290 



PRACTICAL COURSE IN BOTANY 



Fig. 423. — A thicket of i.iii. 



a mixed growth of hard wood trees. 



killed off by fire or other cause, plants of an entirely different 

 cliaracter will nearly always spring up to occupy its place. A 



forest of pine, for in- 

 stance, is rarely fol- 

 lowed by conifers, 

 but by a growth of 

 hardwood trees, and 

 vice versa — nature 

 thus giving an im- 

 pressive example as 

 to the effectiveness 

 of a rotation of crops. 

 Succession may be 

 influenced by a va- 

 riety of causes. Two 

 of the most efficient are : (1) the exhaustion of the soil by the 

 long-continued growth of one formation (CO), thus causing 

 a deficiency of mineral material suited for the support of 

 plants of that kind ; (2) the migration of new species into 

 the denuded territory where those which have different re- 

 quirements as to min- 

 eral nutrients from the 

 former inhabitants will, 

 other things being equal, 

 have the best chance to 

 succeed. 



328. Invasion. — A 

 rapid and widespread 

 occupation of any terri- 

 tory by a new species is 

 called an invasion. No- 

 table examples of inva- 

 ders are those of the 

 Russian thistle in the 

 northwestern states of 



Fig. 424. — A successful invasion — Japanese 

 honeysuckle covering the banks of a ravine and 

 climbing over shrubs and tree tops. 



the Union, and the '' bitterweed " 

 ( Helenium tenuifolium) that has almost driven out the hardy 



