^0.493] THE I'll EXOGAMOI'S PAR A Si TE S 



leading an honest life in the soil, but so firmly is their 

 predatory habit established by heredity that they never do 

 so. Their seeds will germinate successfully only on the 

 bark of living trees, and their embryos, although struc- 

 turally perfect, are evidently unable to develop in the soil. 

 When germination of the seed begins the radicle pierces 

 through the dry bark of the host as if driven by some ex- 

 traneous force; and it sometimes enters the bark of a 

 branch from its under side, showing that gravity is not 

 that impelling force. It lifts the strong bark by its in- 

 crement beneath, and sends the sinkers into the growing 

 layers. The cells of those layers and the cells of the sink- 

 ers become vitally commingled much as do the somatic 

 cells of the scion and stock in common grafting, but not 

 quite so harmoniously. This parasitic root-grafting is 

 remarkable because the parasites and their usual hosts 

 differ from each other in botanical relationship far more 

 than do any scions and stocks that can be artificially 

 grafted with success. 



Because the mistletoes obtain full nourishment from 

 their hosts their parasitism is complete, and yet, unlike 

 other completely parasitic phenogams, they produce elilo- 



by the mistletoes is apparently due to the fact that they 



notwithstanding their parasitismi. While the mistletoes 

 have retained more of the structure and functions of nor- 

 mal plants than have other completely parasitic pheno- 

 gams, their draft upon the vitality of their hosts is great, 

 and it doubtless would be more apparent if the latter were 



Group III 



having the embryo differentiated into 

 iele and plumule, germinate upon the grot, 

 duce plants which begin to grow in the 

 d manner. By their earlier roots they a 

 •asitic after the manner of group I, bu 



the whole plant b 

 »dily by burrowing 



