22 ANNALS NEW YORK ACADEMY OF SCIENCES 



Self-incompatibility is so strongly developed in the species that at least 

 many plants of seedling origin are the result of cross-pollination be- 

 tween plants that differ in coloration. The Europa clou is evidently 

 heterozygous for color of flowers, but there is no very conclusive evidence 



that it is a hybrid between two distinct species. Among the wild plants 

 obtained from China and grown at The N. Y. Botanical Garden there 

 have been some that very closely resemble the Europa don, especially 

 in respect to the color and shape of the flowers. 



The abnormalities seen during sporogenesis in plants of the Europa 

 clou closely resemble those common in hybrids, especially in the non- 

 pairing, the nondisjuction, the irregular distribution of the chromosomes 

 and the formation of extra numbers of pollen grains. The omission of 

 the reduction division and the multiplication of chromosomes during 

 the divisions are also quite similar to irregularities observed in certain 

 known hybrids. 



3. A rather extensive literature has recently appeared concerning the 

 sterility of polyploids. It appears that the odd numbers of chromosomes 

 in triploids, pentaploids and various aneuploids promote irregularities in 

 meiosis which result in the abortion of spores. (See especially Blakeslee 

 and Cartledge. l!t^7.) But here, as in certain hybrids, abortion may 

 also develop in forms having sets of chromosomes of even number, which. 

 however, seem to be "unbalanced" in the relations of meiosis. 



Sterility in polyploids is undoubtedly a chromosome phenomenon com- 

 ing to expression during meiosis. As a general rule, it develops in quite 

 the same degree for both microspores and macrospores and hence is 

 quite similar to sterility from hybridity. In certain cases, as in Fragaria 

 (Longley, 1 i • '<? 7 ) , dioecious plants are found to be polyploid, but, a- in 

 such cases the chromosomal complex of a plant is fully potent in one 

 sex, it would seem that such one-sided abortion is not merely due to the 

 number of chromosomes involved. 



Plants of the Europa clon are clearly not polyploid. In the BQmatic 

 tissues the normal number of chromosomes is L2, which, it now appears, 

 is the basic diploid number for the genus. In the chromosome irregular- 

 ities there is an increase in the number of chromatin units, bo that many 

 of the cells become polyploid and aneuploid, and in this reaped the 

 abortion of spores may be similar to that which occurs in plants whose 

 somatic cells are uniformly polyploid. 



1. Abortion of spores is the rule for plants whuh produce seeds by 

 parthenogenesis or by apogamy. The cytologicaJ studies of such plants 



