No. 463.] STUDIES ON PLANT CELL. VI. 467 



They are accompaniments of sexual processes which may always 

 be expected but nevertheless are not the essential characteris- 

 tics. The essence of the sexual act (fertilization) is the union of 

 germ plasm with such possibilities of new developments as come 

 from the inheritance of mixed characters from two lines of ances- 

 try. And the more diverse and complex are the characters of 

 the parents the more conspicuous are the essential features of 

 the sexual act. Among lowly organisms and in simpler types of 

 sexual processes the growth stimulus becomes exaggerated in 

 our attention because the features of heredity are not so promi- 

 nent as in the higher forms. But in the higher groups the 

 varied characters of offspring express clearly the subtle factors 

 concerned with the mingling of diverse germ plasm in the proc- 

 ess of fertilization and the growth stimulus recedes into the 

 background. 



For these reasons it seems to me that the term fertilization 

 should only be used for the mingling of germ plasm with the 

 possibilities of new combinations in the potentialities of the 

 resulting sexually formed cell and that the growth stimulus should 

 be treated as an accompaniment but quite apart from the essen- 

 tials of the sexual act. And for these reasons I was careful to 

 include in Section IV under the caption " Sexual Cell Unions 

 and Nuclear Fusions " only illustrations in which the sexual 

 nature of the phenomena was clearly shown by applying a mor- 

 phological or phylogenetic test to the elements concerned in the 

 process of cell fusion. The phylogenetic test seems to me the 

 only sure way of determining the sexual nature of the members 

 of a cell fusion and there are very few cases in which there can 

 be any hesitation in deciding whether or not such elements are 

 morphologically gametes. 



I included under "Asexual Cell Unions and Nuclear Fusions" 

 in Section IV a number of cases in which the sexual nature of 

 the act is under dispute for the reason that none of these satisfy 

 the phylogenetic test. It is perfectly clear that the growth stim- 

 ulus is a conspicuous feature of these cell and nuclear fusions 

 and that in this feature they resemble sexual processes but this 

 does not, to my mind, make them acts of fertilization or the 

 equivalent of sexual processes. The union of sporidia in the 



