GENERAL ARGUMENT AND CONCLUSION _Pages_ 515-523
APPENDIX _Pages_ 525-605
LIST OF ILLUSTRATIONS
PLATE.
The Fijian species of Rhizophora _Frontispiece._
FIGURES.
TO FACE PAGE
Diagrams illustrating some of the causes of 111
seed-buoyancy
Figures illustrating the development of the seed 452-453
and the germinating process of Rhizophora and
Bruguiera
Diagrams illustrating the structure of the growing 574
seeds of Barringtonia
Diagram illustrating the prevailing 585
cloud-formations of Mauna Loa
MAPS.
Oceania 12
The Ocean Currents 61
Trade routes of the Pacific Ocean (intended to 66
illustrate the distances traversed by floating
seeds in that ocean)
The West Coast of South America 474
Rough plan of the Gulf of Guayaquil 484
ADDITIONS AND CORRECTIONS
Page 5 and subsequent pages. _For_ Ipomea _read_ Ipomœa.
Page 68. _For_ Hippomanes _read_ Hippomane.
Page 68. _For_ Conocarpus erecta _read_ Conocarpus erectus.
Page 122. Sir W. Buller includes the fruits of the Puriri tree (Vitex
littoralis, according to Kirk) amongst the food of the New Zealand
fruit-pigeons.
Page 177. _For_ Entata, in the head-line, _read_ Entada.
Page 266. The fruits of Oncocarpus vitiensis have been found in the
crop of a Fijian fruit-pigeon (Carpophaga latrans). _See_ Hemsley’s
_Bot. Chall. Exped._, Introd., 46, and iv. 308; also Newton’s
_Dictionary of Birds_, p. 724.
Page 368. Sernander (p. 185) observes that the fruits of Naias marina
have little or no floating power.
Page 416. For the first eight lines read as follows:—“Of these, 22
occur in Continental regions on both sides of the Pacific; 12 are
found in the Old World alone; one is peculiarly American, and two are
confined to the Australian and Polynesian regions. A few of these can
be regarded as exclusively American in their origin, though the bulk
of them hail evidently in the first place from the Old World. But
from the circumstance that all or most of the other species of the
genus concerned are confined to America, it may legitimately be
inferred that Waltheria americana, Ageratum conyzoides, and Physalis
angulata are American-born species. Teucrium inflatum is a peculiar
instance of an American weed collected in Polynesia before apparently
it had been recorded from the Old World.”
Page 438. _For_ Conocarpus erecta _read_ Conocarpus erectus.
Page 417. Add after Cardiospermum halicacabum.... “Its seeds, as my
experiments show, possess little or no capacity for dispersal by
currents, since they sink at once or within a few days, even after
drying for months.”
Page 455. Omit the reference to figure 6 in the centre of the page.
Page 498. _For_ Hippomanes _read_ Hippomane.
Page 508. Amongst my Solomon Island collections identified at Kew were
the fruits of a species of Litsea from the crop of a fruit-pigeon
(Hemsley’s _Bot. Chall. Exped._, IV. 295.)
Page 533. _For_ Commelyne _read_ Commelina.
Page 539. At foot of page, _for_ Thames sea-drift, _read_ Thames
seed-drift.
Page 581. _For_ Crambe maritimum _read_ Crambe maritima.
Page 618. Under Mascarene Islands add Myoporum to the plants linking
them to the Pacific Islands.
------------------------------------------------------------------------
OBSERVATIONS OF A NATURALIST
IN THE PACIFIC
CHAPTER I
INTRODUCTION
The study of insular floras.—Their investigation in this work from the
standpoint of dispersal.—The significance of plant-distribution in the
Pacific.—The problems connected with the mountain-flora of Hawaii.—The
persistence of dispersing agencies at the coast, their partial
suspension on the mountain-top, their more or less complete suspension
in the forest, and the effect on the endemic character of plants.—The
connection between the endemism of birds and plants.—The relative
antiquity of plants of the coast, forest, and mountain-top.—The
genetic relation between coast and inland species of the same
genus.—The ethics of plant-dispersal.—Evolution takes no heed of modes
of dispersal.—The seed-stage is the price of Adaptation.
TO proceed from the general to the special is the only method of dealing
with insular floras. A broad and comprehensive grasp of
plant-distribution, such as is only acquired by a life-time of research
aided by travel and the handling of large collections, is a necessary
foundation for the study; yet in the nature of things such
qualifications can be possessed by but a few. To direct an inquiry in
the opposite direction, and endeavour to attack the problem of
continental floras through the insular floras would result merely in the
investigation of a few of the many questions connected with
plant-distribution.
The panoramic sketch of the surveyor on the mountain-top aids him in a
thousand ways when after months of tedious labour he plots the details
in his chart. Without such a panoramic view of the plant-world in his
mind’s eye, an observer like myself can only look for guidance to the
writings of those who have generalised on the foundations of a far
broader experience, such as those of Bentham, De Candolle, Gray, Hooker,
Schimper, and others.
It would be quite possible for a botanist possessing a profound general
acquaintance with the plant-world to dispense altogether with actual
observation and experiment on modes of dispersal. It would be quite
possible for him to arrive at conclusions, which, even if they did not
always come into line with results of observation and experiment, we
should be compelled to prefer. It is only from his more elevated
position that a general can follow the course of a battle; whilst the
private with his experience confined to a limited area of the field of
conflict may form the most erroneous ideas of the progress of the fight.
So it is with observers whose employment it is to struggle with the
details and secondary principles of plant-distribution, and so it is
with the generaliser who has already roughly mapped out the principal
features of the main problem.
When Mr. Bentham in 1869, remarking on the paucity of species common to
tropical Asia and America, characterised them either as plants wholly or
partially maritime and spread by the currents, or as weeds dispersed by
cultivation over the warm regions of the globe, he mentioned amongst the
plants in the former category, Gyrocarpus jacquini. This tree presents
one of the mysteries connected with the Pacific islands; and I don’t
imagine that this eminent botanist could have known anything except
inferentially as regards the mode of dispersal of its fruits. Yet
experiment shows how well founded the inference was, whilst behind it
lay a life-time of botanical research.
The author thus approaches the subject of the floras of the Pacific
islands rather as a plotter of detail than as a delineator of great
designs. However much we may study the means of dispersal, we have
behind them the great facts of distribution, serving like the main
stations of a trigonometrical survey, and with these we have to make our
lesser facts and observations square. One is conscious all the time that
much of what seems new in one’s researches has already been foreseen by
the generaliser, and that one can do little else than assist in
confirming some of his results. This is all that I can lay claim to in
this work.
The floras of the islands and coasts of the tropical Pacific are here
regarded entirely from the standpoint of plant-dispersal. The fruits and
seeds rather than the flowers have been the subject of my
investigations; and although there is much to please the eye in the
flora of a Pacific island, it was always with a sense of disappointment
that I turned away from some pretty flowering plant that failed to
present me with its seed. Amongst the wonders of the plant-world rank
the Tree Lobelias of the Hawaiian Islands; yet their greatest charm to
me lay not so much in their giant-flowers and their arborescent habit as
in the mystery surrounding the home of their birth and their mode of
arrival in these islands. When I first stood under the shade of the
lofty Dammara vitiensis, the Kauri Pine of Fiji, all my interest lay in
its cones lying on the ground; and I remember how eagerly I handled my
first specimen, and how anxiously I watched its behaviour when
experimenting on its capacity for different modes of transport. When a
strange plant presented itself on a beach, my first care was to
ascertain the fitness of its fruits or seeds for transport by the
currents; and all inland plants with fruits likely to attract
frugivorous birds were at once invested with a special interest for me.
The mangrove swamps were always great places of interest, and months of
my sojourn in the Pacific must have been passed in exploring their
creeks and in examining their vegetation. Botanists usually avoid these
regions; but the observation of the germination of the Rhizophora fruits
on the trees and the inquiries connected with their methods of
distribution over the oceans were pursuits so engrossing that I ignored
the numerous discomforts connected with the exploration of these gloomy
regions. The magnificent mangrove forests of the Ecuador coast of the
Pacific will live longest in my memory, though the risks were
considerably greater and the discomfort of existence extreme. But the
mangrove swamps present us with glimpses into the conditions of plant
life during the warmer epochs of the earth’s history, when perhaps the
seed-stage was largely dispensed with, whilst an atmosphere, laden with
moisture and screening off much of the sun’s light, enveloped most of
the circumference of the globe.
The plant world viewed only from the standpoint of dispersal may lack
much that is pleasing to the eye, though it abounds with small and great
problems fascinating to the reason. Matters of great moment are here
involved, and in the case of the Pacific islands they concern not only
the source of the oceanic floras, but the story of the islands
themselves; whilst behind these there rise up questions of yet deeper
import, questions that are bound up with the beginnings of genera and
species, and with other mysteries of life on the earth. The distribution
of plants presents something more than a problem of means of dispersal,
or a problem of station, or a problem of plant migration connected with
climatic changes. It is something a great deal more than all three,
since it is indissolubly connected with a past, of which unfortunately
we know very little. Let us take it to be a question of means of
dispersal, and then in imagination transporting ourselves to the
Scandinavian coast, let us gather up the stranded West Indian beans of
Cæsalpinia, Mucuna, and Entada, that have been drifted there for ages by
the Gulf Stream, and lie in some cases semifossilised in the adjacent
peat-bog. Was ever dispersal so utterly purposeless as this? Yet here
lies a principle of plant-dispersal that is fundamental. We see it in
the thistle-seed floating seaward in the wind. Nature never intended its
pappus for such an end. It was formed for quite another purpose, yet it
aids largely the dispersion of the plant. What can be more significant
than that?
Or let us take it to be a matter of station. Given time and the
recurrence of the same conditions, with others I once imagined that we
could explain most things in plant-distribution, whether of plants at
the coast or of plants inland, whether of plants of the alpine peaks or
of plants of the plains, or of plants of the river or of the pond. Time,
it was held, had long since discounted the means of dispersal, and
distribution became merely an affair of station. But the supplanting of
many indigenous species of a flora by introduced species is a common
story in the plant-world; and such a view needs no further discussion
here. Nor is distribution only concerned with plant-migration. Any
theory of the origin of alpine floras on tropical mountains will have to
explain the presence of the temperate genera, Geranium and Sanicula, not
alone on the summits of the mountains of Equatorial Africa and
Madagascar, but on the uplands of Hawaii in mid-Pacific, where also are
found Ranunculus, Vaccinium, Fragaria chilensis (the Chilian
strawberry), and Drosera longifolia.
Taking genera of different stations each in their turn, and following up
the clues thus afforded, it would be possible to find support for all
the reputable views relating to plant-distribution. The wide range of
aquatic plants under conditions that completely change the character of
the terrestrial vegetation, such, for instance, as Myriophyllum and
Ceratophyllum, might be plausibly attributed to the relative uniformity
of the conditions of aquatic life both in time as well as space. The
occurrence of Vaccinium on mountain-tops over most of the world, even on
the highlands of Samoa, Tahiti, and Hawaii in the Pacific Ocean, would
be rightly regarded as evidence of active dispersal of the seeds through
the agency of birds from one mountain-summit to another, whether in
mid-ocean or in the centre of a continent. The prevalence of the same
beach-plants over most of the globe in the same climatic zones would
point unmistakably to the predominant agency of currents. But with many
plant-genera, some of which range the world, whilst others again may be
restricted to a single group of islands in the Pacific, there is often
no question either of means of dispersal, or of station, or of
plant-migration, and problems of a very different nature are opened up.
When we leave the beach and the mountain-top, the river and the pond,
all the troubles of distribution begin; and since but a small proportion
of plants in a typical flora belong to these stations, it follows that
difficulties will dog our steps with the large majority of the plants.
The agencies of dispersal now working around us, the current, the wind,
the insect, the bird, and the bat, will explain many of the features of
littoral and alpine floras and of the vegetation of ponds and rivers.
Here we have in so many cases wide-ranging genera with the means of
dispersal ready to hand. We can connect the wide range of Vaccinium with
the wide range of birds of the grouse and other families that feed on
the berries. We can associate the great areas of aquatic or sub-aquatic
genera, like Potamogeton and Sparganium, with the migratory habits of
the ducks in the stomachs of which we find their seeds. We can connect
the great ranges of beach plants like Ipomœa pes capræ in the tropics,
and Convolvulus soldanella in the temperate regions with the currents,
and the almost cosmopolitan range of many ferns and lycopods with the
winds and other agencies.
When, however, we enter the forests we find genera that are often much
more restricted in their areas, and species that are yet more limited in
their range. There is very little dispersal going on here. The birds are
strange. Their distribution is usually very local. They look lazily down
at us from the branches, as they disgorge the seeds and stones of the
fruits they have eaten, which cover the ground around. We can almost
fancy that they say:—“Our work is done. We rest from the toil of our
ancestors. They carried seeds to far-distant Hawaii, Tahiti, and Savaii.
Our work is done.” And as we walk through those noiseless forests, where
the machinery of species-making is ever in silent motion, we become
aware that we are treading one of Nature’s great workshops for the
manufacture of species and genera. Outside the forest all is bustle and
hurry. We are in the streets, or rather in the distributing areas of the
plant-world. We hear the noise of the breaker, the roar of the gale, the
cry of the sea-gull, the flapping of a myriad pairs of wings of some
migrating host overhead, and we know that the current, the wind, and the
bird are actively at work; but their operations are confined mainly to
the beach, the mountain-top, the river, and the pond.
Let us take a well-wooded Pacific island several thousand feet in
height. We find on its beaches the same littoral plants that we have
seen before on the tropical shores of Malaya, of Asia, of Africa, and of
America. We find in its ponds and rivers the same species of
water-plants, such as Ceratophyllum demersum, Ruppia maritima, and Naias
marina, that are familiar to us in the cool and tepid waters of much of
the globe. On its level summit, if it remains within the clouds we find
in the boggy ground, where Sphagnum thrives, genera that are represented
in Fuegia, New Zealand, and the Antarctic islands, such as Acæna,
Lagenophora, and Astelia, and the world-ranging Drosera longifolia. In
other elevated localities we find Ranunculus, Geranium, Sanicula,
Artemisia, Vaccinium, and Plantago, chiefly genera of the temperate
regions of the northern hemisphere; whilst there are also found Gunnera,
Nertera, and Uncinia, all hailing from the south and belonging to the
Antarctic flora characterising all the land-area around the globe in the
latitude of New Zealand and Fuegia. The Hawaiian species of Nertera and
of Uncinia occur also in New Zealand, and the first-named is found also
in Tristan da Cunha and in South America. In the Hawaiian uplands there
is also to be seen Deyeuxia, a genus of grasses found in the Tibetan
highlands and in the Bolivian Andes at elevations of 16,000 to 19,000
feet; and the same species that exists in Australia may be found in the
mountains of Hawaii. Here also, both in Hawaii and Tahiti, occurs Luzula
campestris.
In making the foregoing remarks on the alpine plants of a Pacific
island, I have had Hawaii in my mind, but we find the elements of a
similar widely-distributed mountain-flora in the less lofty peaks of
Tahiti and Samoa, and traces even in Fiji, where the mountains, however,
have only a moderate elevation. But the point I wish to lay stress on is
the cosmopolitan yet temperate character of the mountain-flora of an
island lying in the midst of the tropical Pacific. As he shifts his
station on this mountain-summit, the observer might at different times
imagine himself in the Sierra Nevada of California, on a Mexican
tableland, on a peak of the Andes, or in the lowlands of Fuegia. Other
plants that I have not mentioned, such as Coprosma, would bring back to
him New Zealand. He might even be on a mountain-top in Central Africa,
or on a Madagascar plateau; whilst in the boggy region of an elevated
Hawaiian tableland he would meet with not only the physical conditions,
but also several of the plants found on the higher levels of Tristan da
Cunha.
It is, however, to be noted that although these mountain-tops in the
mid-Pacific have been stocked with genera from the four quarters of the
compass, the species as a rule are restricted to that particular
archipelago. Whilst the beach and the river in most cases possess plants
that have very wide ranges over the earth, a good proportion of the
species on the mountain-summit are not found elsewhere. This implies a
partial suspension of the means of dispersal on the mountain-top, whilst
the currents and waterfowl are still actively distributing the seeds of
the littoral tree and of the aquatic plant. We here get a foreshadowing
of another great principle, or of another line along which Nature has
worked in stocking these islands of the Pacific with their plants, a
subject concerning which much will be said in later pages.
Hitherto, we have dealt only with a small proportion of the flora, and
with but a small portion of the area of the island. We have yet to deal
with the intermediate region between the sea-border and the summit of
the island, or, in other words, with the forested mountain slopes. This
is the home of many of the peculiar species and peculiar genera, both of
plants and birds; and it is with this zone that we shall be mainly
concerned when we come to contrast the floras of the several
archipelagoes of the tropical Pacific. Here the agencies of dispersal
have, to a large extent, ceased to act; and the question will arise as
to the connection between the endemic character of the plants and the
endemic character of the birds. We shall have to ask why this island,
after receiving so many plants, ceased to be centres of dispersal to
other regions. It is possible that these seeds or fruits have lost their
capacity for dispersal; but only a few instances of this change present
themselves. Rather it may be supposed that the birds that originally
brought the seeds to the island came to stay; and this at once suggests
another query as to the cause of the change of habit. I am alluding here
not to the plants with minute seeds, such as Sagina and Orchis, which
Mr. Wallace, in his _Darwinism_, regards as capable of being transported
by strong winds over a thousand miles of sea; but to those numerous
plants found in the Fijian, Tahitian, and Hawaiian forests, where the
seeds and “stones” are large and heavy, measuring often as much as a
quarter of an inch (6 mm.), and sometimes nearly an inch (25 mm.) in
size. The reader will be surprised to learn how little “size” has
determined the distribution of seeds and fruits in the Pacific. He will
have to appeal to the habits of pebble-swallowing of the Dodo, the
Solitaire, the Goura pigeon, the Nicobar pigeon, &c., if he desires to
find a parallel in the habits of birds.
It is here assumed that the reader is already acquainted with the
principles involved in a discussion of island-floras, principles clearly
laid down in the writings of Hooker, Wallace, Hemsley, and others. As a
general rule in an island or in a group of islands where there are a
large number of plants not found elsewhere, there is also a large
endemic element in the avifauna, and where none of the plants are
peculiar, endemic birds are either few or wanting. As an example of the
first we may mention Hawaii, and Iceland affords an instance of the
second. But there is no hard and fast rule connecting the endemic
character of the plants and birds of an island with its distance from
other regions. Even the small group of Fernando Noronha, lying only some
200 miles off the coast of Brazil, possesses its peculiar birds and its
peculiar plants; and we can there witness the singular spectacle, as
described by Mr. Ridley, of an endemic bird, a frugivorous dove, engaged
in scattering the seeds of endemic plants over the little group. This is
the only fruit-eating bird in the islands, remarks the same botanist in
the _Journal of the Linnean Society_ (vol. 27, 1891); and “when one sees
the number of endemic species with edible fruits, one is tempted to
wonder if it were possible that they were all introduced by this single
species of dove, or whether other frugivorous birds may not at times
have wandered to these shores.” This inter-island dispersal in a
particular group of peculiar plants by peculiar birds is a common
spectacle in the Pacific. The contrast between the large number of
plant-genera possessing fruits that would be dispersed by frugivorous
birds and the poverty of fruit-eating birds in the avifauna is well
displayed in Hawaii.
The island of St. Helena would seem to offer an exception to the rule
that endemic birds and endemic plants go together, since, though its
flora possesses a very large endemic element, there are scarcely any
endemic or even indigenous birds recorded from the island. We can never
know, however, how much of the original fauna disappeared with the
destruction of the forests. It would nevertheless appear that but few of
the genera possessing peculiar species of plants were adapted for
dispersal by frugivorous birds. The lesson to be learned from this
island concerns the Compositæ, often arboreous, that constitute the
principal feature of its flora. St. Helena retains almost more than any
other island evidence of the age of Compositæ which has left its impress
on many insular floras; and when we discuss the original modes of
dispersal of the endemic Hawaiian genera of the same order we shall look
to the flora of this Atlantic island for assistance in the matter. To
the age of Compositæ belong the beginnings of several insular floras.
To return to the main line of our argument, it would seem that in a
Pacific island there is a constant relation between free means of
dispersal and the preservation of specific characters. The ocean-current
and the aquatic bird are in our own time actively engaged in dispersing
the seeds of shore-plants and water-plants, and we see the same species
ranging over the world. On the other hand on the mountain-top the
agencies of dispersal are beginning to fail, and as a result many a
mountain has some of its species restricted to its higher regions. In
the forest zone there has been a more or less complete suspension of the
activity of the dispersing agencies, and new genera are formed whilst
peculiar species abound. Free means of communication with other regions
restrains but does not arrest the differentiating process that is ever
in progress throughout the organic world. Isolation within certain
limits gives it play.
It is in this connection interesting to reflect that during the
differentiation of the inland flora the littoral plants have lagged
behind or have remained relatively unchanged. The currents have been
working without a break throughout the ages; and the cosmopolitan
Ipomœa, that now creeps over the sand of the beach, or the wide-ranging
Rhizophora, that forms the mangroves of the coast-swamp, must have
witnessed the arrival of the ancestors of several of the endemic inland
genera. The swamp-plants of the littoral flora are probably older,
however, than the beach-plants which have been recruited from time to
time in one region or another of the tropics from the inland flora. Yet
as a body the littoral plants have lagged far behind the inland flora.
We might thus expect that in a Pacific island, excluding the
wind-distributed plants, such as the ferns and the lycopods, the most
ancient types of the plants would be found at the coast, the most modern
in the forests, whilst the plants of the mountain-summit would represent
an intermediate age.
But true as this may be, the composition of a strand-flora is a very
complex one. Although, as Prof. Schimper remarks, the mangrove formation
is more isolated than the beach formation, and affords evidence of a
much earlier separation, the beach-plants as a body are anything but
homogeneous in their character. Their physiognomy varies to some extent
with the alteration in the characters of the inland flora, changes to
which the mangrove formation makes a very slow response. Yet amongst the
plants of the beach we find strangely assorted forms that are as ancient
denizens of the coast as the mangroves themselves. Take, for instance,
Salsola Kali, that thrives alike on a beach in Chile, on the sea-shore
in Devonshire, and in the salt-marshes of the interior of Tibet. Then,
again, there is a type of littoral plant, of which Armeria vulgaris and
Plantago maritima may be taken as examples, which is equally at home on
the beach and on the tops of inland mountains. We might in a sense apply
the wrecker’s motto,
“What the sea sends and the land lends,”
to the history of a littoral flora. Yet on the other hand the inland
flora in its turn receives a few recruits from the littoral flora; and
it is the relation between the inland and coast species of the same
genus that offers one of the most fascinating studies in the botany of
the Pacific Islands.
This introductory chapter may be concluded with a few remarks on what
may be termed “the ethics of plant-dispersal.” Not that this is in any
way a suitable phrase, but it best expresses my sense of the lack of
propriety in some things connected with this subject. It is odd, for
instance, that we speak of the dispersal of plants and animals in the
same breath, as if the process was in both cases identical. Seeing that
from this point of view we judge a plant only by its seeds and fruits,
it is apparent that we are following quite a different method than that
which we employ in the study of the dispersal of animals. Whilst the
zoologist classifies the units of dispersal, the botanist does nothing
of the kind; and the two systems of classification are at the outset
fundamentally distinct. The student of plant-dispersal thus often finds
himself placed in an awkward dilemma. For him a family is a collection
of allied genera having similar seeds or fruits and fitted often for the
same mode of dispersal. A family like Sterculiaceæ, possessing such a
variety of seeds and fruits suitable for very different modes of
dispersal, is from his standpoint a collection of dissimilar units.
Genera like Commersonia, Waltheria, Kleinhovia, Sterculia, and
Heritiera, that he so often meets with in the Pacific Islands, have in
these respects frequently very little in common; and yet one of the
earliest determining influences in plant-life must have lain in the
capacity for dispersal.
Yet chance seems to reign in the processes of plant-dispersal ever going
on around us. In the floating seed, in the achene with its light pappus
blown before the gale, in the prickly mericarp entangled in the plumage
of a bird, in the “stone” of the drupe disgorged or ejected by the
pigeon, in the small grain that becomes adhesive in the rain, in the
tiny rush-seed enclosed in the dried pond-mud on the legs of some
migratory bird, in all these we see the agencies of dispersal making use
of qualities and of structures that were developed in quite another
connection and for quite another purpose. That such characters have been
so to speak appropriated by these agencies is a pure accident in a
plant’s life-history. If the evolutionary force had been in operation
here, it would have selected some common ground to work on. There would
have been some uniformity in its methods, whereas the modes of dispersal
are infinite. The qualities and characters that happen to be connected
with dispersal belong to a plant’s development in a particular
environment. They can never have been adapted to another set of
conditions that lie quite outside that environment. There is a relation
of a kind between the specific weight of wood and the density of water,
and this, in a sense, sums up the connection between a seed and its
distributing agencies.
Evolution has never concerned itself directly with means of dispersal.
Evolution and Adaptation represent the dual forces that rule the organic
world, the first an intruding force, the last a passive power
representing the laws governing the inorganic world. To these laws the
intruding power has often been compelled to bend, and it has had to pay
its price, and sometimes it has succumbed, and sometimes it has turned
its defeat into a victory. Nature, so watchful over the young plant, as
represented by the seed, is finally compelled to let it go, and
dispersal begins where evolution ends, or rather when the evolutionary
power fails. The seed-stage itself is the price of adaptation. The death
of the individual may also be regarded from the same standpoint. It
represents a defeat of the evolutionary force, which, however, has been
retrieved by the gift of reproductive power.
CHAPTER II
THE FLORAS OF THE PACIFIC ISLANDS FROM THE STANDPOINT
OF DISPERSAL BY CURRENTS
The initial experiment.—The proportion of littoral plants.—The two great
principles of buoyancy.—The investigations of Professor Schimper.—The
investigations of the author.—The great sorting process of the
ages.—Preliminary results of the inquiry into the buoyancy of seeds
and fruits.
IN the previous introductory chapter some of the numerous questions
affecting insular floras were briefly referred to. I will now ask my
reader, if he has had the patience to read it, to consign that chapter
for the time at least into oblivion, and to proceed with me to our
Pacific island with the intention of investigating its flora from the
standpoint of dispersal. We will together take up the subject _de novo_,
after banishing from our minds all preconceptions that we may have
possessed.
After having been over the island gathering specimens of all the seeds
and fruits, we return to our abode on the beach. But we are puzzled
where to begin. The problem presents itself as a tangled skein, and our
difficulty is to find an “end” that we can follow along with some
chances of success. In our trouble we look around us; and at that moment
we see a number of floating seeds and fruits carried by the current past
the beach. This presents us with a clue and our investigation begins.
[Illustration:
OCEANIA
John Bartholomew & Co., Edin^r.
]
We place all our seeds and fruits in a bucket of sea-water and notice
that many of them sink at once. In a few days we look again and observe
that many more are at the bottom of the bucket, only a small percentage
remaining afloat. We then remark to our surprise that nearly all of the
floating seeds and fruits belong to coast plants, those of the inland
plants, which indeed make up the great bulk of the flora, having, as a
rule, little or no buoyancy. After a lapse of weeks and months the seeds
and fruits of the coast plants are found to be still afloat. In the
results of this experiment we see the work of the ages. There has been,
in fact, a great sorting process, during which Nature has “located” the
plants with buoyant seeds or seed-vessels at the sea-coast, placing the
others inland. This is the clue that we shall follow up during many
chapters of this book; and having in this manner introduced the reader
to the subject, I will now refer to the general results of my
investigations in this direction in the Pacific Islands.
In Fiji there are about eighty littoral plants out of a total of at
least 900 species of indigenous flowering plants, that is to say about
nine or ten per cent. (Note 1), the littoral grasses and the sedges
being with one or two exceptions excluded. These shore plants belong to
the sandy beach and to the coast swamp, and most of them are distributed
over the tropical shores of the Indian and Pacific Oceans, whilst not a
few occur on the coasts of tropical America. They form the
characteristic plants of the coral atoll, and many of them have long
been known to be dispersed by the currents. From the list given in Note
2 it will be seen that these eighty species belong to about seventy
genera. Nearly all of them (95 per cent.) possess seeds or seed-vessels
that float at first in sea-water; whilst three-fourths of them (75 per
cent.) will float unharmed for two months and usually much more, and
several of them will be found afloat after a year or more, being still
capable of reproducing the plant (Note 3).
The prevalence in the Fijian strand-flora of Leguminosæ, which are
included in my list under the divisions Papilionaceæ, Cæsalpinieæ, and
Mimoseæ, is very significant. They make up about 29 per cent. of the
total. Excluding weeds and a few other introduced plants, there are some
fifty species known from the Fijian Islands, and of these almost half
belong to the littoral flora, which as we have seen constitutes only a
fraction (one-tenth) of the whole flora. If we regard the genera, we
find that out of some thirty Leguminous genera twenty are littoral and
in most cases exclusively so. This conspicuous feature in the
constitution of the strand-flora is of prime importance as concerns the
question of adaptation to dispersal by currents, since nearly all the
Leguminosæ with buoyant seeds offer themselves as defiant exceptions to
any such law.
I will now contrast the Fijian inland flora with that of the coast from
the point of view of the buoyancy of the seed or fruit, according as it
presented itself for possible dispersal by currents. Rather over a
hundred plants were experimented upon (Note 4). After excluding some
introduced plants there remain some ninety species belonging to about
sixty genera, and of these quite 75 per cent. sank at once or in a few
days. I may add that all kinds of fruits are here represented, the
capsule, the achene, the coccus, the berry, the drupe, &c. Of the
buoyant residue few possess seeds or fruits that will float uninjured
for any length of time. Not many gave indications directly in opposition
to the principle that whilst the seeds or fruits of shore-plants
generally float, those of inland plants usually sink, since as pointed
out in Note 5 most of the difficulties are removed during the subsequent
developments of the principle discussed in the later pages of this work
or are to be explained on other grounds stated in the note.
We pass now from Fiji as typical in its flora of the Western Pacific to
Tahiti as representing in its flora the more strictly oceanic groups of
Eastern Polynesia. In the Tahitian region, which is taken as including
in a general sense the Society Islands, the Marquesas, and the Paumotus,
there are only between 50 and 60 littoral plants, excluding the
occasional additions from the inland flora. As indicated by the letter T
preceding the species in the list of Fijian shore plants, nearly all are
to be found in Fiji, and the few not yet recorded from that group, which
I have referred to in the remarks following the list, will probably be
found there by some subsequent investigator. In Tahiti also between 75
and 80 per cent. of the strand plants have seeds or seedvessels that
float for months; and here also Leguminosæ predominate, forming about 30
per cent. of the total. A conspicuous negative feature in the Tahitian
strand-flora is concerned with the absence of the mangroves and their
numerous associated plants, which together form the mangrove formation
in Fiji. This remarkable character in the distribution of shore plants
in the Pacific is discussed in Chapter VI.
Not having visited Tahiti, I can only deal inferentially with the inland
plants, as in the case of the strand-flora. Here also the plants are in
the mass Fijian in a generic and often in a specific sense, and there is
no reason to believe that the principle involving the non-buoyancy of
the seeds or fruits of inland plants does not as a rule apply to Tahiti
as well as to Fiji.
The Hawaiian Islands, standing alone in the North Pacific, form a floral
region in themselves, a region that is the equivalent not of one group
in the South Pacific, such as that of Fiji or of Tahiti, but of the
whole area comprising all the groups extending from Fiji to the Paumotu
Archipelago. Lying as it does mainly outside the zone of influence of
the regular currents that would bring the seeds of tropical plants to
its shores, Hawaii possesses a strand-flora that is meagre in the
extreme. Not only does it lack the mangrove formation so characteristic
of Fiji, but it lacks also many of the plants of the beach formation
that are found both in Fiji and in Tahiti, plants that give a peculiar
beauty to the reef-girt beaches all over the South Pacific. Its poverty
is sufficiently indicated in the number of its species, thirty in all,
barely more than half of the number found in Tahiti, and not much over a
third of those occurring in Fiji. Though coral reefs with their
accompanying beaches of calcareous sand are relatively scanty, the
characteristic littoral plants have not been numerous enough to hold
their own against intruders from the inland flora, and endemic species
have taken a permanent place amongst the strand plants. The Hawaiian
strand-flora has thus quite a facies of its own, and it will be found
discussed in Chapter VII., whilst a list of the plants is given in Note
28. It will thus not be a matter for surprise that the littoral flora of
Hawaii follows the principle of buoyancy only in a modified degree. It
is true that about two-thirds of the species of the present beach flora
possess seeds or seed-vessels that float for months; but since there are
reasons for believing that several of them are of aboriginal
introduction, this proportion is reduced to a third. In the list of the
Fijian shore plants given in Note 2, those occurring also in Hawaii are
preceded by H.
When we look to the Hawaiian inland flora for indications respecting the
principle of the non-buoyancy of the seeds or seed-vessels of inland
plants, we find that so far as it has been there tested this principle
receives fresh support from the plants growing on the slopes of the
Hawaiian mountains. Although the author was only able to sample the
inland flora, we have in the list given in Note 6 all kinds of plants,
from the forest-tree to the herb, and most varieties of fruits.
Excluding a few introduced plants, there are in this list about fifty
species of indigenous plants belonging to about forty genera. Of these
plants quite 80 per cent. possess seeds or fruits that sink either at
once or in a week or two. Of the “buoyant” residue very few have seeds
or fruits that will float for months. These apparent exceptions to the
principle are in great part capable of being explained on the grounds
referred to in Note 5 in connection with the Fijian inland plants; and I
have alluded to them in Note 7.
The littoral flora of Fiji is essentially Malayan and Asiatic, and for
our purpose is eminently typical. Its plants are found far and wide on
the tropical coasts of the Old World, and sometimes also in the New
World. In more than half the species we are concerned with the dispersal
by currents of more or less dry indehiscent fruits that range usually in
size from a marble to a cricket-ball, as illustrated by those of
Hernandia peltata and Barringtonia speciosa, whilst with most of the
rest the currents distribute large seeds, several of which are
Leguminous, as in the case of Mucuna, Cæsalpinia, and Entada, with
others of the Convolvulus type, as in the instance of Ipomœa pes capræ.
It is remarkable that in selecting plants with buoyant seeds or
seed-vessels for a station at the coast Nature has generally ignored
those with very small seeds. When such small seeded plants, as Sesuvium
portulacastrum, occur on the beach, the seeds have as a rule no
buoyancy. Pemphis acidula is, however, an exception; but its case is a
very rare one. It will be established in the next chapter that the
non-buoyancy of small seeds is generally true also of plants growing by
the river or by the pond.
The point at which we have arrived in our inquiry concerning the general
collection of seeds and seed-vessels that we placed in sea-water is that
_the plants with buoyant seeds or seed-vessels have been for the most
part “located” at the coast_. But if we look a little more closely at
the sunken and floating seeds, we find that in the same genus there are
species with seeds or seed-vessels that sink and species with those that
float. We look again and then perceive that the same general principle
is true of different species of the same genus growing inland and at the
coast. We learn now that as a rule when a genus possesses both littoral
and inland species, the seeds or fruits of the former float in sea-water
for a long time, whilst those of the latter have little or no floating
power. But we have yet to examine the structure of the coverings of the
buoyant seed or fruit; and we shall then discover that the different
behaviour in water is often associated with corresponding structural
differences of a striking character. The structural causes of buoyancy
are dealt with in Chapter XII.; and we will now content ourselves with
enunciating the second principle that _in a genus comprising both coast
and inland species, only the coast species possess buoyant seeds or
seed-vessels_.
The important principle above indicated was not altogether new to me, as
is shown in the next chapter. But it was new in the case of the floras
of the Pacific Islands. When it first presented itself in Hawaii I was
engaged in trying to find a connection between the inland and littoral
species of Scævola; and its discovery led me to form a plan worthy
almost of Don Quixote, namely, to cultivate the beach species of Ipomœa,
Scævola, and Vitex in the interior with the hope of finding them
converted into inland species when I returned to Hawaii after a lapse of
years. Little matters often determine a career, and for a while my
future movements and probably the remainder of my life were largely
centred around my interests in the well-being of Scævola Kœnigii. The
scheme was actually undertaken, and I had fixed on a little plot of land
at the foot of the mountains rising behind Punaluu in Kau. The
transaction was on the point of completion when the owner changed his
mind and the plan fell through. Subsequent observation and reflection
have led me to believe that in most cases no connection exists between
the littoral and inland species of a genus; and I have dwelt on this
incident merely to show the importance that I rightly attached to this
distinction, whilst misinterpreting its meaning.
But to return to my own investigations. Had I indeed read more carefully
Professor Schimper’s work on the Malayan strand-flora, this subject
would have been found discussed by an observer far abler than myself,
though from a very different standpoint, that of Adaptation and Natural
Selection. He points out (pp. 179-182) that with a number of these
tropical genera possessing both littoral and inland species, such as
Barringtonia, Calophyllum, Clerodendron, Cordia, Guettarda, and
Terminalia, greater buoyancy of the fruits of the shore species is
associated with certain structural characters in the fruit-coverings,
whilst with the inland species, where the floating power of the fruits
is either much diminished or entirely absent, these structural
characters are either less developed or lacking altogether.
The question of structure and the debateable matters concerned with it
are treated at some length in Chapters XII. and XIII., and Professor
Schimper’s views are there given. I will content myself with remarking
that the genus Terminalia was especially studied by him in this respect.
He tested the buoyancy of the fruits of ten species, and found that the
flotation period varied from nothing to 126 days and more. By far the
best “floaters” were the fruits of Terminalia Katappa, the only littoral
species tested, all the others being inland species with less buoyant
fruits, and diminished ranges, some of the fruits sinking at once,
whilst the others sank usually in a few days or in a few weeks. It was
also ascertained that, although the buoyant tissue in the fruit-coats
varied in amount generally with the floating-powers, it was rarely
absent altogether in the inland species, a very significant conclusion,
as will subsequently be pointed out.
Several other striking examples of this principle came under my notice
in the Pacific, and perhaps the most significant is that of Scævola, a
genus of the Goodeniaceæ, confined mainly to Australia and the Pacific
islands, but possessing also a littoral species, S. Kœnigii, that is
found on tropical beaches all round the globe. It is associated in both
Hawaii and Fiji with inland species, none of which are common to the two
archipelagoes, and in the case of the Hawaiian species not found outside
the group. All the species have fleshy drupes, both coast and inland
plants, the “stone” in the littoral species possessing a thick covering
of buoyant tissue, which is absent or but slightly developed in the
inland species. The fruits of the shore species float for many months;
whilst those of the inland species experimented on by me (S.
Chamissoniana and S. Gaudichaudii in Hawaii, and S. floribunda in Fiji)
sank at once or within a few hours. Here we are only concerned with the
difference of buoyancy between inland and littoral species. The several
other questions involved concerning this genus will be dealt with later
on in this work.
The genus Morinda offers another good example of this principle. It
includes one widely-spread littoral species (M. citrifolia), found not
only in all the Pacific archipelagoes, but also over much of the
tropics. It is associated in all the large groups with one or more
inland species, some of which are endemic and others more generally
distributed. The littoral species displays in its pyrenes a singular
air-cavity, the nature of which is discussed in Chapter XII., which
endows them with great floating powers. This cavity is not found in
inland species, and the pyrenes have in consequence no floating power
(see Note 8).
Calophyllum Inophyllum, an Old-World littoral tree, spread far and wide
over the Pacific islands, has very buoyant fruits. In the groups of the
South Pacific it is associated with inland species that are commonly
found in the forests, namely, C. spectabile and C. Burmanni, the fruits
of both of which, according to my observations in Fiji, have limited
floating powers, sinking after periods varying from a few days to four
weeks, and lacking in great part the buoyant coverings of the littoral
species. Professor Schimper obtained similar results with inland species
from other regions (Note 9).
The fruits of the two Fijian coast trees, Barringtonia speciosa and B.
racemosa, possess great floating powers; whilst those of an undescribed
species that I found in the mountains of Vanua Levu sink at once.
Another Fijian inland species (B. edulis, Seem.) that is often planted,
has fruits that float heavily for about a month. This difference in
buoyant powers is also associated with characteristic differences in the
structure of the fruits. It would be interesting to learn what floating
capacity belongs to those of the Samoan endemic species (B. samoensis,
Gray). Professor Schimper’s observations on the genus in the Malayan
region point in the same direction, but more than one difficulty awaits
its solution in the re-examination of the genus. He says, however, that
B. excelsa, Bl., a Malayan species, sometimes cultivated and growing
both inland and at the coast, has fruits that floated for one hundred
days after drying (p. 173).
A striking instance of this principle is afforded in the case of the two
Fijian species of Tacca, the wide-ranging littoral species, T.
pinnatifida, where the seeds float for several months, and the inland
species, T. maculata, Seem., found also in Australia and Samoa, where
the seeds sink at once or in a few days. The seeds of the shore plant
owe their buoyancy to the spongy tissue in their coverings, which is
either absent or much less developed in those of the inland species.
This point might also be determined for the new Samoan inland species
described by Reinecke, the German botanist, as T. samoensis.
Another good illustration is afforded by the two species of Premna of
the South Pacific, though here the buoyancy of the “stone” of a drupe is
concerned. With P. taitensis or P. integrifolia, a small littoral tree
or shrub, these stones possess great floating-power, and are often found
in the floating seed-drift of the Fijian estuaries and in the stranded
drift of the beaches. In the case of Premna serratifolia, an inland tree
of moderate size, the stones have as a rule little or no buoyancy. As
shown in Note 32, where this genus is discussed in detail, the buoyancy
is mainly due to empty seed-cavities.
Other instances might be given in illustration of this principle; but it
will have been noticed that already many of the familiar trees and
shrubs of a tropical beach have been mentioned in this connection either
by Professor Schimper or by myself. There are other genera that afford
similar indications but in a less direct fashion.
For instance, there are three widely spread Leguminous beach plants of
the Pacific, Erythrina indica, Canavalia obtusifolia, and Sophora
tomentosa, none of which are found in Hawaii; but in that group the
genus is represented in each case by an inland species, Erythrina
monosperma, Canavalia galeata, and Sophora chrysophylla, the last two
species being peculiar to those islands. The seeds of the three littoral
species will float for a long time in sea-water, whilst those of the
three Hawaiian inland species have no buoyancy. I may say that some very
interesting questions relating to the origin of these inland species are
here raised. They will be discussed in a later chapter (Chap. XV.).
There are a number of plants belonging to the Convolvulaceæ in these
islands that behave in an irregular way in flotation experiments ; but
their inconstant behaviour can in most cases be explained in accordance
with the principle that in the same genus the shore species have buoyant
seeds and the inland species non-buoyant seeds. Thus, whilst the seeds
of the littoral species, Ipomœa pes capræ, I. grandiflora (Lam.), and I.
glaberrima (Boj.), can float for long periods, and those of the inland
species, I. pentaphylla, I. tuberculata, and I. Batatas (Sweet Potato),
have no buoyancy, the seeds of other inland species, I. insularis
(Steud.), I. bona nox (L.), and I. turpethum (R. Br.), are inconstant in
their behaviour. The three last-named species are, however, to be found
also flourishing at times at and near the coast, and the varying
floating powers of their seeds may probably be connected with their
varying stations. This is indeed suggested by the case of Argyreia
tiliæfolia in Hawaii, in which in my experiments the seeds of plants
growing at the coast floated, sometimes for months, whilst those from
inland plants sank.
This behaviour of the Convolvulaceæ becomes yet more intelligible, and
more in accordance with the principle, when we reflect that the cause of
buoyancy is not concerned with the seed-coats or with the nucleus,
neither of which are able to float, but with the air-spaces left by the
incomplete filling-up of the seed-cavity by the crumpled embryo. The
extent to which the seed-cavity is filled up varies not only between
different genera and between different species of the same genus, but
also amongst individuals of the same species. Even the seeds of Ipomœa
pes capræ, amongst the most typical of floating seeds, display this
variation, and they show it also in their floating power, since about a
third of the seeds usually sink during the first month or two of the
flotation experiments. We can thus explain also why in the case of
Ipomœa insularis seeds from Fiji floated for months, whilst those from
Hawaii had no floating power.
The seeds of the different species of Hibiscus also appear to behave
very irregularly; but even here most of the difficulties can be removed,
when we come to consider a further extension of the principle. Thus,
whilst the seeds of Hibiscus tiliaceus, a wide-ranging littoral tree
known to be dispersed by the currents, float for a long time, those of
H. Youngianus (Gaud.), an endemic Hawaiian species, and of two
wide-ranging species, H. diversifolius (Jacq.) and H. Abelmoschus (L.),
also float for some time. The Hawaiian plant, however, grows in wet
places; and this applies also to H. diversifolius which grows in swamps
at and near the coast. The extension of the principle to water-side
plants generally, which is discussed in the next chapter, will explain
the difficulties connected with these two species. But we have in H.
Abelmoschus a remarkable exception to any rule of buoyancy, since it
grows in dry situations, is often cultivated, and yet possesses a
special layer of buoyant tissue in the seed-coats to which the floating
power is due. The seeds of Hibiscus esculentus (L.), the widely spread
cultivated plant of the tropics, have no buoyancy.
Some curious indications are supplied by Cæsalpinia, a Leguminous genus,
containing two wide-ranging shore species. Speaking generally the rule
applies; and I found in Fiji that whilst the seeds of the two littoral
plants (C. Bonducella and C. Bonduc) were as a rule buoyant, those of an
inland mountain species sank. But it is very remarkable that although
the seeds of C. Bonducella have long been known to be transported by the
currents, and are often stranded by the Gulf Stream on the coast of
Scandinavia, when it grows in Hawaii, where it is as a rule an inland
plant, the seeds lose their buoyancy. This is quite in accordance with
the general principle; but I must refer the reader for a general
treatment of this genus to Chapter XVII. There also will be found the
instance of another Fijian littoral plant, Afzelia bijuga, a common
littoral tree with buoyant seeds which also lose their buoyancy when the
tree grows inland. A similar instance is afforded by Kleinhovia Hospita,
the seeds of which seem to lose their buoyancy in inland stations. Not
all littoral plants, however, lose the floating power of the seeds when
grown away from the coast. The seeds of Ipomœa pes capræ retain it in
spite of the change of station. This point is dealt with in Chapter XIII
and in Note 44.
In concluding this general sketch of the first results obtained by
testing the buoyancy in sea-water of a collection of seeds and fruits
from a mountainous Pacific island, such as we find in Fiji, I must
remind the reader that the subject has only been lightly treated.
Enough, however, has been said to illustrate the character of the
sorting-process by which in the course of ages the plants with buoyant
seeds or seedvessels have been gathered at the coast. This is
indicated:—
(1) By the far greater proportion of species with buoyant seeds and
seedvessels amongst the shore plants than among the inland plants.
(2) By the circumstance that almost all the seeds or fruits that float
unharmed for long periods belong to shore plants.
(3) By the fact that when a genus has both inland and littoral species,
the seeds or fruits of the coast species as a rule float for a long
time, whilst those of the inland species either sink at once or float
only for a short period.
These results, therefore, justify our dividing the flora of our island
into two groups, the one including the plants with buoyant seeds or
fruits and comprising most of the littoral plants, the other including
the plants with non-buoyant seeds or fruits, a group which contains
almost all the inland plants and indeed nine-tenths of the flora. This
classification is a very crude one; but it enables us at once to assign
a value to the agency of currents in stocking a Pacific island with its
plants. Yet this is but the initial step in an inquiry that branches off
in a thousand different ways, even if restricted to the littoral plants.
There are a host of difficulties connected with the history of the
strand-flora of such an island which can only be properly gauged when
viewed from various standpoints.
CHAPTER III
THE LESSON OF THE BRITISH FLORA
Results of observations on the buoyancy of over 300 British plants.—The
small proportion of plants with buoyant seeds or seedvessels.—Their
station by the water-side.—The great sifting experiment of the
ages.—Summary.
THE singular relation between station and seed-buoyancy that exists in
an island of the tropical Pacific, such for instance as Vanua Levu,
Tahiti, or Hawaii, would lose much of its significance if it stood alone
in the economy of plant-life. It must be true not only of tropical
floras generally, but of those of the temperate regions; and there can
be little doubt that it prevails all over the world. Displayed to us at
first in a Pacific island, it acquires a new significance when we study
it in the light of numerous observations made in Europe. It exhibits
itself then as part of a far wider method pursued by Nature in
determining the stations of plants. It is not only at the coast, but
also at the river-bank and at the lake-side that Nature “locates” the
plant with the buoyant seed or seedvessel. This relation is indeed as
well exhibited in inland districts as it is at the coast.
In this connection I have the results of my own investigations on the
buoyancy of the seeds and fruits of British plants and on the
composition of the seed-drift of ponds and rivers, which were carried on
in the years 1890-96. Some of them were published in a short paper on
the seed-drift of the Thames, read before the Linnean Society of London
in June, 1892, and in the columns of _Science Gossip_ for April, May,
and October, 1895; but the mass of the observations remain in my
notebooks. Nor do my observations of the period since elapsed lead me to
alter the position then adopted. I have since pursued the same line of
inquiry in Hawaii, Fiji, on the Pacific coast of South America, and in
Sicily, and with the same results.
Since the elaboration of my notes was begun in 1900, Dr. Sernander, the
Swedish botanist, has published (1901) his work in Swedish on the
Dispersal-biology of the Scandinavian plant-world, in which the
seed-drift of river, pond, and sea is exhaustively treated. Although
this author has dealt with plant-dispersal from a somewhat different
standpoint, I have perused his pages with the keenest interest and with
great profit, having gone over much of the same ground with respect to
the seed-drift of ponds and rivers. Yet the introductory remarks to my
paper in _Science Gossip_ in 1895 are as apposite now as they were then,
and the reader will, I trust, pardon my reproducing them.
“By following up the path of inquiry that is concerned with the
flotation of seeds and seedvessels, we are guided into other fields of
research that give promise of interesting discoveries in connection with
plant-life. We are led in the first place to consider the question of
utility, and to ask whether the buoyancy of the seed or fruit has been a
matter of moment in the history of the species. Nature is ever engaged
in telling off the plants to their various stations. She places the
yellow iris at the river’s side and assigns to the blue iris its home in
a shady wood. Under her direction the common alder thrives at the
water’s edge, whilst its fellow species live on the mountain slope.
These and similar operations are carried on daily around us, and we know
but little of the wherefore and the how. We are induced, therefore, to
inquire whether by pursuing the line of investigation above indicated we
may be able to get a glimpse at the methods adopted by Nature in
selecting stations for plants.”
I possess the results, which are given in Note 10, of buoyancy
experiments and observations on the seeds and seedvessels of about 320
British flowering plants belonging to about 65 families. Of these about
260 are included in my own results, the data for the rest being obtained
from the writings of Darwin, Martins, Thuret, Kolpin Ravn, and
Sernander. In the great proportion of cases, 240, or 75 per cent.,
sinking took place at once or within a week; whilst 80, or 25 per cent.,
floated for a longer period, usually a month or more; and about 60, or
nearly 20 per cent., floated for several months. It is to this last
small group that belong the seeds or seedvessels that float through the
winter in our ponds and rivers.
If the grasses had been properly represented, the grains of which
possess as a rule but little buoyancy, except through air-bubbles
temporarily entangled in the glumes, the proportion of seeds and fruits
that sink at once or in a few days would probably have been about 80 per
cent. Then again, since the plants from stations where buoyant seeds and
seedvessels are most frequently found—that is at the river-side, the
pond-margin, and the sea-coast—are much more completely represented in
these experiments than those from other stations, it would seem that
even 80 per cent. is too low a figure. Even if the 80 plants with the
buoyant seeds or seedvessels included all the species thus
characterised, which they certainly do not, we should obtain an estimate
for the British flora (rather over 1,200 species of flowering plants) of
about 93 per cent. with non-buoyant seeds or fruits. This is, of course,
too high. It is, however, very probable that the proportion of plants
with non-buoyant seeds or seedvessels for the whole British flora is
about 90 per cent.
This proportion of plants with non-buoyant seeds or seedvessels, that is
to say, of those that sink at once or within a week, is also
approximately correct for the flora of one of the larger islands of the
tropical Pacific. The data at my disposal only enable me in the cases of
Fiji and Hawaii to fix it at between 95 and 85 per cent., or on an
average 90 per cent. With the floras of continental regions the
proportion would doubtless be markedly higher. That seeds and
seedvessels as a rule possess but little buoyancy was a sound conclusion
of Darwin, and one, as he remarked, that is in accordance with the
common experience of gardeners. Thuret, after experimenting on the
buoyancy in sea-water of the seed or seedvessels of 251 species of
plants, belonging to 77 families and to various regions, found that
scarcely two per cent. had any powers of flotation, all the rest sinking
at once or in a few days, a result that led De Candolle in a note to
this memoir to reiterate his opinion regarding the inefficacy of
currents as plant distributors. Thuret, however, did not select many of
his plants from stations where buoyancy is most frequently exhibited,
and his estimate errs, therefore, in imputing too little buoyancy to
seeds in general. The power of seeds and fruits to germinate after
prolonged flotation in sea-water has long been well established, and it
is often illustrated in this work, so that there is no need to dwell
upon it here. (See Note 11.)
Of the 240 species of British plants where sinking took place at once or
within a week, in about 50 per cent. the plants had dry indehiscent
fruits, such as we find in the genus Ranunculus and in the Umbelliferæ,
the Compositæ, and the Labiatæ; whilst in about a third the plants had
dehiscent fruits with small seeds, such as are characteristic of the
Cruciferæ, the Caryophyllaceæ, and the Juncaceæ. Plants with large
seeds, such as those of Nuphar luteum and Convolvulus arvensis, make up
only six per cent. of those of the non-buoyant group, the remainder
comprising plants with berries, such as Solanum, and others with
miscellaneous fruits.
Of the 80 plants where the seeds or fruits floated more than a week,
usually for several weeks, and often for months, 70 per cent. possessed
dry, indehiscent fruits, such as those of Hydrocotyle vulgaris, Bidens
cernua, Lycopus europæus, Carex, &c., whilst only 6 or 7 per cent. had
dehiscent fruits with small seeds, such as we find in Lysimachia and
Menyanthes, the remainder being generally characterised by large seeds,
such as those of Convolvulus sepium, C. soldanella, Iris pseudacorus,
Calla palustris, &c. It would thus appear that, in so far as buoyancy is
concerned, Nature has for the most part ignored the small seed and has
confined herself mainly to the dry indehiscent fruit. We have already
seen that this is also true of the same great sorting-process in the
tropical islands of the Pacific, and it doubtless applies all over the
world.
We have now to learn the significance of this distinction amongst
British plants between those with and those without buoyant seeds or
seedvessels. When we regard the stations of these 80 plants of the
buoyant group we find that about 70 per cent. of them are placed by the
river, or the pond, or the sea, the fresh-water stations much
predominating. But if we include the plants of the moist meadows
adjoining the rivers, such as Ranunculus repens, Rhinanthus crista
galli, some Cyperaceæ, &c., the buoyant fruits or seeds of which are
regularly swept into the stream in the time of flood, we shall raise the
proportion possessing a water-side station to 80 per cent. On the other
hand, about two-thirds of the 240 plants of the non-buoyant group, which
are enumerated in Note 10, live away from the water-side; but the
proportion of plants with a relatively dry station would be considerably
higher than this figure for the whole flora, since my investigations
were especially directed towards plants frequenting wet stations, and
the number of them is excessive in the list.
Supposing, however, that our materials were restricted to the 260 plants
tested by myself, we should obtain highly instructive results, since in
a general sense the floating powers of their seeds or fruits were tested
to the finish. We place them, let us say, in a bucket of water, and
after six months we find that in not more than forty plants are the
seeds or seedvessels still afloat. These forty plants, excluding two or
three littoral plants, are nearly all plants of the borders and vicinity
of rivers and ponds. (They are indicated in the list given in Note 10 by
the numbers vi. and xii., the last being those where the flotation
experiment was prolonged to a year and over.)
It would thus seem—I am now quoting mainly from my paper in _Science
Gossip_ for May, 1895—that there are gathered at the margins of rivers
and ponds, as well as at the sea-border, most of the British plants that
could be assisted in the distribution of their seeds by the agency of
water. This great sifting experiment has been the work of the ages, and
we here get a glimpse at Nature in the act of selecting a station. But
the curious character of the sorting process becomes yet more apparent
when we discover that the buoyancy of the seeds or fruits of species of
the same genus may become a matter of station.
We will first take the four British species of Stachys (arvensis,
betonica, sylvatica, and palustris). Of these the fruits of S. palustris
alone possess any buoyancy, being able to float for weeks. It is the
only species that finds its characteristic home at the water-side; and
as observed by Sernander its reproductive shoots occur in the
Scandinavian fresh-water drift.
Galium illustrates the same principle. Whilst in my experiments the
fruits of G. aparine and of another species growing in a dry station
displayed little or no floating power, those of G. palustre, which alone
grows at the water-side and in wet situations, have great buoyancy. As
my observations show, they float unharmed through the winter in our
ponds and rivers, and, according to Sernander, are often found in the
Baltic sea-drift. (See Note 12.)
The achenes of Potentilla afford another example. Those of P.
tormentilla and of another species from dry situations have but little
floating power. On the other hand, those of P. comarum float
indefinitely. The last also came under my notice in the floating drift
of ponds in February; and we learn from Sernander that they occur in the
fresh-water and salt-water drift of Scandinavia.
As a further instance, I will take the two British species of Iris. The
familiar river-side Iris pseudacorus has seeds that float unharmed in
our ponds and rivers from the autumn to the spring, and often for a year
or more. On the other hand, the seeds of Iris fœtidissima, which has its
home in the shady wood, sink at once even after drying for months.
The nature of the sorting-process is especially well shown in some of
the families, as for instance with the Labiatæ. Let the reader put on
one side the four species with buoyant fruits, namely, Lycopus europæus,
Mentha aquatica, Scutellaria galericulata, and Stachys palustris, and on
the other side all the species with non-buoyant fruits, such as Salvia
verbenaca, Thymus sp., Calamintha officinalis, Nepeta glechoma, N.
cataria, Prunella vulgaris, Stachys arvensis, S. betonica, S. sylvatica,
Galeopsis tetrahit, Ballota nigra, Lamium purpureum, L. album, Teucrium
scorodonia, and Ajuga reptans, and he will at once perceive that he has
separated the regular water-side plants from those growing in drier
stations.
If he does the same with the Umbelliferæ he will find that when he is
separating Hydrocotyle vulgaris, Cicuta virosa, Œnanthe crocata, and
Angelica sylvestris from Æthusa cynapium, Pastinaca sativa, and
Chærophyllum sylvestre, on account of their buoyant fruits, he is also
distinguishing them on account of their stations. On the other hand,
there are apparently weighty exceptions to this rule in the non-buoyancy
of the fruits of the three British species of Apium (graveolens,
nodiflorum, inundatum), which grow in streams and marshes. Or, again, if
we look at the sea-coast representatives of the family, we find that
whilst the fruits of the Samphire (Crithmum maritimum) float buoyantly
for months, those of Eryngium maritimum seemingly set the law at
defiance, and all sink in less than a week or ten days, even after
months of drying. To regard these as exceptions, however, is to miss the
essential point of the principle concerned. It is not thereby implied
that all water-side plants, whether by the sea or by the river or by the
pond, have buoyant fruits or seeds, but that nearly all plants with such
fruits or seeds have been gathered at the water-side. It will be shown
in the next chapter that several other influences go to determine the
station of a plant on a beach or by a river. This is true of the
Compositæ, which, if we except our two species of Bidens (cernua and
tripartita), come under the play of other determining causes, as
indicated by the little or no buoyancy displayed by the fruits of Aster
tripolium, Senecio aquaticus, and Carduus palustris.
Within the limits of a genus we can, however, point to other examples of
this principle. Take, for instance, Convolvulus arvensis, the common
weed of our fields. Its seeds, whether fresh or dried for months, have
no buoyancy. On the other hand, those of Convolvulus soldanella float
unharmed in sea-water for half a year and more. Its seeds have come
frequently under my notice among the stranded drift of the Devonshire
beaches, and also on the coasts of Chile; whilst Sernander includes them
amongst the drift of the beaches on the Norwegian coasts. It is
remarkable that Convolvulus sepium, which accompanies C. soldanella over
much of its great range, has seeds that are sometimes able to float
unharmed for long periods, even for years (Notes 13, 41, 49). Though not
strictly a water-side plant, it grows commonly over other plants on the
banks of the Thames; and when it fruits its seeds occur typically in the
floating drift of that river. According to Gray, it is almost a
river-side plant in the United States, where it is found “especially on
the moist banks of streams.” Not all the seeds of C. sepium, however,
are buoyant; and in its varying behaviour in this respect it resembles
the inland species of Ipomœa, which are referred to in the previous
chapter.
The British species of Euphorbia also seem to behave in accordance with
the principle that when a genus has littoral and inland species, the
first-named alone possesses buoyant fruits or seeds. Thus, whilst the
sound fruits of E. helioscopia and of another species found commonly as
a garden weed are non-buoyant, those of E. paralias, the familiar
beach-plant, float for several weeks, and are to be noticed among the
stranded drift of the coasts frequented by this plant. (See Note 90 for
later results.)
The structural characters connected with the buoyancy of the seeds or
seedvessels of some of the British plants are dealt with in Chapter XII.
Here it may be remarked that this capacity is often associated, as with
the Pacific island plants, with a “buoyant” tissue, that is either
absent or less developed in the case of the non-buoyant group.
Enough has now been said to show in a general fashion how Nature through
the agency of buoyant seeds and fruits has affected the stations of
plants of the British flora. Allowing this line of inquiry to develop
itself as the work proceeds, we will here pause and close the chapter
with a reference to some of the principal points that have been brought
into prominence.
(_a_) The proportion of flowering plants of the British flora that
possess buoyant seeds or seedvessels is very small, probably not more
than 10 per cent.
(_b_) In so far as buoyancy is concerned, Nature has for the most part
ignored the dehiscent fruit with small seeds, such as we see in the
Cruciferæ and the Caryophyllaceæ, and has chiefly endowed with floating
power the dry indehiscent fruit, such as we see in the Umbelliferæ and
in the Labiatæ.
(_c_) In the great sorting-process that has been in operation through
the ages, nearly all the plants with buoyant seeds or seedvessels have
been located at the water-side, principally by ponds and rivers, but
also on the sea-beach. On the other hand, the great majority of the
plants with seeds or seedvessels that sink have found a home in drier
stations.
(_d_) The character of the operation is well displayed in certain genera
possessing species of the water-side and species of drier situations,
and in the case of genera having both coast and inland species. In both
instances the species by the water-side possesses buoyant seeds or
fruits, whilst that of the station in a drier locality or removed from
the coast has seeds or fruits that sink.
(_e_) Yet it is necessary to remember that the principle involved is not
that all water-side plants have buoyant seeds or fruits, but merely that
plants thus endowed gather at the water-side. There are many plants with
non-buoyant seeds or fruits on our beaches and beside our ponds and
rivers.
(_f_) We have now learned from the British flora that the “locating” of
plants with buoyant fruits or seeds on the beaches of the tropical
islands of the Pacific, and indeed of tropical regions generally, is but
a part of a much wider principle by which plants thus endowed are placed
at the water-side, whether by a river or a pond or by the sea.
(_g_) It is with this distinction between a fresh-water and a salt-water
station that we shall be occupied in the next chapter; and it is of
great interest, since it leads us to discover that the wider principle
is in its turn part of a far larger scheme.
* * * * *
_Note._—It must be clearly understood that by water-side plants the true
aquatic plants, such as the Water-lilies, the Myriophylls, the
Potamogetons, &c., are not implied. It will be seen from the list in
Note 10 that in most cases the seeds or fruits of aquatic plants have
little or no floating power. This is true, for instance, of Ranunculus
aquatilis, Nymphæa, Nuphar, Myriophyllum, Ceratophyllum, Callitriche,
Naias, Zannichellia, Ruppia, and half the Potamogetons.
CHAPTER IV
THE LESSON OF THE BRITISH FLORA (_continued_)
The choice of station of the water-side plant possessing buoyant seeds
or seedvessels.—Determined by its fitness or unfitness for living in
physiologically dry stations.—In the internal organisation of a plant
lies the first determining influence of station.—The grouping of the
British strand-plants.—Whilst the Xerophyte with buoyant seed or fruit
finds its station at the coast, the Hygrophyte similarly endowed makes
its home at the river or pond side.—The grouping of the plants of the
river and the pond.—Summary.
BY following up the clue supplied by the floating seed, we have arrived
at the conclusion with respect to the British flora that plants with
buoyant seeds or fruits gather at the water-side. But we have yet to
inquire why some of these plants are “located” at the sea-coast and
others on the borders of ponds and rivers. Mere buoyancy aided by chance
has not determined the choice. There are definite principles at work in
the economy of plant-life that make the selection for each plant.
Rivers in all parts of the world carry to the sea in great abundance the
seeds and fruits of the plants that are stationed at their borders; and
such seed-drift is found in quantity washed up on the beaches in the
vicinity of the estuary. One finds, for instance, on such beaches in the
South of England the stranded fruits and seeds of Bidens cernua, Alnus
glutinosa, Sparganium ramosum, Iris pseudacorus, &c., mingled with those
of true beach plants like Cakile maritima, Convolvulus soldanella,
Euphorbia paralias, &c. Yet we would be much surprised if either the
Bidens or the Alder or the Sparganium were to establish itself on the
sandy beach, even though they have had through the ages innumerable
opportunities of doing so. We thus see that mere buoyancy of fruit or
seed cannot determine a station on a sea-beach, and that some other
factor makes the choice. The nature of this factor I will now endeavour
to explain; but in so doing it will be necessary to employ a few
technical terms, which it is not easy to dispense with altogether.
It may be doubted whether Professor Schimper could have conferred a
greater benefit on the student of plant-distribution than in his clear
delineation of the connection between the habit or organisation of a
plant and its station. Nature has imposed an important structural
distinction between plants that have been endowed with the means of
checking excessive transpiration or water-loss in stations where there
is risk of drought, as in deserts and in similar arid localities, and
those that live in stations where such safeguards are not needed. Hence
arises the distinction between Xerophytes on the one hand, and
Hygrophytes on the other. This contrast is shown not only in minute
structural features, but also, as my readers are aware, in the external
characters, as in hairiness, succulency, a leathery cuticle, the
occurrence of thorns, and in several other characters of the plants of
the steppe and the desert. This important subject is dealt with by
Professor Schimper in his recent work on Plant-Geography; but it was
from his earlier work on the Indo-Malayan strand-flora that I learned
this valuable lesson in plant-distribution.
It has been ascertained, however, that a safeguard against excessive
water-loss by transpiration is not only needed by plants living in arid
localities, but also by those placed at the coast. Both the shore plant
and the plant of the steppe and the desert present the same xerophilous
organisation, provision against excessive transpiration being also
required by the beach plant to prevent the injury of the green cells
from the accumulation of salt in the tissues. It would thus appear that
plants of the Hygrophytes that possess buoyant seeds or fruits are
gathered at the borders of ponds and rivers, whilst those of the
Xerophytes that are similarly endowed find their station on the
sea-shore. This important distinction penetrates very deeply into the
conditions defining the stations of plants. The connection between the
plant of the coast and the plant of the steppe or the desert is
strikingly shown on those occasions when the beach plants extend inland
over parched and arid plains, such as occurs for instance in North
Africa, and in the larger islands of Fiji, as described in Chapter V.
The causes of the buoyancy of fruits and seeds, as pointed out in
Chapter XII, are so various, that it appears at first sight impossible
to connect them with the xerophilous or hygrophilous organisation of a
plant, or, in other words, with any structural characters associated
with particular stations; yet behind all lies the general principle
that, given a plant of the buoyant group, if it is a Xerophyte it finds
its way to the coast, and if a Hygrophyte it makes its home by ponds and
rivers. In the case of a tropical littoral flora, such as we find in a
Pacific island, the large proportion of plants with buoyant fruits or
seeds gives so much prominence to the subject of their distribution by
currents that the question of “station” is often masked. On the other
hand, in the shore-flora of a temperate region like that of Great
Britain, the plants with buoyant seeds or fruits are in the minority,
and the question of “station” is the first to obtrude itself.
In establishing the principle that most of the plants with buoyant seeds
or fruits have been gathered at the water-side, it was never implied
that all the plants by the river or by the pond or at the coast are thus
characterised. There is much to learn from the circumstance that whilst
nearly all plants with buoyant seeds or fruits are placed at the
water-side, not all water-side plants have buoyant seeds or fruits. In
the first place, it is to be inferred in the light of what has been said
above that the first determining principle in the selection of a station
is concerned not with the buoyancy of the seeds or fruits, but with the
xerophytic or hygrophytic organisation of a plant. In other words, it is
the fitness or the unfitness of a plant for living in situations where
the loss of water by transpiration requires to be checked that primarily
determines the station at the coast. We thus see in the internal
organisation of the plant the primary determining influence on station.
Buoyancy of seed or fruit comes subsequently into play, the Xerophyte
and the Hygrophyte, thus endowed, ultimately finding their way, the
first to the beach, the second to the bank of the river or to the margin
of the lake or pond.
In the next place, when we regard the composition of the British
coast-flora, and examine the distribution of the plants in other
situations than on the beach, we obtain some interesting results. There
is first a group of plants, including such as Armeria vulgaris,
Artemisia maritima, Cochlearia officinalis, Erodium maritimum,
Matricaria inodora, Plantago coronopus, Polycarpon tetraphyllum,
Raphanus maritimus, Spergularia rubra, Silene maritima (see Note 15),
and others, all of which occur not only at the coast and on the adjacent
hill-slopes, but also often far inland, and sometimes at considerable
elevations in mountainous districts, as in Central Europe. It is on this
occurrence of certain shore-plants in alpine regions that Prof. Schimper
lays much stress in his memoir on the Indo-Malayan strand-flora (p. 28),
and in his later work on Plant Geography (Engl. edit., p. 716), when
pointing out that here temperature does not play a determining part, and
that in both stations, whether on the sandy beach or on the
mountain-top, the same xerophilous organisation is needed to obviate the
risk of impeded water-supply. He quotes in this connection the
observation of Battandier that many alpine species from the Atlas
Mountains occur on the Algerian beaches, but not in intervening regions.
Mr. Druce, in his discussion of the British species of Sea-Thrifts and
Sea-Lavenders (Armeria, Statice), brought the subject of the occurrence
of maritime plants on mountain summits again to the front; but he did
not advance any general explanation, and seems to regard it as the
result, as it doubtless is, of the recurrence of suitable stations
(_Jour. Linn. Soc. Bot._, Dec. 1900).
Very few of these plants have any capacity for dispersal by currents, a
subject dealt with in Note 16. Several of them have dehiscent,
small-seeded fruits which, as pointed out in the previous chapter,
hardly ever come into the buoyant category. I have experimented on the
greater number of them, and in only one species, Matricaria inodora
(var. maritima), do the results indicate a capacity for dispersal over
wide tracts of sea.
If we look again at a list of British shore-plants, we find another
group of plants frequenting salt marshes and muddy shores, and found
also often far inland, as in the saline plains of Central Asia. Here we
have such plants as Aster tripolium, Glaux maritima, Plantago maritima,
Salicornia herbacea, Salsola kali, Samolus valerandi, Scirpus maritimus,
Suæda fruticosa, S. maritima, Triglochin maritimum, T. palustre, &c. It
becomes in this connection a subject of peculiar interest to the student
of plant-distribution when he reads in Mr. Hemsley’s paper on the flora
of Tibet (_Jour. Linn. Soc. Bot._, vol. 35) that amongst the British
shore-plants above-named the two species of Triglochin and the same
species of Glaux and Salsola occur in the salt marshes of the Tibetan
uplands at elevations of 15,000 to 16,000 feet, Scirpus maritimus also
being found in the swamps of the lower levels. We have the same thing,
affecting much the same plants, illustrated in America. Thus we learn
from Asa Gray that Salicornia herbacea, Scirpus maritimus, Triglochin
maritimum, &c., which are common in salt marshes on the coast of the
United States, occur also in the interior of the continent in the
vicinity of salt-springs.
Facts of this sort are well known, and I merely refer to them here in
order to emphasise the importance of this little group of British
littoral plants, those of the salt marsh. Their very wide distribution
is connected with the frequent recurrence of suitable conditions, not
only in space, but what seems of greater import, also in time. One can
scarcely doubt when the Saltwort (Salsola kali) is seen on the
Devonshire coast, on a beach in Chile, and in the elevated regions of
Central Asia that here a very ancient type of plant finds its still more
ancient conditions of existence. In the capacity which most of the
plants of the salt marsh possess of germinating in sea-water, this group
of littoral plants is sharply distinguished, as far as my observations
show, from the other groups of British shore-plants. For instance, in my
experiments the seeds of Aster tripolium, Salicornia herbacea, and
Triglochin maritimum germinated freely in sea-water, whilst those of
Spergularia rubra, Cakile maritima, Convolvulus soldanella and others
failed to do so (see Note 19). It will also be noticed with respect to
this group of littoral plants that, except in the case of Scirpus
maritimus, the seeds or fruits have little or no floating power, the
exception offered by Salsola kali being not very striking. This feature
is brought out in the Table given in Note 10; but some of the details of
my observations are given in Note 17.
There yet remains a third group of the British shore-plants, namely,
that comprising the plants that rarely stray far from the beach and
often possess seeds or seedvessels that will float for months. Here we
have such species as Arenaria (Honckeneya) peploides, Beta maritima,
Cakile maritima, Crambe maritima, Crithmum maritimum, Convolvulus
soldanella, Eryngium maritimum, Euphorbia paralias, Glaucium luteum,
Lathyrus maritimus, Polygonum maritimum, &c. The seeds or seedvessels of
quite half of these species will float for months unharmed in sea-water,
but in a few, as with Cakile maritima and Eryngium maritimum, they float
for only a week or two, whilst in others again like Glaucium luteum they
have no buoyancy. (Some details of the buoyancy experiments on these
plants are given in Note 18; and the long list in Note 10 may be first
consulted.)
It is not necessary to enter here into more detail with respect to
British shore-plants. Enough has been said to disclose cleavage-lines in
what might have appeared as a homogeneous plant-formation. We can thus
discern the elements of at least three groups amongst the plants of our
beaches, each group bearing the impress of an independent history:—
(_a_) The plants of the beach and of the inland plain or of the distant
mountain peak, excluding those of the salt marshes. Armeria vulgaris,
Silene maritima, and Spergularia rubra may be taken as examples. The
currents here as a rule take little or no part in their dispersal.
(_b_) The “saline” group, including the plants of the saline plains and
the salt marshes of the interior of continents. Of these Glaux maritima,
Salsola kali, and Triglochin maritimum are examples. The capacity of
germinating in sea-water is a distinguishing character of most of the
plants; and but few of them possess seeds or seedvessels that are
markedly buoyant.
(_c_) The true beach plants that rarely stray far from the beach, of
which Arenaria peploides, Cakile maritima, and Convolvulus soldanella
are examples. Many of them have buoyant seeds or fruits capable of
dispersion over wide areas through the agency of the currents.
The reader will be able to extend this subject for himself if he is so
inclined, but we have gone far enough together to learn that the plants
with buoyant seeds or fruits are in the minority on our beaches,
scarcely a third of the total being fitted for dispersal by the currents
over broad tracts of sea. The British strand-flora thus differs
strikingly from the littoral flora of a Pacific island, or indeed of any
ordinary tropical coast, and in this respect it is to be regarded as
typical of the temperate regions. It has been remarked before that on a
beach in the tropics we would expect to find that quite three-fourths of
the plants are provided with buoyant fruits or seeds distributed far and
wide over the tropical seas by the currents.
We pass on now to briefly discuss from the same standpoint the British
plants that find their homes on the borders of rivers and ponds. It is
here that the hygrophytes with buoyant seeds or fruits gather together,
just as the xerophytes with similar seeds or fruits collect on the
beaches. We have seen before that only a portion of the beach plants
belong to the buoyant group, and the same applies to the plants at the
edges of rivers and ponds. The plant-formation is no more homogeneous
there than it is in the case of the strand-flora. Let us see if we can
discern some lines of division there also, or in other words let us
endeavour to connect the absence or presence of floating power in the
fruits and seeds with some variations in the placing of the plants. We
still pursue the clue to the study of the complicated problems connected
with plant-stations by taking the floating seed as our guide.
We will carry ourselves in thought to the Thames-side between Teddington
and Twickenham at the end of August, 1892. The river is at the
high-water level, and we see flourishing at the margins, sometimes a
little above the water and sometimes a little within its reach,
Ranunculus repens, R. sceleratus, Spiræa ulmaria, Lycopus europæus,
Scutellaria galericulata, different species of Rumex, Alnus glutinosa,
Iris pseudacorus, Sparganium ramosum, and different species of Carex,
with several other plants, all contributing their seeds or fruits to the
drift that floats in the river from the autumn to the spring.
But besides these plants there are a number more or less submerged in
the stream, including Nasturtium amphibium, N. sylvestre, Stellaria
aquatica, Myosotis palustris, and Veronica beccabunga; and as the water
falls other plants still more submerged come into view on the exposed
flats, such as Nasturtium officinale, Apium nodiflorum, and Polygonum
hydropiper. None of these plants are represented by their seeds or
fruits in the floating river-drift. Several of them possess dry
dehiscent fruits with small seeds, such as Nature ignores in the matter
of buoyancy, and the small fruits of Myosotis, Apium, and Polygonum have
little or no floating power.
We have thus here a clear dividing line between the plants with buoyant
seeds or fruits that were more or less exposed above the high-water
level, and those that were more or less submerged at that state of the
tide. That which occurs in the Lower Thames twice in the day within the
reach of the tide represents what happens in the higher part of the
river during the seasonal floods, but in the last case the effects
cannot be so readily distinguished. We thus perceive that the buoyant
seed or fruit is as a rule only characteristic of the plants of the
river-side that grow more or less exposed above the water, whilst those
plants liable to periodic submergence have seeds or fruits that sink.
In this connection it is of especial interest to observe that as a
general rule the truly aquatic plants of English rivers contribute
little or nothing to the floating seed-drift. I pointed this out several
years ago, in my paper on the Thames, as an agent in plant-dispersal,
and it has been already noticed in this work (page 30). We look in vain
amongst the floating winter drift of our rivers for the seeds or fruits
of Ranunculus aquatilis, Nuphar luteum, Nymphæa alba, and of the species
of Myriophyllum, Limnanthemum, Callitriche, Ceratophyllum, Zannichellia,
and of several of the Potamogetons, all of which give character in
summer to the aquatic vegetation of the river. In their place we find
only the seeds and fruits of the plants growing on the banks.
There is, however, another small group of river plants, which in their
structure and habits and in the behaviour of their floating fruits come
between the true aquatics and the plants of the river-banks. They belong
mostly to the Alisma family, and Alisma plantago and Sagittaria
sagittifolia may here be specially mentioned. Their fruits display great
variation in their floating power; and on this point M. Kolpin-Ravn,
writing to me in 1895, made the following interesting suggestion, that
since these plants approach true aquatics in structure they may be also
regarded as approaching them in the inconstancy of the buoyant capacity
of their fruits, those of aquatics having typically little or no
floating power.
Seed-buoyancy, however, does not play quite such an important part in
the plant-economy of a river as the examination of the floating drift
would lead one to expect. Only a portion of the bank-plants have buoyant
seeds or fruits, whilst amongst the true aquatics, the semi-aquatics,
and the plants periodically submerged, the rule of non-buoyancy
prevails. And, indeed, when we look at all the possible stations for the
plants of the British flora, we discover that seed-buoyancy can rarely
be connected with station. It is, however, in those few stations that
plants with buoyant seeds have mainly gathered. There it is, probably,
that the remnants of a past floral age find a refuge, since it would
seem likely that the tendency has been in the course of geological time
for the development of dry stations for plants at the expense of the wet
stations.
The following is a summary of some of the points discussed in this
chapter:—
(1) In the case of the strand-flora of a Pacific island, and indeed in
that of an ordinary tropical region, the large proportion of plants with
buoyant seeds or fruits tends to mask all other issues, and we are
seemingly only concerned with dispersal by currents.
(2) But in the British strand-flora where plants with buoyant seeds and
fruits are in a minority, constituting less than a third of the total,
it is seen that the issue is primarily an affair of station, an
inference that may be applied generally to temperate regions.
(3) All British shore-plants may be regarded as owning certain
characters in common which may be collectively designated the
xerophilous habit, and we may extend this view to other temperate
strand-floras.
(4) But this xerophilous habit is also characteristic of inland plants
in certain localities, as of those of the steppe, the desert, the rocky
mountain-top, and of other exposed situations, in all of which checks to
the loss of water by transpiration are required. Whilst the risks of
drought are thus guarded against in the case of plants stationed in arid
localities, the risk of injury to the plant from the accumulation of
salt in the tissues is obviated in the instance of the plants of the
coast.
(5) On the other side we have the hygrophilous habit characteristic of
plants living under conditions where checks to transpiration are
relatively little needed. All the plants of the margins of rivers and
ponds belong here, and indeed all plants living under moist conditions.
(6) This distinction between the xerophilous and hygrophilous habits
penetrates deeply into all questions connected with stations, and lies
behind all matters relating to the buoyancy of seeds or fruits. It is
the fitness or unfitness of a plant for living in dry situations that
primarily determines the station. If a xerophilous plant has a buoyant
seed or seedvessel it finds its way ultimately to the coast; if it is
hygrophilous and its seeds or fruits can float, then it is finally
established on the side of a pond or river.
(7) The composite character of the British strand-flora is to be
explained on the above principles. We have in the first place the plants
confined to the sandy beach, many of which possessing buoyant seeds or
fruits are dispersed by the currents. Next come the plants of the sandy
beach which are found also far inland in open plains and on
mountain-tops; and afterwards come the plants of the salt-marsh and
mud-flats of the coast, which appear again in the saline plains and
swamps in the interior of the continents.
(8) The plant-formation of the river’s border displays also lines of
division, and is by no means homogeneous; and indeed other factors
besides those connected with seed-buoyancy have here been in operation.
(9) In only a few of the possible stations of British plants can a
direct connection be traced with seed-buoyancy. Yet it is at these few
stations, such as at the coast and by the pond or river, that the plants
with buoyant seeds and fruits have mainly gathered.
(10) The plants now frequenting wet stations may often be regarded as
the remains of an age when moist conditions for plant-life prevailed.
CHAPTER V
THE FIJIAN STRAND-FLORA
The inland extension of the beach plants.—The grouping of the coast
plants.—Their modes of dispersal.—The zone of change.—Summary.
HAVING learned from the British flora the real significance of the
buoyant seed or fruit in a littoral flora, we will now return to the
Pacific and proceed to deal with the composition and general character
of the strand-plants.
Speaking of the Malayan strand-plants, Professor Schimper remarks (pp.
11, 12) that both in outward appearance and in anatomical structure they
are xerophilous in character, whether in the case of those of the
mangrove-swamp or in those of the beach. Since the tropical shore-flora
of the Pacific islands is essentially Malayan, the identity usually
extending to the species, the same conclusion may be applied to its
character. The xerophilous habit may show itself externally in a variety
of ways, as in hairiness, leaf-structure, a leathery cuticle,
succulency, &c.
From this xerophilous habit of the Pacific strand-flora we should expect
to find that many of the plants stray far from the coast, wherever the
suitable conditions for their type of organisation occur, whether in the
inland plain or on the mountain-top. This is indeed the case; but in
dealing with this subject it will be necessary to discuss in some
general detail the littoral floras of the Fijian, Hawaiian, and Tahitian
groups in succession.
THE FIJIAN STRAND-FLORA
THE INLAND EXTENSION OF THE BEACH PLANTS
Viewed from the old standpoint of “station,” where one would distinguish
sharply between the coast and the inland plants, the Fijian strand-flora
exhibits a number of inconsistencies, all at first sight extremely
puzzling. When, however, we regard their xerophilous character and
reflect that this habit, and not mere fitness for growing at the coast,
is the primary determining factor of their station, much that is strange
appears normal and plain.
Let me refer in this connection to the impression that the distribution
of the Fijian shore-plants made on Mr. Horne, the director of the
Botanic Gardens of Mauritius, who spent a year in the botanical
investigation of the group about a quarter of a century ago. In his
account of the group (pp. 59, 60) he says that several of “what are
known as sea-shore plants” are found far in the interior of the larger
islands; and amongst others he names such characteristic beach plants as
Cerbera Odollam, Hibiscus tiliaceus, Ipomœa pes capræ, and Pandanus
odoratissimus. On the other hand, he remarks that several species of
inland plants occur at the coast, and that several plants growing on the
mountain-tops are found near the sea. This apparent confusion of station
he seems to attribute to the circumstance that the mountains of Fiji are
not high enough for the development of an alpine flora. But such a view
could not be held now, since the effect of an alpine flora would be the
introduction of further elements of confusion in the occasional
occurrence of some of the alpine plants on the sea-coast, as we find in
Hawaii.
Yet this apparent mingling of the littoral and inland floras in Fiji
becomes intelligible when we perceive that the seeming confusion of
station is mainly restricted to the xerophilous plants of the arid
inland plains and of the bare mountain-tops. The rank humid forests that
cover so much of the interior of the islands, and the luxuriant
vegetation of the mountain-gorges, are not here concerned. Such a
mingling occurs it is true under certain conditions; but in the general
physiognomy of the flora the distinction between the shore and inland
plants holds good. The same shore plants that are distributed far and
wide over the Pacific here present themselves; and although some of them
extend far inland, where the scantily-vegetated plains descend to the
coast, this does not deprive them of the right of being still regarded
as littoral plants.
Still, when we look at a fairly complete list of the shore-plants of
Fiji, numbering in all about eighty, we perceive that about two-thirds
of them also occur inland, either in Fiji or in some other tropical
region; and if we reflect that many of the residue are plants of the
mangroves that would not be found inland except under estuarine
conditions, it becomes evident that with this reservation there are very
few littoral plants in Fiji that do not at times leave the coast.
Cæsalpinia Bonducella may be taken as a type of those shore-plants that
stray far away from the coast, even into the interior of continents,
since in India it reaches the Himalayas. Although Terminalia Katappa and
Calophyllum Inophyllum often owe their existence inland in different
parts of the tropics to man’s agency, this cannot be said of most
others, as Cassytha filiformis, Casuarina equisetifolia, Cycas
circinalis, Ipomœa pes capræ, Pandanus odoratissimus, Premna tahitensis,
Tacca pinnatifida, Tephrosia piscatoria, Vitex trifolia, &c., when they
occupy the extensive inland plains that slope to the coasts on the lee
sides of the large islands of Fiji. Plants, like Hibiscus tiliaceus, are
found in a Pacific island almost as frequently away from the beach as on
the beach itself; and this is true of most other regions of the tropics
where it occurs.
Other plants that appear to be altogether confined to the sandy beach in
Fiji, break away on rare occasions from their usual station and appear
on the bare rocky summits of hills near the coast, even though the
hill-slopes are densely wooded. On such bare hilltops in Vanua Levu,
varying from 500 to 1,100 feet in elevation, one is surprised at times
to find shore creepers and climbers like Canavalia obtusifolia and
Derris uliginosa associated with other beach-plants more frequently
found inland, such as Tephrosia piscatoria and Vitex trifolia, and in
the company of climbing species of Morinda and of small trees of Fagræa
Berteriana. When the “talasinga” (sun-burnt) districts, as the Fijians
term the plains on the north sides of the islands, extend a long
distance from the coast into the heart of the island, they carry with
them their peculiar vegetation and the intruding beach-plants up to
considerable elevations above the sea. We then find familiar
beach-plants like Cerbera Odollam and Ipomœa pes capræ growing far
inland at heights of 1,000 feet and over above the sea. (See Notes 20
and 21.)
One is never quite sure of the behaviour of shore-plants in Fiji when
the “talasinga” plains lie behind the beach, since even Scævola Kœnigii,
usually a steadfast beach-plant, occurs at times some miles inland. (See
Notes 20 and 55.) There are, however, a few that never came under my
notice inland, such as Pemphis acidula, Triumfetta procumbens, and
Tournefortia argentea. The extension of sea-coast plants for any
distance inland depends a good deal on the occurrence of
scantily-vegetated plains, or of scrub-covered, rolling country at the
back of the beaches; and doubtless that which I have described in the
case of Fiji is to be found in other tropical coast-regions. Professor
Schimper informed me by letter that he had noticed a similar inland
extension of the shore-plants in the Seychelles.... I have only here
touched on this subject. In Notes 20 and 21 the reader will find further
details of the inland extension of the beach-plants, and in Note 22 is
given a general account of the “talasinga” plains, in which the
wandering beach-plants mingle with the peculiar vegetation of the plains
themselves. Covered with reeds and bracken, and dotted over with clumps
of Casuarinas and Acacias, with the Cycad and Pandanus distributed
irregularly over their surfaces, such level districts possess, as
remarked by Seemann, a South Australian look.
THE GROUPING OF THE FIJIAN LITTORAL PLANTS.
The littoral plants readily divide themselves into three principal
groups as concerning their station, namely:
(_a_) The “beach-formation,” typically exhibited on the whitish
calcareous beaches of reef-bound coasts.
(_b_) The “mangrove-formation,” found at intervals all along the coasts,
but most fully developed at the estuaries, and for the most part
occupying flats regularly overflown by the tide.
(_c_) The “intermediate formation,” comprising the plants of the tracts
between the beach and the mangrove-swamp and at the borders of the
swamps.
This grouping does not differ materially from that adopted by Professor
Schimper in the instance of the Indo-Malayan strand-flora. (See Note
23.)
To the beach-formation belong, amongst the trees and shrubs,
Barringtonia speciosa, Calophyllum Inophyllum, Guettarda speciosa,
Pemphis acidula, Scævola Kœnigii, Tournefortia argentea, &c., and
amongst the creepers and procumbent plants, Canavalia obtusifolia,
Ipomœa pes capræ, Triumfetta procumbens, &c. To the mangrove-formation
belong the Asiatic and the American species of Rhizophora, and species
of Bruguiera, Carapa, Lumnitzera, &c. Amongst the trees that gather
around the borders of the mangrove-swamp, constituting the intermediate
formation, occur Barringtonia racemosa, Excæcaria Agallocha, Heritiera
littoralis, Hibiscus tiliaceus, and several other species, all of them
being equally at home on the sandy beach, at the border of a
mangrove-swamp, and on the banks of an estuary. The climbers, such as
Entada scandens, Mucuna gigantea, Derris uliginosa, &c., belong more to
the mangrove and to the intermediate formations than they do to that of
the beach. Referring the reader to the more complete lists given in Note
24, I may remark that it is not always possible to distinguish sharply
between the three formations, since some of the plants belong to two,
and a tree like Cerbera Odollam may, in different localities, be
referred to all three formations. The general distinction, however,
prevails in the physiognomy of the coast-flora.
The mangrove-formation comprises, it may be pointed out, many plants
other than mangroves, plants that find a home in the mangrove-swamps of
Fiji, either within their limits or at their borders. It presents,
indeed, a world in itself. When the mangroves establish themselves in a
new locality they carry along with them a host of hangers-on, both
plants and animals, that only find a home under the favourable
conditions of a mangrove-swamp. Thus, the absence of the
mangrove-formation from a Pacific island deprives its littoral flora of
many very striking features. For this reason the Tahitian shore-flora
must seem to a botanist coming from Fiji comparatively tame and
monotonous; whilst that of Hawaii, for this and for other reasons to be
subsequently mentioned, is still less interesting, and scarcely even
gives a character to the coasts.
We are now, therefore, prepared to learn that a large number of the
plants other than true mangroves, that thrive in or around the Fijian
mangrove-swamp, are not to be found in those Polynesian islands where
true mangroves do not exist; and that a law of association here exists.
Many of the plants of the intermediate formation are so closely bound up
with the mangroves in their life-conditions that they are not to be
found where the mangroves are absent, even though their seeds or fruits
are pre-eminently fitted for dispersal by the currents. The influence of
“station” here rules supreme. This matter will be treated more in detail
when discussing the Tahitian and Hawaiian strand-floras in Chapters VI.
and VII.
THE MODES OF DISPERSAL OF THE FIJIAN STRAND-PLANTS.
The predominant influence of the currents having been already
established, there remains for consideration the distribution of the
floating capacity of the seeds or fruits among the different formations.
One can say that almost without exception the seeds or fruits or
seedlings of the mangrove and intermediate formations float for long
periods. In the case of some of the true mangroves, as in Rhizophora and
in Bruguiera, where germination takes place on the tree, it is the
seedling that floats, whilst in others, as in Carapa and Lumnitzera, it
is the seedvessel that floats. The plants with non-buoyant seeds or
fruits that belong to the littoral flora are all confined to the beach
formation, but they do not form more than a sixth of the total. Almost
all the “good floaters” of the beach-plants are widely spread over the
shores of the Pacific and of much of the tropics, and include such
familiar species as Barringtonia speciosa, Cæsalpinia Bonducella,
Terminalia Katappa, and many others mentioned in the lists of Notes 2
and 24.
When, however, we come to the dozen or so of beach-plants that possess
seeds or fruits with little or no floating power, we find that several
of them have a limited distribution in the Pacific, such as Acacia
laurifolia, Drymispermum Burnettianum, Eugenia Richii, &c., whilst
others, such as Casuarina equisetifolia, Tephrosia piscatoria,
Triumfetta procumbens, and Wikstrœmia fœtida, are widely spread. This
small non-buoyant group of the beach-plants has a nondescript
appearance, and it is here that the inland flora is most likely to make
its influence felt by additions to the number. It is here indeed that
the littoral floras of the tropics mostly differ, the accessions from
the inland flora varying in each region. It is in fact the zone of
change.
A number of these plants, such as the species of Drymispermum, Eugenia,
and Wikstrœmia, have probably been dispersed by frugivorous birds;
whilst others, like Triumfetta procumbens, possess fruits that might
have been transported in birds’ plumage. From the frequency with which
Tephrosia piscatoria is associated on hilltops in Fiji with Fagræa
Berteriana and climbing species of Morinda that are well suited for
dispersal by frugivorous birds, it seems likely that it is also
distributed by birds fond of a drier diet. It is possible that the
Polynesians, who much value the wood of Casuarina equisetifolia, have
often assisted in dispersing the tree.
The following is a summary of the contents of the chapter.
(1) The extension inland of the Fijian strand-flora is to be attributed
to the xerophilous organisation of the plants, and to the exceptionally
favourable conditions that are offered to such plants on the plains, and
in other scantily vegetated localities, lying usually on the drier sides
of the larger islands.
(2) Excluding the mangroves and the plants associated with them in the
coast-swamps, there are few littoral plants of the islands of the
tropical Pacific that do not extend inland in one region or another.
(3) The Fijian shore-plants can be rudely arranged in three groups,
those of the mangrove-swamp, those of the sandy beach, and those of the
intermediate districts, the last including those plants that occur
typically at the borders of a mangrove-swamp, though some of them can
thrive equally well on a beach.
(4) There is a law of association connecting many plants with a
mangrove-swamp in such a manner that when the true mangroves are not
represented in a Polynesian group, as in Tahiti or in Hawaii, the plants
in question are also absent, notwithstanding that in many cases, such as
those of Clerodendron inerme and Heritiera littoralis, they possess
seeds or seedvessels of great floating power.
(5) The fruits or seeds or seedlings, as the case may be, of the plants
of the mangrove-swamp and of the bordering districts float almost
without exception for long periods. This is true also of five-sixths of
the beach-plants, whilst the remainder owe their dispersal chiefly to
birds.
(6) The small non-buoyant group of the beach-plants represents that
portion of the strand-flora that is most likely to be recruited from the
inland flora. It is here that exists the zone of change; and it is in
this respect that the littoral floras of the tropics differ principally
amongst each other, the recruits from inland varying naturally with the
floras of different regions.
* * * * *
Though it does not come within my plan to discuss the littoral floras
of the adjacent smaller groups of Tonga and Samoa, it may be remarked
that they reflect most of the principal features of the strand-flora
of Fiji. In particular it may be observed that they possess the
mangrove-formation, but to a more limited extent. Both own the
mangrove genera Rhizophora and Bruguiera, whilst Carapa is also found
in Tonga. The intermediate formation is represented in Tonga by
Clerodendron inerme, Excæcaria Agallocha, and Heritiera littoralis;
whilst in Samoa we find, besides the first-named species, Barringtonia
racemosa and Scirpodendron costatum. In both the beach-formation is
well represented.
CHAPTER VI
THE TAHITIAN STRAND-FLORA
(_From materials supplied mainly by the work of Drake del Castillo_)
Lacks the mangroves and their associated plants.—Possesses mainly the
plants of the coral beach.—Predominant agency of the currents.—Inland
extension of shore-plants.—Summary.
JUST as the littoral plants of Fiji may be regarded as typical of
Western Polynesia, so the strand-flora of Tahiti, or, rather, of the
Tahitian Islands, may be considered as representing Eastern Polynesia.
We have thus the Tahitian area, comprising generally the Cook and
Austral Groups, the Society Islands, the Paumotus, and also the
Marquesas, as contrasted with the Fijian area, including the
neighbouring Samoan and Tongan groups. For the sake of brevity the terms
Fiji and Tahiti are often used as equivalents of the entire areas (see
Note 25).
The littoral flora of this part of the Pacific lacks the mangroves and
most of the plants that are associated in the Fijian region with a
mangrove-swamp, either at its borders or within its interior. Thus we
miss here the true mangroves of the genera Rhizophora, Bruguiera,
Carapa, and Lumnitzera, as well as the accompanying trees and shrubs,
such as Barringtonia racemosa, Excæcaria Agallocha, and Heritiera
littoralis. The climbers and straggling plants that are so
characteristic of the borders of the mangrove-creeks in Fiji proper are
also wanting, such as Clerodendron inerme, Derris uliginosa, and Smythea
pacifica; and we do not find in the Tahitian region the Giant-Sedge
(Scirpodendron costatum) that is so common in the mangrove-swamps of
Fiji, and occurs also in Samoa.
It is not at first sight easy to account for the absence from Tahiti of
the mangrove-formation and of so many of the plants that grow at the
borders of a mangrove-swamp in Fiji. Their absence can scarcely be due
to the want of suitable stations, as is indicated by the common
occurrence in the Tahitian coast-marshes of Chrysodium aureum, the Great
Swamp-fern, that not only abounds in the mangrove belts of Fiji, Tonga,
and Samoa, but is associated with mangrove-swamps over much of the
tropical zone. Nor can it be said that the currents are ineffective, or
that the seeds or fruits of the missing plants possess, as a rule,
insufficient floating powers. Most of the plants of the Tahitian beaches
hail, like those of Fiji, from Malaya, and have been brought through the
agency of the currents; and many of the absent littoral plants that have
the same home, such as Heritiera littoralis and Clerodendron inerme,
have fruits or seeds just as capable of floating unharmed over the same
extent of ocean. It is not any defect in floating-power that has
prevented the establishment of two such plants in the Tahitian area.
Entada scandens, which in some parts of the world is a typical climber
of the mangrove-formation, and in other places thrives well in the
absence of mangrove-swamps, has only been recorded from Rarotonga in
this region by botanists, but I believe Wyatt Gill refers to its
occurrence in Mangaia in one of his books.
On the other hand, it is likely that the floating seedlings of
Rhizophora and Bruguiera, which represent the only means of dispersal by
the currents at the service of these mangroves, would not arrive at
Tahiti in a condition favourable for the establishment of the plants. My
observations, which are described in Chapter XXX., go to show that,
though the seedlings will float uninjured in still sea-water for months,
they will not withstand prolonged sea-buffeting. These two genera of
mangroves, it is most important to remember, supply the pioneers and the
principal components of a mangrove-swamp in the Western Pacific. Where
they fail to establish themselves, the requisite conditions for the
large number of plants and animals that find their home in and around a
mangrove-swamp would not be provided. We thus perceive that the absence
from the Tahitian coast flora of several plants that are associated in
Fiji with the mangrove-swamps depends on a law of association, which has
already been referred to in the preceding chapter, and is not concerned
with incapacity for dispersal by currents (see Note 26).
Whilst the Tahitian coast flora does not, therefore, possess the plants
of the mangrove-swamp and its vicinity, it includes most of the typical
beach-trees of the coral islands and reef-fronted coasts of other parts
of the South Pacific. Thus we find here on the sandy beaches
Barringtonia speciosa, Calophyllum Inophyllum, Cerbera Odollam,
Hernandia peltata, Guettarda speciosa, and numerous other plants that
are indicated by the letter T in the list of Fijian littoral plants
given in Note 2. The total number of Tahitian shore-plants is thus
considerably less than that of Fiji (there are about 55 in Tahiti and
about 80 in Fiji); but in its turn, as will subsequently be shown, it is
much larger than that of Hawaii, where the number is about 30.
Quite three-fourths of the strand-flora of this region have buoyant
seeds or seedvessels capable of floating for long periods; and there is
no difficulty in assigning by far the greater share in the stocking of
the beaches with their plants to the agency of the currents. The
currents in their operations have indeed carried the fruits or seeds of
many of these plants across the South Pacific as far as the islands
extend, namely, to Ducie Island and to Easter Island. There are few more
significant proofs of the efficacy of the currents in distributing
plants over the Pacific than the discovery, by Mr. Arundel, of
Barringtonia speciosa in Ducie Island in association with Tournefortia
argentea (Challenger, Botany, III. 116).
The residue of the Tahitian coast flora possessing fruits or seeds that
are unsuited for dispersal by currents includes such plants as
Heliotropium anomalum, Triumfetta procumbens, Tephrosia piscatoria,
Wikstrœmia fœtida, &c. The small nucules of the first-named are perhaps
dispersed by granivorous birds; the fruits of Triumfetta are probably
transported in birds’ plumage; those of Wikstrœmia are distributed by
frugivorous birds; and the seeds of Tephrosia may be dispersed like
those of Heliotropium.
The recruits or intruders from the inland flora do not appear to be
numerous or to give any special character to the shore flora. (See Note
27.)