pRAKGUR — The Buoyancij of the Seeds of some Britannic Plants. 15 



most important series of observations are those of Guppy, who, in his 

 exhaustive work on seed-dispersal in the Tropics,' furnishes much informa- 

 tion relative to our native plants as well, including a huoyancy-table for 

 273 species. Those portions of his book -which deal with the British flora 

 constitute the main source of information relative to the buoyancy of the 

 seeds of our native plants. His conclusion is that the seeds of 90 per cent, 

 of the British flora sink either at once or within a few days, leaving 10 per 

 cent, which alone possess buoyancy sufficient to render them capable of 

 transference across any but a very narrow stretch of water. It will be 

 noticeil that this figure is identical with that arrived at by Darwin just half 

 a century earlier, as representing the proportion of a flora capable of crossing, 

 by means of currents, a considerable stretch of sea. His study of the 

 buoyancy of Fijian and Hawaiian seeds led Gruppy to adopt the same 

 percentage as representing the seed-buoyancy of the flora of those regions. 



The pkesbnt Experiments. 



The experiments of which the results are given below were undertaken 

 in order to extend our knowledge of the buoyancy of seeds of plants which 

 inhabit the British Islands, especially with a view of obtaining information 

 useful in the study of the immigration and dispersal of our native flora. 

 Until Guppy's time, no special attention had been given to our native plants 

 as regards their seed-buoyancy. Guppy, as stated above, tested the seeds 

 of some 273 of these ; and he added to his table results for about 60 additional 

 species, the seed-buoyancy of wliicli could be obtained from the writings of 

 Darwin, Martins, Thuret, Kolpin Kavn, and Sernander. In the present 

 list, results are given for 786 species. The number of species in Guppy's 

 list which were not tested by me is 114. Adding these figures, we have 

 now buoyancy -results for just 900 species — a number which, though falling 

 far short of the total for the British Isles, still represents a proportion 

 sufficiently large to permit of a generalization regarding the whole flora. 

 Wliile Martins and Guppy experimented mainly with seeds .known or 

 believed to have a high average of buoyancy, and thus obtained slightly 

 abnormal results, I endeavoured to have all kinds of seeds equally 

 represented in my experiments, whether the nature of the seeds themselves, 

 the phylogenetic relationships of the plants which bore them, or their 

 habitats, be taken into account. 



Since power of dispersal is to be measured by its maximum in the case 

 of any species, special care was taken to test the seeds, so far as was possible, 

 in the condition of maximum efficiency which may occur in nature. 



1 H. B. Guppv. Observations of a Naturalist in the Pacific . . . vol. ii : Plant- Dispersal. 1906. 



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