126 



in spite of the unaccustomed shade. ReHc species are frequently 

 very tenacious of Hfe and will struggle for a long time before they 

 succumb. They are usually able to reproduce vegetatively. 



The status of invaders is only a little different. The invader 

 must be able to cope advantageously with the new conditions 

 from the beginning, in order to maintain its life. This may 

 induce extreme variation, which is not mutation because there 

 are usually all stages of transition from the usual form to the 

 new form. Furthermore, when the succeeding association be- 

 comes dominant in an area in which the extreme form originally 

 developed, only usual forms occur. Conclusive evidence is at 

 hand to show that the vegetative structures of a perennial plant, 



Fig. I. Pelalosleiniim purpureum f. arenarium growing among the bunches of 

 Andropogon scoparius in the bunchgrass prairie. Waukegan, HHnois. 



acting as an invader, may be strikingly different from the struc- 

 tures of the same plant after the successful invasion of the 



