56 BEITISH PLANTS 



on land, but which afterwards succeeded in making the 

 water a place of abode. In the course of time they have 

 become, by gradual modification, better adapted to a 

 watery existence, but traces of their terrestrial origin 

 always remain. The oldest aquatics are, naturally, most 

 changed (Lemna, Elodea, Ceratophyllum), while the latest 

 emigrants from the land differ little in organization from 

 ordinary land-plants — e.g., water-buttercup and water- 

 violet. Thus, when we find an aquatic whose vascular 

 tissue has lost all trace of lignification (e.g., Ceratophyllum), 

 which has no stomata or no roots (bladderwort), we know 

 that we are dealing with a very ancient race which has 

 lost most of its land-characters through its long sojourn in 

 the water. Probably the oldest aquatics are the duck- 

 weeds (Lemna), which seem to have lost all the characters 

 of land-plants except their method of flowering. Of aU 

 organs the flower is the most conservative, and, except in 

 a few cases, all water-plants still send their flowering shoots 

 into the air, where the flowers are pollinated by the 

 same means as their relatives on land. Some aquatics, 

 indeed, possess large and quite showy flowers — e.g., 

 water-lobelia, water- violet, bladderwort — all of which are 

 pollinated by insects. 



Some of the very recent inhabitants of the water 

 live equally well on land — e.g., Polygonum amphibium. 

 These are amphibious plants (p. 28), and it is from 

 among the amphibious plants which throng the edges of 

 water everywhere that the aquatic vegetation is con- 

 tinually being recruited. 



