i 9 22] STOUT— STERILITY 127 



alternation of vegetative and reproductive vigor. Flower abortion 

 occurs normally as a transitional stage between the formation of 

 green leaves and the production of functional sporophylls. Those 

 plants which exhibit flower abortion are not able to pass directly 

 from producing green leaves or leaves with branches at the nodes to 

 the production of flowers, and flower abortion occurs as a transitional 

 stage. The abortion of flowers appears in the phase where vege- 

 tative vigor is waning, but before reproductive vigor is fully in 

 evidence. There is also a marked agreement among the various 

 branches of a plant as to the grade of development reached at any 

 one date of blooming (figs. 1-3), which indicates a definite relation 

 between the condition causing flower abortion (and also normal 

 flower formation) and a condition of the plant as a whole. These 

 phenomena, therefore, have many aspects characteristic of physio- 

 logical correlation. 



The arrested development of flowers at the ends of branches 

 after a period of vigorous blooming of the plant is obviously due 

 to an extreme waning of vigor and the approaching death of the plant 

 as a whole, and of course is a phenomenon prevalent in all sorts of 

 plants. Axial proliferation from the pistils is to be considered as a 

 resumption of vegetative growth after the differentiation of the 

 pistils has been accomplished. 



Turning to the functional relations of the sex organs in these 

 two species of Brassica, at least to the compatibility in self- 

 fertilization, it is seen that they also exhibit a periodicity on their 

 occurrence which forms a very definite cycle. A total of 718 plants 

 that were self-compatible to some degree have now been observed 

 in these two species and in hybrids between them. With the excep- 

 tion of a few individuals in which pods developed irregularly, the 

 maximum of self-compatibility was reached during the mid-bloom 

 of the plant (figs. 2,3, 5-7). Previous to and following this period, 

 the self-compatibility grades into complete self-incompatibility or 

 into a much weaker grade of self-compatibility. Furthermore, the 

 climax of self-compatibility is remarkably coordinated among the 

 different branches according to the time of blooming quite as is 

 the earlier development of flower abortion. 



The remarkably uniform development of self-compatibility 

 during the time of mid-bloom in Brassica chinensis, B. pekinensis, 



