90 Self-Incompatibility in Hermaphrodite Plants 
different results if reciprocal matings were made at different times 
during the period of bloom. 
These cases of difference in the compatibility of reciprocal matings 
point very clearly to a source of fluctuating variability by no means 
sufficiently recognized, and that is the complexity of the fertilization 
processes as revealed by cytological study. It may well make a difference 
which parent furnishes the male and which the female when we realize 
the possibility of variation offered in the complex processes of cyto- 
plasmic fusion, nuclear fusion, pairing of homologous chromosomes and 
the arrangement of the pairs with reference to each other, to the nucleus, 
and to the cell as a whole. 
Sirks recognizes that the conditions in V. phoeniceum indicate that 
“ auto-incompatibilité ” is a phenomenon of physiological sex differentia- 
tion which cannot be ascribed to fixed genotypic constitution nor to the 
inheritance of specific line stuffs. He suggests that the poor growth of 
pollen tubes very generally observed in cases of incompatibility may 
involve osmotropism. 
Evidence that self-sterility is somewhat exclusive of, and more 
specific than, cross-sterility is given by Sutton (1918). The evidence 
of a wide range of variation in self-compatibility among the various 
cultivated (propagated asexually) varieties of plum, of cherry and of 
’ apple has been confirmed by studies of varieties commonly grown in 
England. Self-fertile, partially self-fertile and self-sterile varieties are 
reported in each of these quite as have been found by other investigators. 
Sutton finds, however, no evidence of cross-incompatibility between 
varieties except in crosses between the Jefferson variety and the 
group of varieties of plums. She concludes that otherwise inter-varietal 
cross-fertility under field conditions depends solely on the production of 
plenty of pollen and on simultaneous blooming. The varieties of the 
cherry reported cross-incompatible by Gardner (1913) were not studied 
by Sutton. 
Sutton’s data show clearly that a distinction is to be made between 
fruitfulness involving only parthenocarpy, and fruitfulness with and 
dependent upon seed reproduction. In the case of the navel orange the 
ort and quality of the carpels are quite independent of any process of 
fertilization, or even of parthenogenetic production of seed; the pistils 
however contain normal ovules, and when pollination occurs fertilization 
results and seeds are formed even in the accessory carpels (Shamel, 
1918). Sutton finds in plums and cherries that well formed fruits 
