92 A NATURALIST IN THE PACIFIC CHAP. 



groups ; some, indeed, that have a specific weight approximating 

 to that of fresh water, or to that of sea-water, or fluctuating 

 between them, and presenting such evidence of a fine adjustment 

 that the observer, forgetting that they are members of a series, 

 might be apt to regard them as specially adaptive in their origin. 

 It will thus be seen that this subject is gradually assuming a 

 statistical character ; and in truth we shall ultimately recognise 

 here the play of the laws of numbers. 



As an example of the plants where the specific weight of the 

 seeds or fruits is near that of fresh water, Alisma plantago may 

 be taken. In the course of an experiment, by lowering the 

 density of the water from 1*025 to 1*020, I sent a shower of 

 floating carpels to the bottom. The results vary considerably, 

 as one might expect ; but, generally, during the first few days of 

 an experiment about twice as many (sometimes in all as much as 

 80 per cent.) sank in fresh water as in sea-water, a few only 

 floating in either water for long periods. . . . The seeds of 

 Arenaria peploides present an example where the specific weight 

 is between that of fresh water and of sea-water. For the purposes 

 of dispersal they may be considered as heavier than fresh water 

 and lighter than sea-water. The details are given in Note 18 ; 

 but it may be remarked here that plants possessing seeds or fruits 

 that sink in fresh water and float in sea- water are very rare. As 

 indicated below, this is what we might look for on statistical 

 grounds. 



Plants whose seeds or fruits are not much lighter than sea- 

 water are exceptional. In such cases the effect of increased density 

 of the water is to extend the period of flotation. Thus, in my 

 experiments on the nutlets of Scirpus maritimus, the majority of 

 the fruits floated in fresh water only eight to ten days ; whilst 

 in ordinary sea-water they floated in most cases two to three 

 weeks; but when the density was raised to 1*050, the greater 

 number of them were afloat after two months. In a few plants, as 

 with Spiraea ulmaria, the effect of the difference in density between 

 fresh and sea- water was not to extend the period of flotation, but 

 to increase the number that floated for a given period, the extreme 

 limit of the buoyancy of the carpels in either water with this 

 species being about three weeks. 



Amongst tropical plants, as illustrated by those of the Pacific 

 islands, cases also came under my notice where the mean specific 

 weight of the seed is somewhere between those of fresh water and 

 sea-water. The seeds of Afzelia bijuga, an inland as well as a 



