Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 2 / Plant-Dispersal

by H. B. Guppy

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].

Diagrams illustrating some of the causes of seed-buoyancy.

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.]