits from

different localities, since variations in this way sometimes occur. It

is necessary that the experiments should be made on seeds or fruits

collected at the same time and place, and that they should be

simultaneous and carried on under the same conditions. As the discussion

proceeds, the reader will perceive that many interesting points are

opened up, and that such an investigation, instead of being, as the

title of this chapter might suggest, an abstruse and disconnected

inquiry, is of considerable importance in relation to the dispersal of

plants through the agency of currents.

Guided by the results of my experiments in this direction I will proceed

to lay down certain general principles:—

(A) In the first place it may be accepted as a general rule that _seeds

or seedvessels that sink in fresh water sink also in sea-water_, the

difference in density between the two being rarely a factor of any

importance in determining buoyancy. The great majority of seeds and

fruits come under this category, since, as is pointed out in Chapter

VIII., only a small proportion of the whole, say a tenth, possess

floating power. We might cite, as illustrative of this principle in

temperate regions, almost all the 240 species included in the

non-buoyant group of the British plants experimented on (see Chapter

III. and Note 10). As a general rule this is true alike of the small

seeds of the Cruciferæ and Scrophulariaceæ, of the nutlets of the

Labiatæ and Boragineæ, of the genus Scirpus, and of the dust-like seeds

of Juncus. The results of my experiments on the plants of the tropical

Pacific are no doubt typical of other tropical regions; and if I wished

to quote instances, I should have to enumerate not only most of the

plants without buoyant seeds or fruits that are mentioned in the Fijian

and Hawaiian lists given under Notes 2, 4, and 6, but also to appeal to

tropical regions generally.

(B) One can carry the principle above-named yet further and say that not

only as a rule do seeds or fruits that sink in fresh water sink also in

sea-water, but that so far as tested _many of them sink in water of much

greater density than that of ordinary sea-water_ (1·026). Thus, for

instance, the seeds of Nuphar luteum, Scrophularia aquatica, and

Stellaria aquatica, the nutlets of Polygonum persicaria, and the achenes

of Aster tripolium sank in sea-water the density of which had been

raised to 1·050, the limit of the experiment. The minute seeds of Juncus

communis and J. glaucus and the larger seeds of Luzula campestris, even

after drying for six months, sank in salt water having a density of

1·075. It would, however, seem probable that for most of these small

seeds and seedvessels a density of 1·100 would prove to be the critical

point. If this is so, then most of those that sink in sea-water would

float in the dense water (1·160) of the Dead Sea.

However, my investigations have only gone a small way in this direction;

and perhaps some of my readers will pursue the inquiry. I will take the

case of the nutlets of Scirpus palustris. They sink in fresh water and

in sea-water, or may float in rare cases for a day or two. Out of 100 of

these seed-like fruits, 25 floated in salt water of a density of 1·075,

13 in water of 1·050, 7 in sea-water (1·025), and 3 in fresh water,

(1·000). It would thus appear that the proportion of buoyant nutlets is

doubled with every increase of ·025 of the density scale. At this rate

of increase they would all float in salt water of a density of 1·125,

which may be regarded as the suitable medium for the flotation of the

fruits of this Scirpus.... The seeds of Glaucium luteum, the Sea-Poppy,

have no buoyancy either in fresh water or in sea-water even after

prolonged drying. They all sank in water of a density of 1·050, but 18

per cent. floated when the density was raised to 1·075. At the rate of

increase noticed in the case of Scirpus palustris, all the seeds would

float in water of a density of 1·130-1·140.... The acorns of the Common

Oak (Quercus robur) have usually but little buoyancy unless they have

been long drying. After soaking in fresh water for half an hour 100

mature fruits, without the cupule, that had been kept a fortnight, I

found that only 2 floated in fresh water, 6 in sea-water (1·025), and 18

in water of 1·050. At this rate of increase all would float in water

having a density of 1·080-1·090.

(C) There is also another general rule, and it is this:—_Seeds or fruits

that float for a long time in sea-water usually float almost as long in

fresh water_. Here belong the greater number of buoyant seeds and

fruits, those only able to float for a few weeks being comparatively

few. Now with the long-floating seeds and fruits, those for instance

that float in the drift of English rivers from the autumn to the spring,

or those that are transported by currents over the tropical zone, there

is, as a rule, but a slight difference between their flotation periods

in fresh water and sea-water. If one of them sinks after floating for

several months in fresh water, it will sink in sea-water a few days

after. Fruits of Scævola Kœnigii, pyrenes of Morinda citrifolia, and

seeds of Thespesia populnea, Ipomœa grandiflora, Cæsalpinia bonducella,

and of different species of Mucuna, that had been kept afloat for a year

in sea-water, floated just as buoyantly in fresh water at the close; and

in those cases where any sank during the course of the experiment, it

was ascertained that they were able to float in fresh water almost to

the end.

That many of the seeds and fruits of tropical littoral plants that are

known to be dispersed by the ocean-currents will float well in fresh

water is shown in the constant occurrence in the floating drift of

Fijian estuaries, where the water may be quite fresh or brackish, of the

seeds and fruits of plants like Cerbera odollam, Clerodendron inerme,

Entada scandens, Heritiera littoralis, Ipomœa pes capræ, Morinda

citrifolia, Mucuna, Vigna lutea, &c. In the same way I noticed afloat in

the Guayaquil River in Ecuador, when the water was quite fresh, seeds

and fruits characteristic of the sea-drift, such as those of Anona

paludosa (seeds), Entada scandens, Ipomœa, Mucuna, Vigna, &c.; and when

we supplement observation with experiment, as for instance in the case

of Anona paludosa, we find that they will float equally long in fresh

and sea-water.

The same rule prevails with most of the buoyant seeds and seedvessels of

plants of the British flora—seeds and fruits, as I may remind the

reader, that are mostly to be found in river and pond drift. I am not

able to distinguish any difference of importance in the results of the

separate fresh-water and sea-water experiments. Thus with the seeds or

seedvessels of Bidens cernua, several species of Carex, Galium palustre,

Iris pseudacorus, Lycopus europæus, Ranunculus repens, and numerous

others, the difference after a flotation of many months was but slight.

If the results of the separate experiments were to be compared, there

would be at least ninety afloat in fresh water for every hundred afloat

in sea-water; and if at the end of a sea-water experiment, whether

occupying three, six, or twelve months, the seed or fruits were to be

placed in fresh water, quite nine-tenths and sometimes more would remain

afloat. A striking illustration of the principle that the excess in

density of sea-water, as compared with fresh water, adds but little to

the floating capacity of seeds is to be found in the results given in

Note 41 of simultaneous experiments made some years since by Mr. Millett

and myself at Marazion and in London on the seeds of Convolvulus

soldanella.

(D) In their relation, therefore, to the density of fresh water and

sea-water, most seeds and seedvessels may be placed in _two principal

classes, the first including quite four-fifths of the total, where they

are much heavier than sea-water, and the second comprising most of the

remainder, where they are much lighter than fresh water_.

(E) It would be surprising, however, if there were not some seeds or

seedvessels that come between these two extreme groups; some, indeed,

that have a specific weight approximating to that of fresh water, or to

that of sea-water, or fluctuating between them, and presenting such

evidence of a fine adjustment that the observer, forgetting that they

are members of a series, might be apt to regard them as specially

adaptive in their origin. It will thus be seen that this subject is

gradually assuming a statistical character; and in truth we shall

ultimately recognise here the play of the laws of numbers.

As an example of the plants where the specific weight of the seeds or

fruits is near that of fresh water, Alisma plantago may be taken. In the

course of an experiment, by lowering the density of the water from 1·025

to 1·020, I sent a shower of floating carpels to the bottom. The results

vary considerably, as one might expect; but, generally, during the first

few days of an experiment about twice as many (sometimes in all as much

as 80 per cent.) sank in fresh water as in sea-water, a few only

floating in either water for long periods.... The seeds of Arenaria

peploides present an example where the specific weight is between that

of fresh water and of sea-water. For the purposes of dispersal they may

be considered as heavier than fresh water and lighter than sea-water.

The details are given in Note 18; but it may be remarked here that

plants possessing seeds or fruits that sink in fresh water and float in

sea-water are very rare. As indicated below, this is what we might look

for on statistical grounds.

Plants whose seeds or fruits are not much lighter than sea-water are

exceptional. In such cases the effect of increased density of the water

is to extend the period of flotation. Thus, in my experiments on the

nutlets of Scirpus maritimus, the majority of the fruits floated in

fresh water only eight to ten days; whilst in ordinary sea-water they

floated in most cases two to three weeks; but when the density was

raised to 1·050, the greater number of them were afloat after two

months. In a few plants, as with Spiræa ulmaria, the effect of the

difference in density between fresh and sea-water was not to extend the

period of flotation, but to increase the number that floated for a given

period, the extreme limit of the buoyancy of the carpels in either water

with this species being about three weeks.

Amongst tropical plants, as illustrated by those of the Pacific islands,

cases also came under my notice where the mean specific weight of the

seed is somewhere between those of fresh water and sea-water. The seeds

of Afzelia bijuga, an inland as well as a littoral tree in Fiji, offer

an interesting example. If we place 100 seeds of a littoral tree in

sea-water, we find that on the average about 70 float. If then we lower

the density gradually, some of the seeds begin to sink at once; and on

the removal of the survivors to fresh water, about 47 will remain

afloat. The results may thus be stated:—Out of 100 littoral seeds, 30

are specifically heavier than sea-water (1·025); 23 are between

sea-water and fresh water in specific weight; whilst 47 are lighter than

fresh water (1·000). When, however, we take 100 seeds of inland trees,

we find that on the average 87 are heavier than sea-water, 5 are in

weight between sea-water and fresh water, and 8 are lighter than fresh

water. The significance of these figures becomes evident when we arrange

them in curves. The combined result for littoral and inland seeds is

given in the diagram below; and we see there, what is also indicated

with the separate curves that we are dealing with a double series, one

concerned with seeds lighter than fresh water, and the other with seeds

heavier than sea-water. The reader can himself supply the separate

curves for the littoral and inland seeds. The point, however, to notice

is that if a botanist with a statistical bent were to make a

miscellaneous collection of the seeds of the Vesi (Afzelia bijuga) in

one of the Fijian islands, in order to test their buoyancy, he would

obtain such a result as is given in this diagram. Two varieties of the

tree would be at once indicated, and further research would indicate

that these varieties were connected with littoral and inland stations.

This subject is further dealt with in Chapter XVII.

+------------------------------------------------------------------------------+

|Combined results for 200 seeds of Afzelia bijuga (100 littoral; 100 inland). |

+--------------------------------+------------------------+--------------------+

| | | | |

|Percentage.| Heavier than sea- | Between sea-water and | Lighter than fresh |

| | water, or +1·025. | fresh water in weight. | water, or -1·000. |

+-----------+--------------------+------------------------+--------------------+

| | | | |

| 100 | | | |

| | | | |

| +--------------------+------------------------+--------------------+

| | | | |

| 80 | | | |

| | | | |

| +--------------------+------------------------+--------------------+

| | | | |

| 60 | | | |

| | | | |

| +--------------------+------------------------+--------------------+

| | . | | |

| 40 | . | | |

| | . | | |

| +--------------------+------------------------+--------------------+

| | . | | |

| 20 | .| | . |

| | |. | . |

| +--------------------+------------------------+--------------------+

| | | . . | |

| 0 | | . | |

| +--------------------+------------------------+--------------------+

| | | | |

+-----------+--------------------+------------------------+--------------------+

It might seem strange that the seeds of Entada scandens should come into

the category of seeds with a specific weight near that of fresh water;

yet my observations in Fiji indicate that such is the case. In the

discussion of this plant in Chapter XVII. it is pointed out that, as a

rule, not more than a fourth will float in a river when they are first

freed from the pod, and not more than fifty per cent. will float in the

sea. Those that float, however, in either water will usually float

indefinitely. The seeds also of Mucuna gigantea D.C. are not very much

lighter than fresh water. Out of six seeds that floated in sea-water

buoyantly, five floated in fresh water, but heavily.

It is of interest to notice in this connection that the

mangrove-seedlings produced by germination on the tree, as in the case

of Rhizophora and Bruguiera, have a mean specific weight somewhere

between fresh water and sea-water. This is often illustrated in a

curious way, when the seedling has not been prematurely detached from

the tree. Thus in the sea off the coast of tropical America, as well as

amongst the Fijian Islands, the seedlings of Rhizophora mangle are as a

rule to be observed floating horizontally; whilst in the fresh or

brackish water of the estuaries of these regions they assume a more or

less vertical position, only the plumular portion protruding above the

water. This is also true of the seedlings of Rhizophora mucronata, the

Asiatic mangrove, and of Bruguiera rheedii. This subject is discussed in

detail in Chapter XXX.; but it may be here remarked that a good

proportion of Rhizophora seedlings, when detached in the mature

condition from the tree, have no buoyancy, between 20 and 50 per cent.

going to the bottom when they fall into a river, and between 5 and 10

per cent. when they drop into the sea. The navigator might often obtain

an indication of the density of the sea-surface when approaching the

mouth of a large river by observing the floating Rhizophora seedlings (a

foot long) which are carried out to sea in numbers. If he sees them from

the deck of his ship floating horizontally he will infer that the

surface-water is mainly sea-water. In ordinary fresh water when they

float vertically he would not be able to distinguish them from floating

seeds or fruits.

It has only been possible to treat this subject in an illustrative

manner. More details might have been given; but I have gone far enough

to bring the following points into relief and to justify one in drawing

the conclusions to be now stated.

_Most seeds and seedvessels in respect of their floating powers tend to

gather around two centres or means and to form two groups, the sinking

group and the buoyant group._

_In the sinking or non-buoyant group, which includes 80 per cent. of the

whole, the mean specific weight is considerably greater than that of

sea-water (1·026), which would require its density to be raised to 1·100

in order to serve as a floating medium for many of them._

_In the buoyant group the mean specific weight is much lighter than that

of fresh water (1·000); and from this it is to be inferred that in

oceans of fresh water the same fruits and seeds in the mass would be

distributed by the currents that are transported by them at the present

day. Even though it arose from an ocean of fresh water, the coral island

would receive the same littoral plants through the agency of the

currents that it receives under its existing conditions._

The number of plants with seeds or fruits between fresh water and

sea-water in specific weight is very small, probably not over 2 per

cent. of the total. Most seeds or fruits that sink in fresh water sink

also in sea-water, and most that float in sea-water float also in fresh

water. _Nature has thus created a wide gap between the sinking and the

floating seed; and nearly all of the work of the present currents in

plant-dispersal might have been effected, so far as the density is

concerned, in fresh water._ She has not arranged seeds and seedvessels

in what the statistician would term “a good series.” As indicated in the

diagram below, there are two series that meet in the neutral region

where the density is between fresh water and sea-water, but with

culminating points placed on the one side far above the density of

sea-water and on the other far below that of fresh water.

+-------------------------------------------------------------------------------+

| Relation of the specific weight of seeds and fruits to the density of |

| fresh and sea-water. |

+--------------+--------------------+-----------------------+-------------------+

| Percentage. | Heavier than sea- |Between fresh and sea- | Lighter than fresh |

| | water, or +1·026. | water, 1·000-1·026. | water, or -1·000. |

+-------------+--------------------+-----------------------+--------------------+

| | | | |

| 100 | | | |

| | | | |

| +--------------------+-----------------------+--------------------+

| | | | |

| 80 | | | |

| | | | |

| +--------------------+-----------------------+--------------------+

| | . | | |

| 60 | . | | |

| | . | | |

| +--------------------+-----------------------+--------------------+

| | . | | |

| 40 | . | | |

| | .| | |

| +--------------------+-----------------------+--------------------+

| | | . | |

| 20 | | . | |

| | | . | |

| +--------------------+-----------------------+--------------------+

| | | . | . |

| 0 | | . .| |

| | | . | |

| +--------------------+-----------------------+--------------------+

| | | | |

+-------------+--------------------+-----------------------+--------------------+

I do not, therefore, think that the buoyancy of seeds and fruits has had

any relation either in the present or in the past to the density of the

sea. Nor is it to be supposed that any slight variations in density in

the course of ages would have materially affected the dispersal of

plants by currents. It is to be inferred that the physicist and the

geologist would be prepared to grant only small variations, such as a

change from 1·020 to 1·025. It will be gathered from what has been said

before that changes of this nature would have a very slight influence on

the buoyancy of seeds and fruits, since the plants they would affect

would be very few. The change that the student of plant-dispersal would

require to produce any marked alteration in distribution would be in

amount alarming to the physicist.

Whether or not the oceans have been getting fresher or salter in the

course of ages (see Note 42), we will be moderate in our demands, and

will listen to the physicist when he argues that a diminishing density,

for instance, from 1·035 to 1·025, in the course of ages might explain

some of the peculiar features in the present isolation of insular

floras. Many seeds, he would contend, that could float across an ocean

having a density of 1·035 would be unable to accomplish it when the

density fell to 1·025. It has, however, been remarked that the critical

point of density for the flotation of seeds or fruits that sink under

present conditions is probably about 1·100. Cases of such a fine

adjustment to the density of sea-water are too few to endow this

argument with any weight. Or it might be suggested that with a gradual

increase in density in the lapse of ages seeds might float now that sank

before, or they might float for a longer period. Such a change, however,

would not have much effect, since nearly all the seeds and seedvessels

that sink in our rivers sink also in our seas, and a much greater

increase of density is required to make any difference.

Yet, although we might term the sinking of a seed or fruit an accidental

attribute of certain plants, just as we might regard the floating of a

log as an accidental attribute of a pine, since in either case the

specific weight might have been acquired without any direct relation to

the density of water, still the sinking of the seed or fruit signifies a

profound distinction not only, as is stated below, in plant

distribution, but, as we shall see later on, in plant-development.

Especially striking, says Prof. Schimper (p. 153), is the dependence

between an over-sea area of distribution and a station at the coast in

the case of species of the same genus of which some belong to the

littoral and some to the inland flora. In the first place, as has been

often remarked in these pages, we have a wide distribution generally

associated with considerable buoyancy of the seeds or fruits. In the

second case the areas are usually very restricted and there is little or

no buoyancy. The better fitted a seed or fruit is for dispersal by

currents the greater, therefore, is the area of the plant. Whether such

an important relationship depends on an accidental attribute of the seed

or fruit is the question that immediately presents itself. But it is

obvious that in raising such a question we touch on a very vital point

in adaptation, since if attributes developed in one connection have a

profound influence in another we may have to rearrange some of our

fundamental notions of the inner workings of Nature.

Let us, therefore, look a little closer into this matter, and turn again

to the Pacific islands. The present state of things may be thus tersely

described. Whilst the shore-plants dispersed by the currents have

remained relatively the same, changes of all kinds, from the production

of a variety and of a species to the development of a genus, have taken

place in the inland floras. Now, let us imagine that all this is altered

and that every seed or fruit is buoyant. There would then be but little

distinction between the strand and inland floras, since they would be in

a constant state of interchange, and most species would be widely

distributed. A relatively monotonous aspect would belong to all insular

floras, and indeed to much of the plant-world, since isolation, one of

the principal conditions for the origin of new species and new genera,

would often not exist.

On the other hand, let us suppose that all seeds and fruits were

non-buoyant. The agency of birds would then be alone available for

stocking new islands with most of their plants. The conditions of

isolation would be intensified. There would be no widely-ranging

strand-flora, since every island and every stretch of continental

sea-board would possess its own littoral plants that could only reflect

the peculiarities of the inland flora. The only determining factor

between coast and inland plants would be the presence or absence of the

capacity or organisation for occupying a station on the sea-shore.

We have now proceeded far enough to disclose the far-reaching influence

on plant-distribution and on plant-development that the relation between

the specific weight of seeds and fruits and the density of sea-water

must possess. Yet it has been shown that when such a relation is viewed

statistically it has an accidental aspect. We will accordingly devote

the next few chapters to the discussion of the buoyancy of seeds and

fruits from the structural standpoint.

_Summary of the Chapter._

(_a_) The great majority of seeds and seedvessels (quite 80 per cent.)

are much heavier than sea-water, but a noticeable proportion are

considerably lighter than fresh water, whilst those with a specific

weight near that of fresh water or of sea-water are very few.

(_b_) The buoyancy of seed and fruit has no direct relation to the

density of sea-water, and even if the ocean was deprived of all its

dissolved salts, the agency of the dispersal of plants by currents would

not be materially affected.

(_c_) Small changes in sea-density, such as the physicist would allow,

would, therefore, have no appreciable influence on the operations of the

currents as plant dispersers; and only great changes in density, such as

are presented by the waters of the Dead Sea, would add materially to the

number of floating seeds and fruits.

(_d_) Although the specific weight of seeds and fruits in its relation

to sea-density may be regarded as an accidental attribute, their

non-buoyancy in the great majority of plants has had a far-reaching

influence not only on plant-distribution, but on plant-development. The

plant-world would be transformed if all seeds and fruits floated in

sea-water.

(_e_) If the floating seed or fruit displays a quality that, so far as

the density of the sea is concerned, has been developed in quite another

connection, we have next to inquire whether the structure of such

buoyant seeds and fruits also affords evidence of non-adaptation.

CHAPTER XI

ADAPTATION AND MEANS OF DISPERSAL

Nature has never concerned herself directly with providing means of

dispersal.—Fleshy fruits not made to be eaten.—Nor “sticky” seeds to

adhere to plumage.—Nor prickly fruits to entangle themselves in fur

and feathers.—The dispersal of seeds a blind result of the struggle

between the intruding Evolutionary power and the controlling influence

of Adaptation.

BEFORE entering into a discussion of the causes of the floating powers

of seeds, it is necessary that I should state my general position on the

relation between capacities of dispersal in the organic world and the

question of adaptation. Adaptation runs through all the organic and

inorganic worlds, and we cannot conceive an universe without it. The

naturalist who looks only for the end in the purpose served makes but a

partially legitimate use of the phrase. On the other hand, it has been

improperly appropriated by those who hold to the theory of Natural

Selection, as indicating the result of small fortuitous variations that

have chanced to be of service to the species in the struggle for

existence. There is no question here of any end in view. Nature is

represented as working blindly, and the result of such “fortuitous

variation” is termed an adaptation. We cannot, however, pick and choose

only adaptations that are very evident in their character. We must

include everything in the organic world as an adaptation, whether

apparent or not, that is in direct relation with the organism’s

conditions of existence. It is not conceivable that an organism can be

adapted to conditions outside its environment, and yet many so-called

adaptations are of this character.

Nature—and I here confess my belief in a determining agency working

above and through all living and dead matter, but largely controlled and

checked by the laws of the physical world—Nature, as I apprehend, has

never concerned herself directly with providing means of dispersal

either for plants or animals. With regard to plants, she makes no direct

provision for the distribution of their fruits or seeds. If she had done

so, she would have employed some uniformity in her methods, as in the

instance of the means of reproduction; whereas the modes of dispersal

are almost infinite in their variety. When I say that Nature makes no

direct provision for the dispersal of plants and animals, I mean not in

the sense that a bird is adapted for an aerial life, or an aquatic plant

for a more or less submerged existence. That a bird is often able to

distribute its kind over a great area is the “accident” of its

conditions of existence. In a similar way the wide distribution of the

“ticks” that they carry round the world is due to the parasitical habits

of these insects, habits that have been acquired without any view to

their mode of dispersal by birds.

Similarly it cannot be said of seeds or fruits that are transported by

birds, whether adhering to their plumage by means of hooks or hairs, or

through some viscid excretion, or inclosed in soil adhering to the feet

or legs, or carried in the stomach and intestines, that Nature has made

any special provision for their dispersal. The dispersing agencies take

advantage of certain capacities or characters of a seed or fruit that

have been developed in the plant for quite other reasons and in

conformity with quite other principles. There may be mentioned as

examples the mucosity of seeds, the fleshiness of fruits, the occurrence

of hairs and prickles, &c. Yet as far as their connection with dispersal

is concerned, such capacities and characters are blind results in the

history of the plant’s development, the dispersing agencies making use

of what was not intended for them.