iv THE LESSON OF THE BRITISH FLORA 37 



the clue to the study of the complicated problems connected with 

 plant-stations by taking the floating seed as our guide. 



We will carry ourselves in thought to the Thames-side between 

 Teddington and Twickenham at the end of August, 1892. The 

 river is at the high-water level, and we see flourishing at the 

 margins, sometimes a little above the water and sometimes a little 

 within its reach, Ranunculus repens, R. sceleratus, Spiraea ulmaria, 

 Lycopus europaeus, Scutellaria galericulata, different species of 

 Rumex, Alnus glutinosa, Iris pseudacorus, Sparganium ramosum, 

 and different species of Carex, with several other plants, all con- 

 tributing their seeds or fruits to the drift that floats in the river 

 from the autumn to the spring. 



But besides these plants there are a number more or less 

 submerged in the stream, including Nasturtium amphibium, N. 

 sylvestre, Stellaria aquatica, Myosotis palustris, and Veronica 

 beccabunga ; and as the water falls other plants still more sub- 

 merged come into view on the exposed flats, such as Nasturtium 

 officinale, Apium nodiflorum, and Polygonum hydropiper. None 

 of these plants are represented by their seeds or fruits in the float- 

 ing river-drift. Several of them possess dry dehiscent fruits with 

 small seeds, such as Nature ignores in the matter of buoyancy, and 

 the small fruits of Myosotis, Apium, and Polygonum have little or 

 no floating power. 



We have thus here a clear dividing line between the plants with 

 buoyant seeds or fruits that were more or less exposed above the 

 high-water level, and those that were more or less submerged at 

 that state of the tide. That which occurs in the Lower Thames 

 twice in the day within the reach of the tide represents what 

 happens in the higher part of the river during the seasonal floods, 

 but in the last case the effects cannot be so readily distinguished. 

 We thus perceive that the buoyant seed or fruit is as a rule only 

 characteristic of the plants of the river-side that grow more or less 

 exposed above the water, whilst those plants liable to periodic 

 submergence have seeds or fruits that sink. 



In this connection it is of especial interest to observe that as a 

 general rule the truly aquatic plants of English rivers contribute 

 little or nothing to the floating seed-drift. I pointed this out 

 several years ago, in my paper on the Thames, as an agent in plant- 

 dispersal, and it has been already noticed in this work (page 30). 

 We look in vain amongst the floating winter drift of our rivers 

 for the seeds or fruits of Ranunculus aquatilis, Nuphar luteum, 

 Nymphaea alba, and of the species of Myriophyllum, Limnan- 



