290 PRACTICAL COURSE IN BOTANY 
killed off by fire or other cause, plants of an entirely different 
character 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 
Fia. 423.— A thicket of pines that has succeeded tal a iG 
a mixed growth of hard wood trees. . intiuence y 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 (60), 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- ——<—————— - : 
Fie. 424.— A successful invasion— Japanese 
ders are those of the honeysuckle covering the banks of a ravine and 
Russian thistle in the climbing over shrubs and tree tops. 
northwestern states of the Union, and the “ bitterweed ”’ 
( Helenium tenuifolium) that has almost driven out the hardy 
li 
