336 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



well be limited in its application to those causes existing in the 

 plants themselves which prevent fertilization in and between 

 plants with normal reproductive organs and gametes. 



In such cases we may distinguish two quite distinct types of 

 incompatibility: 



1. Anatomical incompatibility due to more or less marked 

 structural differences as: 



(a) Obvious specific differences in structure, such as the com- 



parative length of styles in two such species as Mirabilis 

 Jalapa and M. longiflora. 



(b) Structural differences within a species, such as dimorphism 



or trimorphism as seen in Linum and Lythnim (which 

 may be correlated with true physiological incompati- 

 bility). 



(c) Many structural adaptations that prevent self-fertilization 



and secure cross-pollination (hercogamy). 

 The investigations reported in this paper do not concern this 

 class of incompatibilities, and a further discussion or classification 

 of the almost innumerable floral modifications involved will not 

 be attempted here. 



2. Physiological incompatibility. — When complete potency exists 

 and morphological compatibility is perfect or morphological incom- 

 patibilities and hercogamy are eliminated by proper pollination, 

 sterility may still be present as a result of physiological conditions 

 which make impossible the union of male and female gametes. 



In the flowering plants this incompatibility may make itself 

 manifest in the growth of the pollen-tube through the tissues of 

 the stigma and style, or in the more intricate processes of fusion 

 of the gametes. So far as known, such physiological incompati- 

 bility, at least in the flowering plants, may be due to conditions 

 in either the sporophyte or gametophyte. In lower forms of 

 plant life and especially in those forms with well-marked alterna- 

 tion of generations with an autonomous existence of a monoecious 

 gametophyte or of male and female gametophytes, such incom- 

 patibilities would be more strictly gametophytic. Even in the 

 flowering plants, however, this sort of incompatibility is, it would 

 seem, most intimately concerned with the fundamental processes 

 involved in fertilization, and presents problems whose solution 

 will unquestionably throw nuich light on the nature of sexual 

 fusions in general. 



