90 A NATURALIST IN THE PACIFIC CHAP. 



inquiry. I will take the case of the nutlets of Scirpus palustris. 

 They sink in fresh water and in sea-water, or may float in 

 rare cases for a day or two. Out of 100 of these seed-like 

 fruits, 25 floated in salt water of a density of 1*075, J 3 m 

 water of 1*050, 7 in sea- water (1*025), and 3 in fresh water, 

 (1*000). It would thus appear that the proportion of buoyant 

 nutlets is doubled with every increase of '025 of the density 

 scale. At this rate of increase they would all float in salt 

 water of a density of 1*125, which may be regarded as the 

 suitable medium for the flotation of the fruits of this Scirpus. 

 . . . The seeds of Glaucium luteum, the Sea-Poppy, have no 

 buoyancy either in fresh water or in sea-water even after pro- 

 longed drying. They all sank in water of a density of 1*050, 

 but 1 8 per cent, floated when the density was raised to 1*075. 

 At the rate of increase noticed in the case of Scirpus palustris, 

 all the seeds would float in water of a density of 1*130-1*140. . . . 

 The acorns of the Common Oak (Quercus robur) have usually 

 but little buoyancy unless they have been long drying. After 

 soaking in fresh water for half an hour 100 mature fruits, without 

 the cupule, that had been kept a fortnight, I found that only 

 2 floated in fresh water, 6 in sea- water (1*025), and 18 in water 

 of 1*050. At this rate of increase all would float in water having 

 a density of 1*080-1*090. 



(C) There is also another general rule, and it is this : Seeds or 

 fruits that float for a long time in sea-water usually float almost as 

 long in fresh water. Here belong the greater number of buoyant 

 seeds and fruits, those only able to float for a few weeks being 

 comparatively few. Now with the long-floating seeds and fruits, 

 those for instance that float in the drift of English rivers from 

 the autumn to the spring, or those that are transported by 

 currents over the tropical zone, there is, as a rule, but a slight 

 difference between their flotation periods in fresh water and sea- 

 water. If one of them sinks after floating for several months 

 in fresh water, it will sink in sea-water a few days after. 

 Fruits of Scaevola Kcenigii, pyrenes of Morinda citrifolia, and 

 seeds of Thespesia populnea, Ipomea grandiflora, Csesalpinia 

 bonducella, and of different species of Mucuna, that had been 

 kept afloat for a year in sea-water, floated just as buoyantly 

 in fresh water at the close ; and in those cases where any 

 sank during the course of the experiment, it was ascertained 

 that they were able to float in fresh water almost to the end. 



That many of the seeds and fruits of tropical littoral plants 



