i 9 22] STOUT— STERILITY 113 



many were visited daily), and pollen from fully dehiscent stamens 

 in liberal amounts was placed on pistils of all freshly opened 

 flowers. After the cyclic modification of self-compatibility was 

 recognized, frequent cross-pollinations were made to test the 

 functional power of pistils and stamens during the periods of self- 

 incompatibility preceding and following the period of mid-bloom. 

 The potency of the pollen has been studied by germination tests, 

 and a cytological study of the phenomena of pollen tube growth 

 and fertilization is under way. 



Sterility in Brassica pekinensis and B. chinensis 



Three distinct and quite different types of sterility are in 

 evidence during the period of bloom in both these species. 



I. One type is to be classed in general with impotence (Stout 

 12), but here two very distinct types of impotence may be observed. 

 These may be described as (1) flower abortion of the first flowers, 

 and (2) arrested development of the last flowers that start to form. 



II. In some plants of both species, axial proliferations develop 

 from the pistils of many flowers, and the pistils of such flowers are 

 functionless in respect to fruit production. 



III. Among the flowers that open fully and are capable of 

 functioning in certain relations, various grades of incompatibilities 

 are in evidence, and self-compatibility whenever present is most 

 strong during the period of mid-bloom. 



I. Impotence 



(1) Flower abortion. — Frequently in Brassica pekinensis the 

 first flowers on the main stalk and often also the first flowers on 

 laterals are completely aborted. The flower buds remain small and 

 do not open, but soon become dead and black. A rather character- 

 istic case of flower abortion is shown in fig. 1, which is of a pot 

 grown plant of B. pekinensis blooming in February. Nearly forty 

 of the first flowers of the main branch aborted, after which the 

 flowers were completely formed, normal in appearance, and fully 

 capable of functioning in certain fertilizations. The uppermost 

 lateral branch coming into bloom later than the main axis had only 

 five flowers that aborted, and the next lower lateral had no aborted 



