34 THE EVOLUTION OF PLANTS 



though we do not know what may have gone 

 on in this way among the plants of the past. 



Before leaving the subject of the adaptabil- 

 ity of Angiosperms we must say a word about 

 water-plants. As we all know, a great many 

 Flowering Plants live in fresh water, some 

 with floating leaves, like the Water-lily, some 

 floating altogether, like the Duckweed, and 

 some submerged altogether, like the Hornwort 

 (Ceratophyllum). Such plants hold their own 

 very well and play a more important part in 

 fresh-water lakes, ponds and rivers than the 

 Algae themselves. The water Flowering Plants 

 do not, however, always have it all their own way, 

 for I am told that in some lakes in the Hebrides, 

 the old-world Horsetails are now turning out the 

 more advanced aquatics such as Reeds and 

 Rushes. 



The water-plants, especially those that live 

 quite submerged, have their structure a good 

 deal simplified compared with land-plants, because 

 their requirements are so different. The sim- 

 plification often extends to the flowers, and some 

 botanists have imagined that certain water- 

 plants are really primitive members of the Angio- 

 sperms. As a rule this is very unlikely, for in 

 many cases we can trace quite clearly the families 

 of land-plants to which they belong. Most, at 

 any rate, of the aquatic Angiosperms have taken 



