Reconquest of the Water 



grains of land plants, as well as the spores of rust-fungi, 

 are also spiny, and by these processes stick to the hairs 

 of an insect's body and so get carried from flower to 

 flower. In Elodea the minute spines are kept, but used 

 for the totally different purpose of entangling an air- 

 bubble. A curious fact about freshwater weeds is that 

 many of them are found over enormous areas of the 

 earth's surface. The usual explanation is that such birds 

 as ducks and water-fowl generally are also very widely 

 distributed, and that the seeds are carried in mud stick- 

 ing to the plumage or legs of these birds. But if a 

 bird happened to be starting on a flight of a thousand 

 miles or so, it would surely take good care to wash its 

 feet and feathers before leaving. 



One of the most wonderful arrangements known in 

 water plants is that possessed by a certain Trapella. 

 The small conical fruit has three long fine gracefully 

 incurved spines, which look very well fitted to be en- 

 tangled in a migrating bird's plumage. But this plant 

 is very rare, having been only once discovered by 

 Dr. Henry in one small tarn in China, so that it does 

 not seem to have succeeded in travelling to many 

 places.® 



But these water plants not only live in water but 

 they also have a very important function, that of changing 

 it into dry land. 



The process is orderly and systematic. Each par- 

 ticular group has its own definite part to play, and gives 

 place to another so soon as its work is done. 



The method employed for colonising a shallow lake 

 or the quiet backwater of a river seems very much the 

 same in North America, France, Switzerland, or Great 

 Britain. Probably one would find it going on anywhere 

 in the North Temperate Zone, where the climatic con- 

 ditions are of an average character. 



129 I 



