Note.—If to the last we add the eight British shore plants, the buoyant fruits of which are described in [Chapter XII.], three non-adaptive and five adaptive, we get a proportion of adaptive species for temperate and tropical regions of fifty-one per cent. This is probably fairly typical of the world generally; but it must be remembered by the reader that the author regards them all as non-adaptive. In that case, the table can be used for the numerical results of the three groups which are based only on structural characters without reference to any theory.
NOTE 46 (page [124])
On the Modes of Dispersal of the Genus Brackenridgea.
Seed-vessels of this genus found afloat in the New Guinea drift are described by Mr. Hemsley as having two curved cavities crossing each other one containing a seed, the other empty. “This empty cavity,” it is stated “gives the fruit its buoyancy” (Bot. Chall. Exped., iii., 289; plate 54) Dr. Beccari, in the English edition of his Wanderings in Borneo, p. 187, speaks of the closed air-containing cavities in the seed-vessels, or rather “stones,” of this genus as probably giving them buoyancy and thus enabling them to be dispersed by currents. He points out that the fleshy covering of these fruits would also aid their dispersal by birds. The Italian botanist implies that the two Bornean species grow in swamps. The Fijian species, as observed by me in flower in Vanua Levu, grew in the dry talasinga districts bordering the Mathuata coast, the locality where Seemann found the plant. One of the most recent accounts of the genus is given by Van Tieghem in his memoir on the Ochnaceæ in Ann. des. Sci. Nat. Bot., tome 16, 1902. According to him there are nine species, all from Malaya and New Guinea, with the exception of one in Fiji. Previous authors have also referred to Queensland and Zanzibar species. However, all the species have a limited distribution, a fact which plainly assigns to birds the principal share in the dispersal of the genus.
NOTE 47 (page [125])
On the Transport of Gourds by Currents
Small calabashes or bottle-gourds are not uncommonly to be found floating in the Fijian estuaries and stranded on the beaches; and I have also found them in the sea off the coasts. They are usually more or less globular, 3 or 4 inches across, and are evidently able to float for very long periods and to carry the seeds unharmed. Most of those I examined from the drift were dry inside and contained the seeds dried together into a loose ball about an inch in size. The seeds are not those figured in Gaertner’s De Fructibus et Seminibus, as belonging to Lagenaria vulgaris, and more resemble those of Cucurbita, but are non-buoyant. One of these gourds, picked up by me in the sea in Fiji, was placed in sea-water, and two months later was still floating buoyantly. After being then kept dry for seven months, it was broken open; and ten of the seeds were put in soil, two of them germinating in a few days.
In Ecuador gourds similar in size and shape were frequently observed by me floating in the drift of the Guayaquil River and stranded on the sea-beaches. The seeds are similarly caked together in a loose mass in the cavity of the fruit. Their characters indicate that they belong to another species of gourd; and they differ also from the Fijian seeds in their buoyancy, some of them in my experiments floating two months and afterwards germinating.
It has been known since the days of Ström and Gunnerus, two Norwegian naturalists of the 17th century, that gourds and calabashes are from time to time stranded with other Gulf-stream drift on the coasts of Norway. We learn from Sernander that those found are usually worked calabashes; but he alludes to one that was unworked and contained several seeds (see Sernander, p. 119).
It is scarcely likely that a seed-carrying gourd stranded on a beach would be able to establish the plant without the aid of man; but it seems highly probable that gourds have often been introduced into new countries by the currents and that man has afterwards cultivated them. These plants may be contrasted with that remarkable Cucurbit, Luffa insularum, a genuine littoral plant, the seeds of which, and not the fruits, are dispersed in the Pacific by the currents (see page [426]).
NOTE 48 (page [126])
On the Useless Dispersal by Currents of the Fruits of the Oak (Quercus robur) and other Species of Quercus, and also of the Hazel (Corylus avellana)
The fruits of different species of Quercus are of not infrequent occurrence in the seed-drift both of the temperate and tropical regions, being brought down by the rivers to the sea and then stranded on the neighbouring beaches. They were amongst the drift gathered by Mr. Moseley in the open sea, 70 miles off the New Guinea coast (Bot. Chall. Exped., iv., 294). I found them on the beaches of Keeling Atoll where no oak exists, and on the beaches of the south coast of Java; whilst Prof. Schimper noticed them among the stranded drift of the Java Sea, and Prof. Penzig found them stranded on the shores of Krakatoa. They also came under my notice on the Sicilian beaches and on the Italian coast at Cumæ. Those of Quercus robur are to be found on the English beaches and in the autumn drift of the Thames, but they soon sink and disappear from river-drift. They are referred to by Dr. Sernander as frozen with other floating seeds in the ice of the Scandinavian rivers; but he evidently does not regard them as possessing much independent floating power.