234 PLANTS, SEEDS, AND CURRENTS 



in Chapter XII. of my book on Plant Dispersal. Those of Sc. Koenigii, 

 which are described and figured by Schimper in his work on the 

 Indo-Malayan strand flora (p. 172, pi. vii.), belong to a type that 

 includes many of the characteristic littoral plants of the Indian and 

 Pacific Oceans, the buoyant tissue forming part of the fruit- coverings. 

 Since the question of adaptation was raised by Schimper in their 

 case I termed them the " adaptive " group, though not myself in 

 agreement with him on that point. 



However, the interesting thing is that the type of buoyancy 

 represented by the stones of Sc. Plumieri is offered by plants where 

 this question of adaptation could not be raised. It corresponds 

 with the Premna type, which is discussed at length on pp. 112 and 

 561 of my book above quoted. The behaviour of the small drupes 

 of a littoral species of the genus is exactly that of the West Indian 

 Scoevola Plumieri. The Premna drupes floated at first, but the 

 buoyant stones are soon freed by the decay of the soft parts. Neither 

 the seeds nor the substance of the stone are buoyant, the stone 

 deriving its floating power from the fact that three of its four cells 

 are usually empty. The importance of the bearing on the question 

 of adaptation to dispersal by currents of the contrast presented by 

 the two shore species of Sccevola is obvious. If the buoyant quality 

 is accidental in its origin in one species, it is not reasonable to assume 

 that it is adaptive in the other. 



The results of my experiments on the floating powers of the stones 

 of the drupes of these two species of Scaevola now require a few 

 remarks. As long ago as 1888 I tested the buoyancy of those of 

 Sc. Koenigii on Keeling Atoll, the results being given in my paper on 

 the plants of that locality which was published in the journal of the 

 Victoria Institute of London in 1889. Ripe fruits gathered from 

 the plant continued to float buoyantly after fifty days' immersion in 

 sea- water, losing during the early days of their flotation their white 

 fleshy covering. Subsequently three of the stones were sown out by 

 Dr. Treub at Buitenzorg, and out of the six seeds that they contained 

 five germinated in the course of the next two months. Twenty-one 

 months afterwards I put in sea- water in England two fruits gathered 

 on Keeling Atoll. Both floated after a year's immersion, the seeds 

 proving to be quite sound (see Plant Dispersal, p. 531, and Note to 

 the Keeling Atoll paper). 



My first experiments on the fruits of Sccevola Plumieri were carried 

 out in the Turks Islands in 1911, the average result being that about 

 70 per cent, remained afloat after sixty-three days in sea-water. 

 The stones that had lost their soft coverings whilst lying under the 

 bushes on the beach sand were the most buoyant. About 60 per 

 cent, of the fresh stones and about 80 per cent, of the old dry stones 

 floated after six weeks. In all cases the seeds of the floating stones 

 proved to be sound and healthy at the close of the experiments. 

 It was ascertained that the sinking was due to water penetrating the 

 empty cell, the cell containing the seed being usually quite dry. 

 In later experiments made in England on stones that had been 

 collected ten months and still possessed sound moist seeds, I found 

 that 66 or 67 per cent, floated after eighteen weeks in sea-water. 



