HI THE LESSON OF THE BRITISH FLORA 29 



the stranded drift of the Devonshire beaches, and also on the 

 coasts of Chile ; whilst Sernander includes them amongst the drift 

 of the beaches on the Norwegian coasts. It is remarkable that 

 Convolvulus sepium, which accompanies C. soldanella over much 

 of its great range, has seeds that are sometimes able to float 

 unharmed for long periods, even for years (Notes 13, 41, 49). 

 Though not strictly a water-side plant, it grows commonly over 

 other plants on the banks of the Thames ; and when it fruits its 

 seeds occur typically in the floating drift of that river. According 

 to Gray, it is almost a river-side plant in the United States, where 

 it is found " especially on the moist banks of streams." Not all 

 the seeds of C. sepium, however, are buoyant ; and in its varying 

 behaviour in this respect it resembles the inland species of Ipomea, 

 which are referred to in the previous chapter. 



The British species of Euphorbia also seem to behave in 

 accordance with the principle that when a genus has littoral and 

 inland species, the first-named alone possesses buoyant fruits or 

 seeds. Thus, whilst the sound fruits of E. helioscopia and of 

 another species found commonly as a garden weed are non- 

 buoyant, those of E. paralias, the familiar beach-plant, float for 

 several weeks, and are to be noticed among the stranded drift of 

 the coasts frequented by this plant. (See Note 90 for later results.) 



The structural characters connected with the buoyancy of the 

 seeds or seedvessels of some of the British plants are dealt with in 

 Chapter XII. Here it may be remarked that this capacity is often 

 associated, as with the Pacific island plants, with a " buoyant " 

 tissue, that is either absent or less developed in the case of the 

 non-buoyant group. 



Enough has now been said to show in a general fashion how 

 Nature through the agency of buoyant seeds and fruits has affected 

 the stations of plants of the British flora. Allowing this line of 

 inquiry to develop itself as the work proceeds, we will here pause 

 and close the chapter with a reference to some of the principal 

 points that have been brought into prominence. 



{a) The proportion of flowering plants of the British flora that 

 possess buoyant seeds or seedvessels is very small, probably not 

 more than 10 per cent. 



(/;) In so far as buoyancy is concerned. Nature has for the most 

 part ignored the dehiscent fruit with small seeds, such as we see in 

 the Cruciferae and the Caryophyllaceae, and has chiefly endowed 

 with floating power the dry indehiscent fruit, such as we see in the 

 Umbelliferae and in the Labiatae. 



