CH. x THE BUOYANCY OF SEEDS AND SEEDVESSELS 89 



an investigation, instead of being, as the title of this chapter might 

 suggest, an abstruse and disconnected inquiry, is of considerable 

 importance in relation to the dispersal of plants through the agency 

 of currents. 



Guided by the results of my experiments in this direction I 

 will proceed to lay down certain general principles : 



(A) In the first place it may be accepted as a general rule that 

 seeds or seedvessels that sink in fresh water sink also in sea-water, 

 the difference in density between the two being rarely a factor of 

 any importance in determining buoyancy. The great majority of 

 seeds and fruits come under this category, since, as is pointed out 

 in Chapter VIII., only a small proportion of the whole, say a tenth, 

 possess floating power. We might cite, as illustrative of this 

 principle in temperate regions, almost all the 240 species included 

 in the non-buoyant group of the British plants experimented on 

 (see Chapter III. and Note 10). As a general rule this is true alike 

 of the small seeds of the Cruciferae and Scrophulariaceae, of the 

 nutlets of the Labiatae and Boragineae, of the genus Scirpus, and 

 of the dust-like seeds of Juncus. The results of my experiments 

 on the plants of the tropical Pacific are no doubt typical of other 

 tropical regions ; and if I wished to quote instances, I should have 

 to enumerate not only most of the plants without buoyant seeds 

 or fruits that are mentioned in the Fijian and Hawaiian lists given 

 under Notes 2, 4, and 6, but also to appeal to tropical regions 

 generally. 



(B) One can carry the principle above-named yet further and 

 say that not only as a rule do seeds or fruits that sink in fresh 

 water sink also in sea-water, but that so far as tested many of them 

 sink in water of much greater density than that of ordinary sea- 

 water (1-026). Thus, for instance, the seeds of Nuphar luteum, 

 Scrophularia aquatica, and Stellaria aquatica, the nutlets of Poly- 

 gonum persicaria, and the achenes of Aster tripolium sank in sea- 

 water the density of which had been raised to 1*050, the limit of 

 the experiment. The minute seeds of Juncus communis and 

 J. glaucus and the larger seeds of Luzula campestris, even after 

 drying for six months, sank in salt water having a density of 

 1-075. It would, however, seem probable that for most of these 

 small seeds and seedvessels a density of rioo would prove to be 

 the critical point. If this is so, then most of those that sink in 

 sea-water would float in the dense water (ri6o) of the Dead Sea. 



However, my investigations have only gone a small way in 

 this direction ; and perhaps some of my readers will pursue the 



