210 THE NATURE AND WORK OF PLANTS 



found in land which is not well drained that support 

 groups of members from two different kinds of com- 

 munities. This is to' be seen in places covered with 

 water by the spring floods, which last until early 

 summer. During this time the only species living in 

 the place are aquatic, both floating and rooting. 

 With the advance of summer the water evaporates, 

 leaving the soil dry and suitable for some of the 

 rapidly growing members of swamp or meadow 

 societies. A few species of algae, as well as some 

 of the seed plants, are so elastic in their habits that 

 they grow under both conditions. Thus the alga, 

 green velt (Vaucheria), may float or rest on moist soil 

 with equal facility. Species which cannot adapt 

 themselves in this manner live through the unfavora- 

 ble period in the form of seeds, tubers, or corms. 

 The continuation of either the floods by artificial or 

 natural dams, or the complete drainage of the area, 

 will result in the extinction of the members of the 

 group least adapted to withstand the change. 



248. Extermination of a forest. — Find a piece of 

 woodland from which the trees have been recently 

 cut, and note the changes ensuing among the other 

 members of the community. 



