12 



PLANT BIOLOGY 



cleared area. Here and there bushes gain a foothold. 

 Young trees come up ; in time these shade the bushes and 

 gain the mastery. Sometimes the area grows to poplars 

 or birches, and people wonder why the original forest trees 

 do not return ; but these forest trees may be growing unob- 

 served here and there in the tangle, and in the slow pro- 

 cesses of time the poplars perish for they are short-lived 

 and the original forest may be replaced. Whether one 

 kind of forest or another returns will depend partly on the 

 kinds that are most seedful in that vicinity and which, 

 therefore, have sown themselves most profusely. Much 

 depends, also, on the kind of undergrowth that first springs 

 up, for some young trees can endure more or less shade 

 than others. 



Some plants associate. They grow together. This is 

 possible largely because they diverge or differ in charac- 

 ter. Plants asso- 

 ciate in two ways : 

 by growing side by 

 side ; by growing 

 above or beneath. 

 In sparsely popu- 

 lated societies, 

 plants may grow 

 alongside each 

 other. In most 

 cases, however, 

 there is overgrowth 

 and undergrowth : 

 one kind grows beneath another. Plants that have be- 

 come adapted to shade are usually undergrowths. In a cat- 

 tail swamp, grasses and other narrow-leaved plants grow 

 in the bottom, but they are usually unseen by the casual 



FIG. io. OVERGROWTH AND UNDERGROWTH IN 

 THREE SERIES, trees, bushes, grass. 



