Chapter XIX: Section II: The buoyant tissue forms a layer inside the hard test of a

seed or inside the shell of the “stone” of a drupaceous fruit, and to
this cause the floating power is mainly or entirely due.

G. Mucuna gigantea (seed).
S. Hernandia peltata.
S. Excæcaria agallocha.
S. Cycas circinalis.
S. Pandanus odoratissimus.
G. Anona paludosa (seed) of tropical America.

_Note._—I have followed Schimper in respect to Pandanus, but it might be by some placed in the first section of this group.

Here belongs Euphorbia paralias, a British littoral plant, the buoyant seeds of which occur in the stranded seed-drift of English and Mediterranean beaches.

In the following general discussion of the groups, reference will be made only to the plants best illustrating the different varieties of structure connected with buoyancy; whilst mention of the other plants will in some cases be found in other parts of this volume, as shown in the Index; and the matter is discussed at some length in not a few of the species.

THE FIRST GROUP.

Of the first group, where the floating power is due to the unoccupied space in the cavity of the seed or fruit, the Convolvulaceæ offer the most typical examples. Here as a rule the crumpled embryo fills the seed-cavity more or less incompletely; and it is on the relative size of the unoccupied space that the sinking or floating of the seed depends. In those plants where the seed sinks the seed-cavity may be almost filled, as in Ipomœa tuberculata, or densely packed, as in Ipomœa pentaphylla, and in species of Cuscuta. When the seed floats, as with Ipomœa pes capræ, I. glaberrima, &c., the unoccupied space is relatively large; and when, as with I. bona nox and I. turpethum, the behaviour of the seeds is irregular, some floating, and others sinking, a corresponding variation exists in the extent to which the seed-cavity is filled. This applies also to the irregular behaviour of the seeds of Ipomœa peltata and of Argyreia tiliæfolia. A singular instance is afforded by the seeds of Ipomœa insularis, collected by me in Fiji and Hawaii. Those from Fiji were incompletely filled, and consequently buoyant. Those from Hawaii were more densely packed and sank.... The three British species of Convolvulus illustrate the same principle, namely, C. arvensis, with non-buoyant seeds; C. soldanella, with buoyant seeds; and C. sepium, with seeds irregular in behaviour.

In the case of plants of the Convolvulaceæ, possessing buoyant seeds, there is always evidence of marked shrinking of the seed-contents before the final setting and hardening of the seed-coats. The embryo often appears shrivelled and dried up, and is almost brittle, so that large spaces are produced in the seed-cavity. If we partly divide such a seed and place it in water, the embryo absorbs water rapidly, and within an hour is soft, healthy-looking, and much swollen, the interspaces being filled with a jelly-like mucilage. It is therefore evident that absolute impermeability of the seed-coats is essential for the successful transport by sea-currents of the floating seed; and we can only suppose that the shrinking of the seed-contents takes place before the final setting of the tests. That with the buoyant seeds the coats are quite waterproof was illustrated in many of my experiments where, after a period of flotation covering several months, and sometimes a year or more, the seed-contents were still quite dry and shrunken. The limit of buoyancy, as I have shown in Chapter IX., depends on an attempt at germination on the part of the floating seed, which then absorbs water, softens, swells, and sinks.

It is, therefore, not a matter of surprise that non-buoyant seeds of the Convolvulaceæ do not gain floating power after prolonged drying of many months. It is also to be expected that, as we find in Fiji, when a characteristic shore-species with buoyant seeds like Ipomœa pes capræ extends far inland, the seeds retain their floating powers. Seed-buoyancy of this description is, on the face of it, purely mechanical.

EXPLANATION OF THE DIAGRAMS ILLUSTRATING THE CAUSES OF SEED-BUOYANCY

1. _Entada scandens_ (natural size): (_a_), the shell; (_b_), the
kernel; (_c_), the intercotyledonary cavity. The shell consists of
three coats—an outer and an inner hard chitinous coat, and an
intermediate layer of brown cellular tissue containing little or no
air. The buoyancy is due entirely to the central cavity, neither the
seed-tests nor the seed contents possessing any floating power (see
page 181).

2. _Mucuna urens_, from Hawaii (natural size). The kernel (_b_) sinks,
and the shell has no floating power except where it possesses (under
the raphe) a layer of dark brown, air-bearing, spongy tissue (_a_).
This, however, is not sufficiently developed to endow the seed with
buoyancy, which is due to the intercotyledonary cavity (_c_). (see
page 111).

3., 4. _Mucuna gigantea_, from Fiji (natural size). The kernel (_b_)
sinks, and the seed owes its floating power entirely to the existence
in the shell (_a_) of a layer of brown, spongy, air-bearing tissue
which is mostly developed at the circumference and is almost wanting
at the flat sides of the seed (see page 115).

5., 6. _Dioclea_ (_violacea?_), from Fiji (natural size). Here the
kernel (_b_) is buoyant and endows the seed with floating power.
Though the shell (_a_) possesses a thick layer of reddish-brown
cellular tissue, this tissue contains but little air and aids the
floating power but slightly (see page 113).

7. _Strongylodon lucidum_, from Fiji (natural size). The floating power
is due entirely to the buoyant kernel (_b_). There is a very scanty
amount of loose brown tissue (_a_) under the raphe; but it has no
appreciable effect on the buoyancy (see page 113).

8., 9., 10. _Cæsalpinia bonducella_ and _C. bonduc_, from Fiji (natural
size). Neither the seed-tests (_a_) nor the kernel (_b_) have any
floating power in themselves, the buoyancy being connected with a
large internal cavity (_c_), which normally is intercotyledonary, as
in Fig. 8 (C. bonducella). With both plants, but more especially with
C. bonduc (Figs. 9 and 10), there may be a lateral cavity (_d_), or
the kernel may be loose in the shell (Fig. 10), but this does not
necessarily imply buoyancy (see page 194).

11., 12. _Arenaria peploides_ (enlarged: seeds 4 mm. in size). Here the
curved embryo (_a_) sinks, and the spongy air-bearing albumen (_b_)
gives buoyancy to the seed (see page 116).

13. _Euphorbia paralias_ (enlarged: seeds 3 mm. in size). The kernel
(_b_) sinks, and the seed owes its buoyancy to a layer of air-bearing
tissue (_a_) in the shell (see page 116).

14. _Morinda citrifolia_ (enlarged pyrene 7 mm. long). The floating
power is due to the bladder-like air cavity (_a_). The seed (_b_)
proper is enclosed in the woody tissue behind the bladder (see page
112).

15. _Cucurbita_ (seed enlarged), from the Valparaiso beach-drift (see
page 125). The kernel (_b_) has no buoyancy. The shell (_a_) is formed
of two layers of air-bearing tissue, the outer composed of prismatic
cells and the inner of a spongy vacuola-material.

[_To face page 111._

Another type of the buoyant seeds of the first group is presented by several species of Leguminosæ, as with Entada scandens, some species of Mucuna, and Cæsalpinia bonducella. As with the Convolvulaceous seed, the embryo sinks and the seed-shell has no buoyancy; but here the floating power is due to the existence of a more or less symmetrical long central cavity produced by the arching or bending outwards of the large cotyledons which lie usually in close contact with the seed-shell. This arching outward of the cotyledons depends on a shrinking process in the setting or final stage of the maturation of the seed. The stages of the process may be traced in the immature seeds, which are much larger and in some cases twice the size of the mature seed. In this immature condition the seed-coats are soft, and the flabby fleshy and thick cotyledons fill up the seed-cavity. As the hardening and setting process continues, the cotyledons diminish in size, become firmer, and gradually bend outward, leaving a central cavity. This arching outwards is no doubt in part the result of the contraction of the seed-tests during the shrinking process. Considerable variation prevails in the results, and where the cavity is very small the seed sinks. Further details relating to this subject will be given in my treatment of some of the plants, and especially under Cæsalpinia. But it may be here remarked with reference to Hawaiian seeds of Mucuna urens D.C., that although they are strictly referable to this group, they display beneath the hard test, on the side beneath the raphe, a scanty layer of dark spongy air-bearing tissue which is sufficiently buoyant to float up detached portions of the test, but does not of itself give buoyancy to the seed. The significance of this structure will be subsequently pointed out. The seed owes its floating power to the large central cavity, but this layer of spongy tissue adds to its buoyancy.

The section where the buoyancy of the fruit is connected with unoccupied space in the fruit-cavity is extremely heterogeneous in its composition. Every fruit has a method of its own, and the great variety of causes of buoyancy of a mechanical character is here exemplified. For instance, with Gyrocarpus jacquini and Cassytha filiformis the cause of buoyancy is in the main the same as that described in the case of the Convolvulaceæ. The origin of the floating power of the pods of Derris uliginosa is two-fold. In the first place the seed or seeds but partly fill the pod, and in the second place the seed is able to float of itself by reason of its possessing, as in the seeds of Entada scandens, a large central cavity produced by the arching out of the cotyledons during the final stage of maturation. A double cause is also to be assigned to the buoyancy of the fruits of Heritiera littoralis and of Smythea pacifica, where, in addition to the unoccupied space produced by the shrinking of the seed, the fruit-case itself floats, though nothing but a mechanical explanation is to be given of the floating of empty ligneous fruits.

One of the most suggestive types of buoyancy belonging to the first group is presented by those cases, which are, however, not very frequent, where the floating power is to be attributed to empty seed-cavities produced by the abortion of the ovule or failure of the development of the seed. A significant instance of this is afforded by the fruits of Premna taitensis, a coast plant. The buoyant “stone” of the drupe, which is often found afloat in the Rewa estuary in Fiji, is 4-locular, each cell containing normally one seed, but as a rule only one cavity contains a mature seed, the three other cavities becoming more or less empty through the failure of their seeds. It can be proved that neither the seeds nor the substance of the “stone” are buoyant, and that the “stone” owes its capacity of floating for months to the empty cavities arising from the failure in development of three out of the four seeds. In Fiji we see the rivers distributing these small fruits, and we find the “stones” stranded on the beaches and floating in the currents amongst the islands; and there can be no doubt that this is one of the effective modes of dispersal of the species; yet, if there was ever a case of accidental buoyancy concerned with dispersal by currents, we have it here. Further details are given in Note 32.

It is probably also to the abortion of the ovule, or to the failure of the seed, that the remarkable air-cavity (see Note 8) to which the pyrenes of Morinda citrifolia owe their floating power, is to be attributed. To this structure Professor Schimper (pp. 165, 183, 200) attaches considerable importance as an example of special adaptation to dispersal by currents through the influence of Natural Selection. He suggests, however, that possibly its morphological significance may be found in its being a peculiarly modified seed-chamber. The case of Premna taitensis above cited indicates that the latter view is the most probable. The subject awaits a careful microscopical study of the seed-development of the genus Morinda since, as elsewhere remarked, the non-buoyant pyrenes of inland species have not such an air-chamber. An outline sketch of a pyrene of Morinda citrifolia is given in the preceding plate. A good figure of it occurs in Schimper’s _Plant Geography_, p. 28. A very suggestive instance of this nature is described under Brackenridgea in Note 46 and in Chapter XIII.

THE SECOND GROUP.

Here are included those seeds and stone-fruits that possess buoyant kernels. Professor Schimper points out that since this is a feature both with inland as well as coast plants such a character cannot be viewed as an adaptation to dispersal by currents. The plants concerned belong mostly to the Leguminosæ, and we find here some of the most widely spread of strand species, such as Canavalia obtusifolia and Sophora tomentosa, as well as some of the giant climbers of the coast forests belonging to the genera Dioclea and Strongylodon. The kernels when divested of their coverings float buoyantly, but they soon absorb water and sink usually in a day or two, a circumstance indicating that it is to the impervious coverings that they indirectly owe their capacity to keep the seed or fruit afloat. It is noteworthy that seeds of Strongylodon lucidum from Fiji display beneath the raphe a trace of an internal layer of loose cellular tissue which, however, has no appreciable effect on the buoyancy; whilst with seeds of Dioclea (violacea?) from the same locality there is a thick layer of loose tissue which aids the floating power of the kernel but is not of itself sufficiently aeriferous to buoy up the seed.

This leads one to refer to two other plants belonging to this group, Calophyllum inophyllum (Guttiferæ) and Ximenia americana (Olacineæ), where, though the floating power is mainly due to the buoyant kernel, it is also aided by a layer of air-bearing tissue inside the hard shell of the “stone” of the drupe. Professor Schimper places these fruits in the third or adaptive group on account of the layer of buoyant tissue, but it would be more correct to class them according to the predominant cause of their buoyancy. It can be shown that with a non-buoyant kernel the “stone” no longer floats. This double cause of the floating power renders an explanation very difficult, since it would seem indefensible to give conflicting interpretations of their nature. With Ximenia americana there is another great difficulty. Its drupes are known to be dispersed by fruit-pigeons (_Introd. Chall. Bot._ p. 46); and judging from the rare occurrence of the “stones” in the drift there is good reason to believe that bird agency in the Western Pacific is predominant in the dispersal of the plant. It is by such test cases as this that we must put to the proof the reality or non-reality of the influence of adaptation on seed-buoyancy.

THE THIRD GROUP.

We have here those plants where the floating-power is entirely or mainly due to an air-bearing tissue in the seed-tests or fruit-coats. Several of the fruits are figured in Schimper’s _Indo-malayische Strand-flora_, and one or two are figured in the English edition of his work on _Plant-Geography_, p. 29.

In the first section, where the buoyant tissue occurs at the outside or forms the periphery of the seed or fruit, are included several of the most familiar of the littoral trees and shrubs of the Pacific islands, such as Barringtonia speciosa, Cerbera Odollam, Guettarda speciosa, Pemphis acidula, Scævola Kœnigii, Terminalia katappa, and several others named in the synopsis. I cannot enter into detail here, but the reader will find fuller particulars of each plant in most cases in Professor Schimper’s work, and in some instances in my separate discussion of the plants concerned. In nearly all cases we are concerned here with the fruits, and only in a few cases with the seeds, as with Carapa and Pemphis acidula.

This investigator observes that to this sub-group belong the fruits and seeds usually described in systematic works as provided with corky or suberous coverings; but he points out (p. 167) that the resemblance is nearly always quite superficial, and is limited to colour and consistence, suberous tissue occurring in only a few cases, as in the fruit-coats of Clerodendron inerme. The buoyant tissues, he remarks, are often more or less ligneous, and in those cases where there is no lignin reaction they resist the action of sulphuric acid much more effectively than pure cellulose; whilst in their physical characters, as well as in their behaviour with reagents, they differ just as much from ordinary cork. Thus, they are but little elastic and often easily crumble away; whilst in large fruits, like those of Cerbera and Terminalia, they would soon be stripped off entirely when subjected to the “wear-and-tear” of transport by currents, if they were not traversed by numbers of stout, tough fibres which hold the materials together. Where the buoyant tissues are firmer, as with Clerodendron inerme and Cordia subcordata, the fibrous framework is scanty or absent, whilst very small seeds or fruits, like those of Tournefortia argentea and Pemphis acidula, where the “wear-and-tear” would be comparatively slight, often possess no protecting fibres in the buoyant tissues.

In one or two fruits, like those of Cerbera Odollam, these tissues display large intercellular spaces; but in the majority of cases such spaces are insignificant in size or absent altogether. Speaking generally, however, there is, as Professor Schimper observes, great similarity in the structure of the buoyant tissue in the coverings of these fruits and seeds. The cell-walls are thin or only slightly thickened, and detached air-bearing portions of the tissue will float for many weeks. The great floating capacity of these fruits and seeds is stated by this investigator to be entirely due to the tenacity with which the air is retained in the covering tissues. It is, however, noteworthy that in the case of Scævola Kœnigii the fruits are just as well suited for dispersal by frugivorous birds as by the currents, a significant circumstance discussed in the next chapter.

The second section contains those plants where the buoyant tissue occurs inside the hard shell of the fruit or seed, such as is found, for example, in Anona paludosa, Mucuna gigantea, Hernandia peltata, Cycas circinalis, &c. Professor Schimper here includes Calophyllum inophyllum and Ximenia americana; but I have before remarked that the buoyancy of their fruits is mainly due to their buoyant kernels. This aeriferous tissue forms a layer between the seed or nucleus and the hard outer shell. It is described by the above-named authority as soft or friable and dark brown. The cells contain air and may be closely arranged or separated by small interspaces, their walls being neither woody nor suberous.

_The structure of the buoyant seeds and seedvessels of the littoral
plants of the British flora._

The littoral plants with floating seeds or fruits form but a section of the strand-plants of the British flora, scarcely a third, as is pointed out in Chapter IV., of the total number. Though small in number they exhibit great variety in structure; and notwithstanding that as far as they have been examined they may all be referred to one or other of the groups and sections of the classification adopted in the synopsis for the plants of the Pacific islands, nearly every plant presents in the structure of its seeds or seedvessels a type of buoyant structure different from the others.

The first group is represented by the seeds of Convolvulus soldanella, which owe their floating power to the incomplete filling of the seed-cavity. The second group, where the buoyancy arises from the buoyancy of the kernel or nucleus, is illustrated by the seeds of Arenaria (Honckeneya) peploides, but in a fashion quite unique. The test is thin but impervious, and has no buoyancy; the curved embryo also sinks; and the floating power arises from the air contained in the loose spongy albumen, around which the embryo is coiled (see figure). A more normal component of the second group is represented in some Leguminous seeds, perhaps of Lathyrus maritimus, that occur regularly amongst the stranded seed-drift of the north coast of Devon. Here the kernel of the seed is buoyant. The seeds of Euphorbia paralias are indebted for their floating capacity to a layer of spongy tissue containing large air-spaces placed between the kernel and the chitinous outer test, neither of which possess any floating power (see figure). They thus belong to the second section of the third group.

The fruits of Cakile maritima, Crithmum maritimum, Matricaria inodora, and Scirpus maritimus, all belong to the first subdivision of the third group where the air-bearing tissue exists in the peripheral coverings, the seed or nucleus in all cases sinking. With Cakile maritima there is a light spongy outer case of aeriferous tissue, which, however, soon loses the epidermis, a circumstance that probably explains the limited period of flotation of about a week. The walls of the mericarp of Crithmum maritimum are composed of spongy cellular air-bearing tissue with a persistent epidermis, and the floating powers of the fruits are consequently great. The achenes of Matricaria inodora have beneath the epidermis a layer of buoyant tissue, and their structure is similar to that found with the buoyant achenes of littoral species of Wedelia, plants of the same order of Compositæ that are found on the Pacific islands. The cause of the floating power of the fruits of Scirpus maritimus lies entirely, according to Kolpin Ravn, in the air-bearing cells of the epidermis. The reader will find the results of my experiments on the buoyancy of the seeds in Notes 16, 17, and 18.

_Summary of the Chapter._

(1) Following the main lines of Schimper’s classification of those of the Indo-Malayan region which possesses for the most part the same species, the buoyant seeds and fruits of the littoral plants of the Pacific islands are classed in three groups: the _first_ where the cavity of the seed or fruit is incompletely filled, the floating power arising from the empty space; the _second_ where the buoyancy is derived from the buoyant nucleus or kernel; and the _third_ where it arises from air-bearing tissues in the coats of the seed or fruit.

(2) The first and second groups, in which the question of adaptation to distribution by currents through the agency of Natural Selection is not raised, since the same structural characters are found in seeds and fruits of inland plants not dispersed by the currents, are termed the mechanical or non-adaptive groups. The third is distinguished as the adaptive group, because it is here that Schimper finds evidence in favour of the Selection Theory.

(3) The first group is best represented by the Convolvulaceous and the Leguminous types. In the former, which is well illustrated by Ipomœa pes capræ, the seed-cavity is imperfectly filled by the crumpled embryo, the result of the shrinking process during the final setting of the seed. In the latter, which is exemplified by Entada scandens and Cæsalpinia bonducella, the seed displays a large central cavity produced by the arching outward of the cotyledons during the shrinking process accompanying the last stage of the maturation of the seed. As an instance of fruits belonging to the group, those of Heritiera littoralis may be cited. An uncommon type is presented in the “stones” of the drupes of Premna taitensis, and in the pyrenes of Morinda citrifolia, where the buoyancy arises from empty seed-cavities resulting from the failure of some of the seeds.

(4) The second group with buoyant kernels includes mostly widespread Leguminous species, such as Canavalia obtusifolia and Sophora tomentosa.

(5) The third or “adaptive” group comprises many of the characteristic littoral trees and shrubs of the Pacific islands, such as Barringtonia speciosa, Guettarda speciosa, Terminalia katappa, Tournefortia argentea, &c., that contain in their fruit-coverings a buoyant cork-like material often bound together by fibres, but which proves on examination to resemble cork only in appearance. In another type, illustrated by the fruits of Cycas circinalis and the seeds of Anona paludosa, the buoyant tissue forms a layer inside the shell of the seed or “stone.”

(6) Some fruits like those of Ximenia americana and Calophyllum inophyllum illustrate both the so-called mechanical and adaptive principles in their structure; whilst with the first-named species they are as well adapted for dispersal by frugivorous birds and are known to be a favourite food of fruit-pigeons. The same difficulty arises with the fruits of some other characteristic littoral plants, as with Scævola Kœnigii, the drupes of which are equally well fitted for dispersal by birds and currents.

(7) The same general principles have been at work in determining the structures concerned with the buoyancy of the fruits and seeds of British littoral plants. Although the species are few in number they exhibit in this respect great variety, eight species illustrating six or seven types of buoyant structure.