xxiv] SEED DISPERSAL 297 



Even within a single river basin, the question of the seed- 

 dispersal of aquatic plants is by no means a simple one. The 

 expectation might perhaps be formed that aquatics would be 

 characterised by floating seeds or fruits, capable of being water- 

 borne for considerable distances. But, as is often the case, 

 Nature fails to conform to the preconceived notions of the 

 teleologist, and we find, as a matter of actual fact, that although 

 many plants with water-side stations possess buoyant seeds, such 

 seeds are relatively rare among true aquatics. Guppy 1 , who gives 

 the results of experiments on the floating powers of the seeds 

 of more than 300 British plants, records that sinking occurred 

 within a week in the case of 26 aquatics, e.g. Ranunculus aqua- 

 tilis, Hottonia palustris. Lobelia Dortmanna, Lemna gibba, Calli- 

 triche^ and others. He found that the seeds of Limnanthemum 

 nymphoides would float for I to 4 weeks, while Lemna minor, 

 Sagittaria sagittifolia, Alisma Plantago and certain species of 

 Potamogeton were the only hydrophytes whose seeds and fruits 

 were capable of floating for months at a time, and, of these, 

 the Alismaceae should perhaps be reckoned, in this connexion, 

 as water-side rather than as aquatic plants. 



It is true that the seeds of those aquatics that sink very 

 rapidly may yet sometimes be carried a short distance -by the 

 wind. For instance, the slender infructescences of Hippuris 

 vulgaris are swayed to and fro by the breeze, and the fruits may 

 be jerked a little way 2 , but the migrations thus achieved can 

 never be extensive. 



If the dispersal of hydrophytes within a single river basin 

 can only be explained with difficulty, this is still more the case 

 when we come to consider migration from one country to 

 another. As Guppy 3 has pointed out, Ceratophyllum demersum 



different river-basins, and if that is right then the species held in common 

 ought to include all those growing in the head-springs, e.g. in England, 

 Callitriche aquatica, Nasturtium officinale, Ranunculus aquatilis, etc., etc." 

 (By letter, February 3rd, 1918.) 



1 Guppy, H. B. (1906). See also Praeger, R. L. (1913). 



2 Fauth, A. (1903). 3 Guppy, H. B. (1893). 



