424 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



Numerous cytological studies of such cases show that the mechani- 

 cal processes of the reduction division and the reorganization of 

 the daughter nuclei of the spores are abnormal. What appears 

 to be chromatin repulsion may be an expression of intra- and inter- 

 cellular incompatibilities that arise in a hybrid from differences 

 in the particular types of sex differentiation possessed by the 

 two parents. Even when the parents exhibit close agreement in 

 morphological differentiation a wide range of difference may 

 exist in regard to the relative time of development of sex organs 

 of which proterandry and proterogyny are marked types. Such 

 parental differences may first clash when the processes of sex 

 differentiation in a hybrid are set in motion, resulting in various 

 grades and types of impotence. This is evidently, in part at least, 

 what Tischler includes in his view (see especially '07) that the 

 impotence of hybrids is the result of differences in the "Ent- 

 wicklungsrichtung oderTendenz" of the parental germ cells 

 \\hich interfere with the normal ontogeny of the hybrid especially 

 during the critical time of the generative phase of the development. 



The production of such conditions as the result of inbreeding 

 or of continued self-fertilization is certainly very infrequent as 

 compared with the development in the progeny of wide cross-fertili- 

 zation. Such impotence on the part of plants was long considered 

 as an evidence of hybridity, and as a direct result of dissimilarity 

 in the cells involved, indicating the specific rank of the parents. 

 Jeffrey ('14) has quite recently emphasized in special reference 

 to the Oenotheras the familiar conception that such impotence is 

 a sign of hybridity. 



Davis ('156) has also pointed out the high degree of "zygotic 

 sterility" (embryo abortion) and pollen and ovule sterility (im- 

 potence) that prevails in numerous Oenotheras and has suggested 

 that a selective fertilization (physiological incompatibility) may 

 also be in operation. He speaks also of "gametic sterility," but 

 it is not clear whether he refers solely to impotence or to physio- 

 logical incompatiljility (as I have used the terms) or to both. 

 His general conception that in these cases similarities are elimi- 

 nated and dissimilarities are perpetuated is, however, purely an 

 assumption which he brings forward to assist in accounting for 

 the sporadic and irregular inheritance in the Oenotheras on more 

 nearly Mendelian assumptions. I lis investigations are highly 



