38 A NATURALIST IN THE PACIFIC chap. 



themum, Callitriche, Ceratophyllum, Zannichellia, and of several 

 of the Potamogetons, all of which give character in summer to the 

 aquatic vegetation of the river. In their place we find only the 

 seeds and fruits of the plants growing on the banks. 



There is, however, another small group of river plants, which 

 in their structure and habits and in the behaviour of their floating 

 fruits come between the true aquatics and the plants of the river- 

 banks. They belong mostly to the AHsma family, and Alisma 

 plantago and Sagittaria sagittifolia may here be specially 

 mentioned. Their fruits display great variation in their floating 

 power ; and on this point M. Kolpin-Ravn, writing to me in 1895, 

 made the following interesting suggestion, that since these plants 

 approach true aquatics in structure they may be also regarded as 

 approaching them in the inconstancy of the buoyant capacity of 

 their fruits, those of aquatics having typically little or no floating 

 power. 



Seed-buoyancy, however, does not play quite such an important 

 part in the plant-economy of a river as the examination of the 

 floating drift would lead one to expect. Only a portion of the 

 bank-plants have buoyant seeds or fruits, whilst amongst the true 

 aquatics, the semi-aquatics, and the plants periodically submerged, 

 the rule of non-buoyancy prevails. And, indeed, when we look at 

 all the possible stations for the plants of the British flora, we 

 discover that seed-buoyancy can rarely be connected with station. 

 It is, however, in those few stations that plants with buoyant seeds 

 have mainly gathered. There it is, probably, that the remnants of 

 a past floral age find a refuge, since it would seem likely that the 

 tendency has been in the course of geological time for the develop- 

 ment of dry stations for plants at the expense of the wet stations. 



The following is a summary of some of the points discussed in 

 this chapter : — 



(i) In the case of the strand-flora of a Pacific island, and 

 indeed in that of an ordinary tropical region, the large proportion 

 of plants with buoyant seeds or fruits tends to mask all other 

 issues, and we are seemingly only concerned with dispersal by 

 currents. 



(2) But in the British strand-flora where plants with buoyant 

 seeds and fruits are in a minority, constituting less than a third of 

 the total, it is seen that the issue is primarily an affair of station, 

 an inference that may be applied generally to temperate regions. 



(3) All British shore-plants may be regarded as owning certain 

 characters in common which may be collectively designated the 



