400 PLANT PECULIARITIES 



276. Plant Succession. — When a swamp is drained, a 

 forest cleared, or a desert irrigated, plant conditions are 

 changed. Thus it becomes impossible for some plants to 

 thrive in their former habitat, and possible for others to 

 grow where before they could not. The replacing of one 

 plant society by another is termed plant succession. When 

 a forest is cleared and the tract burned over, the plant 

 called fireweed appears in large numbers, even if a culti- 

 vated crop is planted. After a year or two the fireweed 

 gives way to a growth of blackberry and raspberry bushes, 

 which are later replaced by grasses and weeds of various 

 kinds. 



Another example of plant succession is seen in regions 

 covered by fresh lava from a volcano. At first nothing 

 grows. Probably bacteria and fungi appear before other 

 plants are noticed, but lichens are usually the first to be 

 observed. These die and decompose, and their remains, 

 together with bits of lava loosened by frost, wind, or 

 water, accumulate in depressions and form a soil in which 

 mosses can grow. The remains of the mosses add to the 

 organic matter in the slowly increasing soil, and, in the 

 course of time, ferns and larger plants can grow. The 

 last finally replace the mosses as they replaced the lichens. 



277. Summary of Our Interest in Plants. — Our first 

 interest in plants is economic, that is, we think of them 

 first in terms of their usefulness or harmfulness to us. 

 As every animal in the world is dependent directly or 

 indirectly upon plants for food, it becomes obvious to what 

 a degree we are benefited by the ability of plants to make 

 food out of the air and the soil. 



Man could live comfortably on what three plant families 

 furnish, — the grasses, which include all the cereal foods 

 and sugar; the pulse family, which furnishes most of our 

 vegetable nitrogen ; and the rose family, which includes the 



