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 
