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

by H. B. Guppy

(2) Of this nature, it is contended, is the relation between fruits and seeds and the agencies of dispersal.

(3) If, however, the structure or mechanism is made more effective by the new function, such a modification may be regarded as an “adaptation” in the language of the theory of Natural Selection.

(4) It is held by Professor Schimper that the structures connected with the buoyancy of the fruits or seeds of several tropical littoral plants are, in the above sense, adaptations; and he points to several genera where the buoyant tissues in the coverings of the fruits or seeds of the coast species are scantily represented or absent in the inland species of the same genus, a difference corresponding with the loss or diminution of the floating powers.

(5) This contrast in structure and in floating capacity between the fruits or seeds of inland and coast species of the same genus is beyond dispute, and the author adduces fresh data in support of it.

(6) But he contends that it is not proved that the relatively great development of buoyant tissues in the case of littoral plants is the effect of adaptation; and that if the selecting process had been confined to sorting out the xerophilous plants with buoyant seeds or fruits and to placing them at the coast, the same contrast would have been produced.

(7) In support of this contention he points out that when such littoral plants extend inland the floating capacity and the buoyant tissues are as a rule retained; and that in those exceptional cases where inland plants possess buoyant fruits or seeds these tissues are sometimes well developed under conditions in which they could never aid the plant’s dispersal.

(8) But the most serious objection against the adaptation view is that admittedly only about half of the shore-plants with buoyant fruits or seeds come within its scope. Therefore a second explanation has to be framed for the other plants concerned.

(9) As showing the difficulties raised by regarding some of the structures connected with buoyancy as “adaptive” and others as “accidental,” it is pointed out that some fruits possess the two kinds of structure. It is also shown that in several cases fruits endowed with buoyant tissues are just as well adapted for dispersal by frugivorous birds; and the instance of Ximenia americana is cited where a drupaceous fruit, known to be dispersed by fruit-pigeons, possesses also in its “stone” both the “adaptive” and “non-adaptive” types of “buoyant structures.”

(10) It is urged that whatever is the relation between the buoyancy of the seeds and fruits of shore-plants and dispersal by currents, there has been a uniform principle affecting all.

(11) The weight of evidence is regarded as adverse to the Natural Selection theory, an inference which is consistent with the conclusion arrived at in [Chapter X.] that there is no direct relation between the density of sea-water and the buoyancy of seeds and fruits, the floating capacities being as a rule far greater than the adaptation view would explain. Nature, it is held, has never made any provision for dispersal by currents, the buoyancy of seeds and fruits being, as concerns the currents, a purely accidental quality.