226 LUTHKK BURBAVK 



in exceptuxui ctiet it is pos«tbk to effect the 

 graft where efforts at crossing have prove<l 

 futile. Such was the case, for example, with my 

 grafted tomato and potato vine. But, in general, 

 tlie plant that refuses to mate with another plant 

 refuses also to accept its stem as a companion 

 organism when grafted or budded. 



However carefully the grafting experiment 

 may be performed in such a case, the uncon- 

 geniality between stock and don is soon made 

 manifest. The surfaces do not unite; or if union 

 takes place there is but slight tendency to grow; 

 or the don does not thrive, and is presently 

 blighted. 



There are all gradations — from actual {M)ison> 

 ing in which there is no tendency whatever to 

 unite, to a partial or even temporarily complete 

 union, followed by separation even after years of 

 growth — according to the degree of antagonism. 



These chemical and mechanical antagonisms 

 between the tissues of the plants themselves 

 ufTord the surest evidence of the long period of 

 time during which the two spedes have lived un- 

 der more or less divergent conditions, and have 

 been occupied, each in its own way, in the devel- 

 opment of new characteristics. Yet that such 

 intimate differences of constitution should obtain 

 l)etween species that show many outward points 



