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

by H. B. Guppy

It has been before established that in the Pacific islands, and indeed in the tropics generally, the plants with buoyant seeds or seedvessels are mainly stationed at the coast. It has also already been shown that this littoral station is often associated with a special buoyant-tissue in the coverings of the seed or fruit; and it will now be pointed out that this tissue is, as a rule, absent or but scantily developed in the case of inland species of the same genus. Of great importance, remarks Professor Schimper (p. 179), in relation to the Selection Theory and the development of adaptations, is the comparison of the fruits and seeds of strand-plants with those of allied inland species; and he finds here evidence in support of the Darwinian view. He takes the cases of the genera Terminalia and Calophyllum, which contain both inland and littoral species; and he shows that although the same buoyant-tissue occurs in the fruit-coats of inland species, it is there much diminished, and in consequence the floating powers are considerably lessened or lost altogether (see [Chapter II.]). It is not pretended that this tissue has had any connection in its origin with dispersal by currents, but merely that its greater development in the shore species is an adaptation to this mode of transport.

Further testimony is adduced by this investigator (p. 182) in supporting his view in the fruits of the genera Barringtonia, Clerodendron, Cordia, and Guettarda, where the buoyant tissues extensively developed in the coast species are either non-existent or only represented by a trace in the inland species of the same genus, a difference in structure associated with the loss or great diminution of the floating capacity of the fruits concerned. I have been able to establish other examples in the cases of the genera Scævola and Tacca, which will be found referred to in [Chapter II].

Professor Schimper (p. 200) points to the circumstance that the “adaptations” in these fruits all belong to the diagnostic marks of the genera and the species, and contends that these plants abundantly prove the erroneous nature of the contention that Natural Selection could have played no part in the elimination of the strand-flora. My own contention is that Natural Selection has played such a part, but that in doing so it has merely availed itself of characters previously existing, without originating, modifying, or improving them in any way. The foregoing evidence might with equal fitness be employed to show, as pointed out in [Chapter II.], that in the course of ages there has been a great sorting process by which, excluding the mangroves, plants of the xerophilous habit possessing buoyant seeds and fruits have been sorted out and placed at the coast. Direct evidence does not lead us farther than to the establishment of a littoral station for plants thus endowed. The problem whether the characters of their fruits and seeds that are concerned with buoyancy may be regarded as adaptive in the Darwinian sense lies beyond the reach of direct testimony. We can, however, approach it from the outside by several directions, and from some of these we will now proceed to deal with it.

There is first the singular circumstance that in Fiji, when the littoral plants with buoyant seeds or fruits leave the beach and extend far inland, they, as a rule, retain their floating powers and, of course, their buoyant structures. I found this to be true of Cassytha filiformis, Cerbera Odollam, Ipomœa pes capræ, Morinda citrifolia, Scævola Kœnigii, and one or two other plants mentioned in [Note 44], where this subject is discussed. My experiments on these plants indicated that their fruits or seeds floated equally long, whether obtained from coast or from inland plants. This, at first sight, appears to present a serious objection to the adaptation theory; but it was not so regarded by Professor Schimper, who in a letter to me, dated March 8th, 1900, observed that he did not see “why littoral plants growing inland should lose their adaptations to littoral life, especially if those adaptations are not conflicting with the conditions of life beyond the littoral zone, and if the competition does not require special adaptations.”

My view, however, is that any process of adaptation is unnecessary. All these plants, it is contended, were originally inland plants that acquired the buoyant qualities of their seeds and fruits in the inland stations, and ultimately found a station at the coast through the sorting process above referred to. In the case of plants like Ipomœa pes capræ and Cassytha filiformis this would be conceded, since they belong to the acknowledged non-adaptive groups discussed in the preceding chapter. It is only to some of these plants, such as Scævola Kœnigii and Cerbera Odollam, that the adaptation view of Professor Schimper is applied; and the question arises whether we are justified in making such a distinction, or, in other words, whether it is antecedently probable that two independent principles have been at work in determining the fitness of seeds and fruits for dispersal by the currents.

The plants for which the influence of adaptation through Natural Selection is claimed belong, as stated in [Chapter XII.], almost entirely to the third group. It is admitted that with the other two groups the utmost that any sorting or selecting process would effect would be to determine a station at the coast and to extend the area of distribution. The numerical aspect of the question therefore acquires some importance; and the reader’s attention is accordingly directed to the results tabulated in [Note 45], where it is shown (assuming for the time that there is no difference of opinion about the adaptive significance of the seeds and fruits concerned) that the plants of the third or adaptive group make up only about half the total. It would therefore appear that if the agencies of Natural Selection have been at work here either in bettering or in developing buoyant structures, half of the shore-plants with buoyant seeds or fruits have not come within their influence.

But the subject takes another aspect when we reflect that in some buoyant fruits, as with Ximenia americana and Calophyllum inophyllum, the two principles would seem to have been at work. Whilst from this standpoint Natural Selection is regarded as having either developed or increased in amount the layer of buoyant tissue in the fruit-coats, the buoyant kernels are not viewed as adaptive in their origin. In the case of Ximenia americana the dispersing agency of frugivorous birds adds another factor, since, as before stated, its drupes are known to be dispersed by fruit-pigeons. In the cases of Scævola Kœnigii and of Vitex trifolia, two plants belonging to the adaptive group, Professor Schimper (pp. 156, 188) admits also the dispersing agency of frugivorous birds, and he claims it for Morinda citrifolia, in the pyrenes of which he also detects a special adaptation to dispersal by currents. It may be added that, as he also points out, fruits of the non-adaptive group of littoral plants, such as Premna integrifolia (P. taitensis) and Cassytha filiformis, would sometimes also attract birds. In fact, those of the last-named have been found in the crops of pigeons (Introd. Chall. Bot., p. 46).

Looking at all these littoral plants with fruits that are equally fitted for dispersal by birds and by currents, we may now ask, Where does the general principle of adaptation to dispersal lie? Whatever view we adopt, we must apply the same view to all, whether it be a question of dispersal by birds or by currents. We cannot choose between two sets of principles determining the buoyancy of seeds and fruits any more than we can regard a fleshy drupe and a buoyant seed as illustrating different principles regulating the dispersal of plants. Nature works with uniformity in these matters, and if the Natural Selection theory is held to explain one case it ought to account for all. Yet nobody would go so far as this; and this view of dispersal is on many grounds antecedently improbable. These difficulties disappear if we assume that in all cases the dispersing agencies have without modification made use of characters and capacities that were developed, as we now see them, in quite other connections and under quite other conditions.

It will now be necessary to look a little closer into the subject of the buoyant tissue, to the existence of which in their coats about half of the littoral plants concerned owe the floating power of their fruits or seeds. In the first place, it is to be remarked that in the case of some of the seeds of the plants of the non-adaptive groups it is also represented to a small degree in the seed-coats, although, as with Strongylodon lucidum and Mucuna urens, it is not present in sufficient amount to float the seed. In the next place, it should be noted that with some genera possessing, like Terminalia, both inland and coast species it is to be found alike in the fruit-coverings of inland and of littoral plants, though in a less degree in the case of the fruits of inland species, the floating power of which is proportionately diminished. There are, however, a few cases where this buoyant tissue is developed in inland species which belong to genera or subgenera that have no littoral species. This is what we would expect, if Natural Selection has merely concerned itself with placing plants of xerophilous habit possessing buoyant seeds or fruits at the coast. Under such conditions we would now and then expect to find an inland plant possessing buoyant fruits or seeds of this description that has never been able to establish itself at the coast.

A good instance is afforded by Pritchardia Gaudichaudii, a fan palm peculiar to Hawaii, the drupes of which float for several weeks and have a covering of spongy buoyant tissue (see [Chapter XXV.]). The seeds of Hibiscus Abelmoschus, a species distinguished subgenerically from the littoral Hibiscus tiliaceus, offer another example. They float for months, and owe their buoyancy to a layer of air-bearing tissue between the kernel and the test, in this respect differing from the seeds of the littoral species, where the floating power is due to unoccupied space in the seed-cavity. The buoyancy of the seeds of Hibiscus Abelmoschus thus offers another example of ineffectual floating power, since it is not a littoral plant, is often cultivated, and has accompanied aboriginal man over much of the tropical zone.