70 THE BIOLOGICAL PROBLEM OF TO-DA Y 



union will not take place between plants most 

 near akin in classification, most alike in external 

 characters ; while it will occur between plants 

 most different in outward aspect and belonging to 

 different genera or even families. In other words, 

 external characters give no certain index to the 

 degree of vegetative affinity or of sexual affinity 

 between two kinds of plants. 



As an example of this, Vochting, in his treatise 

 upon transplantation of plant-tissues, takes the 

 tribes of pear-trees. Grafting between these and 

 apple - trees takes place only with difficulty, 

 although the apple is a close kinsman and belongs 

 to the same genus. On the other hand, most of 

 them graft easily upon the quince, although that 

 belongs to a different genus. In this case, also, 

 there is no sexual affinity between the pollen and 

 the ova. Hybrids are not formed between the pear 

 and the apple. 



It seems probable to me, although as yet I can- 

 not get complete proof of it, that sexual and vege- 

 tative affinity, that is to say, the relationship 

 between the egg- cell and the pollen of two species, 

 and the relation between twig and stem, depend 

 upon the same intrinsic qualities of that elementary 

 organism the cell. 



Vochting distinguishes as harmonic or dis- 

 harmonic the modes of union between twig and 

 stem, according to whether or no they reach the 

 formation of functional unity. Among cases of 

 disharmony there are several interesting grada- 

 tions. Generally speaking, in the case of plants 



