Chapter XII

THE CAUSES OF THE BUOYANCY OF SEEDS AND FRUITS OF LITTORAL PLANTS WITH
ESPECIAL REFERENCE TO THOSE OF THE PACIFIC ISLANDS

The classification of buoyant seeds and fruits.—The first group, where
the cavity of the seed or seedvessel is incompletely filled.—The
second group, where the kernel is buoyant.—The third group, where
there is air-bearing tissue in the seed-tests or fruit-coats.—The
buoyant seeds and seedvessels of the littoral plants of the British
flora.—Summary.

IN the following pages I have adopted in its main features the classification of buoyant seeds and fruits employed by Professor Schimper in his work on the strand-flora of the Indo-Malayan region. The causes of buoyancy, as he points out, are very various, but they can be arranged in a few categories; each category, however, usually admitting great variety within its limits. It is this want of uniformity that first attracts our attention when we come to study the structure of seeds and fruits from the standpoint of their buoyancy. Whilst in the Pacific I went over most of the field traversed by Professor Schimper in Malaya (the majority of littoral plants of these regions being common to both), and as a result I have added not a few plants to his original groups.

It will be seen from the following synopsis that there are three principal groups. The first group includes those seeds and fruits where the buoyancy is derived from unfilled space in the seed or fruit cavity. The second group comprises those seeds or fruits where the floating power is due to the buoyant kernel or nucleus. The third group includes those where the buoyancy arises from the existence of air-bearing tissue in the coverings of the seed or fruit.

The first two groups I will term the mechanical or non-adaptive groups, not only on account of the structure inducing the buoyancy, but because, as Professor Schimper remarks, the same structure often occurs with inland fruits and seeds possessing little or no floating power. In many of these cases, as he points out, the question of adaptation to dispersal by ocean currents cannot, therefore, be raised. The third group may be named the adaptation group, because it is on these examples of buoyant seeds and fruits that this investigator chiefly based his contention that in the main the structures concerned with buoyancy represent adaptations to dispersal by currents effected through the agency of Natural Selection. It is accordingly to this group that Professor Schimper especially directed his attention, and the result of his observations made in the home of the plants and of his investigations in the laboratory has been the elucidation of many difficult points in the structure of their fruits and seeds. To the two “mechanical” groups he did not pay the same attention; and as their examination came more within the limits of my own capacity as an inquirer I have worked them out with some detail, the subdivisions of the first group being my own as well as much of the material.

_Synopsis of the buoyant fruits and seeds of littoral plants of the tropical Pacific classified according to the cause of buoyancy._ (The authorities are indicated by the initial letter, S = Schimper, G = Guppy. Details are given under some of the species in latter part of volume.)

FIRST GROUP.—The floating power is derived from unoccupied space in the cavity of the seed or fruit, no part of the seed or fruit as a rule possessing independent floating power.

SUB-GROUP I., where the seed is concerned.

SECTION I. The seeds have little or no albumen, and neither the tests
nor the seed-contents have any buoyancy. The cotyledons are generally
large, foliaceous, and crumpled or folded, or otherwise arranged, so
that the seed-cavity is incompletely filled.

S. G. Hibiscus tiliaceus.
G. Hibiscus diversifolius.
S. G. Thespesia populnea.
S. Suriana maritima.
G. Kleinhovia hospita, _variable_.
S. G. Colubrina asiatica.
S. Dodonæa viscosa.
G. Argyreia tiliæfolia, _variable_.
G. Ipomœa bona nox, _variable_.
G. Ipomœa glaberrima, Boj.
S. G. Ipomœa grandiflora.
S. G. Ipomœa pes capræ.
G. Ipomœa turpethum, _variable_.
G. Cassytha filiformis.
S. Euphorbia atoto.

_Notes._—The species marked “variable” have seeds that sometimes sink and sometimes float. With the exception of Kleinhovia they are only at times littoral in station.

The plants of the British flora are represented by Convolvulus soldanella and C. sepium, the last being “variable” and not a littoral species.

SECTION II. All the seeds belong to the Leguminosæ. Neither the tests
nor the seed-contents have any buoyancy, the floating power arising
from a large central cavity produced by the bending outward of the
cotyledons during the final shrinking stage of the maturation of the
seed.

S. Mucuna (generically).
G. Mucuna urens D.C. (Hawaii).
G. Mucuna, species of.
S. G. Vigna lutea.
S. G. Cæsalpinia bonducella.
G. Cæsalpinia bonduc.
G. Entada scandens.

SUB-GROUP II., where the fruit is concerned.

SECTION III. The seed only partially fills the fruit-cavity, and as a
rule is not buoyant. The fruit shell, usually woody, may be also
buoyant.

S. G. Heritiera littoralis.
G. Smythea pacifica.
G. Dalbergia monosperma.
S. G. Derris uliginosa.
S. G. Pongamia glabra.
G. Desmodium umbellatum.
G. Gyrocarpus jacquini.

SECTION IV. The floating power is derived from empty seed-cavities,
where owing to abortion of the ovule or some similar cause the seed
is not developed.

S. G. Morinda citrifolia.
G. Premna tahitensis.

_Note._—Professor Schimper, in the case of Morinda citrifolia, holds the view that we have here a special adaptation to dispersal by currents.

SECOND GROUP.—Here the floating power is due mainly or entirely to buoyant kernels. In the case of seeds the tests are non-buoyant; but with “stones” the floating capacity may be aided by a layer of air-bearing tissue inside the shell.

SECTION I. Non-Leguminous.

S. G. Ximenia americana (drupe).
S. G. Calophyllum inophyllum (drupe).

_Note._—Professor Schimper would place these two plants in the second section of the third group on account of the layer of air-bearing tissue inside the shell of the “stone”; but they are assigned to this section, since the floating power is mainly due to the buoyant kernel.

Arenaria (Honckeneya) peploides, a British beach plant, belongs here.

SECTION II. Leguminous seeds.

G. Dioclea.
G. Strongylodon lucidum.
S. Canavalia (generic).
G. Canavalia sericea.
S. G. Canavalia obtusifolia.
S. Erythrina (generic).
S. G. Erythrina indica.
P. Erythrina ovalifolia (Penzig).
S. G. Sophora tomentosa.
G. Afzelia bijuga.
G. Lathyrus?

THIRD GROUP.—The floating power is due to the presence of air-bearing tissue in the seed-tests or fruit-coats.