Mar. II. 1918 Sterility in the Strawberry 657 



origins of which are unknown but which dififer cytologically in having 

 8, 16, and 24 chromosomes as the reduced number, it is significant that 

 the most frequent irregularities in cell division during reduction, and 

 most complete pollen sterility occurs in the two varieties having the 

 greater chromosome numbers. Thus, pollen abortion may or may not 

 be the result of irregularities at reduction division, but is apparently 

 related to hybridity and is associated with heterozygosity. 



After working with several hybrid plants showing both normal divi- 

 sions and irregularities during reduction and formation of tetrads, 

 Tischler (41, p. 144) concluded that — 



Die Sterilitat bei Hybriden hangt nicht von irgendwelcher Chromatin repulsion ab. 



He concluded further that irregularities during tetrad division can not 

 be considered as characteristic only of hybrids. He thought that ste- 

 rility of hybrids was due to the coming together of two sex cells which 

 did not contain identical developmental tendencies and that these were 

 expressed at the critical time of the formation of the reproductive organs. 

 Actual abortion of the grains he thought was due to insufficiency of cyto- 

 plasm in the enlarged microspores. 



One of the most striking cases of sterility following hybridization, the 

 cytological details of which have been worked out, is that of the hybrid 

 Drosera longifolia X D. rotundifolia, reported by Rosenberg (jj). The 

 striking feature of this hybrid is that it is between parents having differ- 

 ent chromosome numbers, the diploid number of D. longifolia being 40, 

 while that of D. rotundifolia is only 20. As a consequence the hybrid 

 contains 30. At reduction division in both the megaspore mother cells 

 and pollen mother cells there appeared 10 pairs of chromosomes and 10 

 single ones, the pairs supposedly being made of the 10 D. rotundifolia 

 chromosomes paired with lo from the D. longifolia parent, while the 10 

 single chromosomes were the remaining 10 D. longifolia chromosomes. 

 Reduction division resulted in the separation of the paired ones, these 

 being drawn regularly to opposite poles. The unpaired chromosomes, 

 on the other hand, were either drawn to one or the other pole or were 

 left in the cytoplasm to form another small nucleus. The homoeotypic 

 divisions took place normally. Following the organization of the micro- 

 spores within the tetrad, many proceeded to increase in size; in some, 

 division of the microspore nucleus proceeded normally, and then in 

 practically all cases abortion of the pollen took place. Following tetrad 

 formation in the feinale reproductive organs, three of the tetrads usually 

 aborted, as is common, while the other proceeded to form the egg sac. 

 Egg-sac formation was carried to various stages, but it was only very 

 rarely that a perfect egg sac, capable of further development, was formed. 

 Rosenberg concluded {p. 39) that because of the fact that the micro- 

 spore division was able to proceed normally, the degeneration of the 

 pollen grains was not the result of the irregular distribution of chromo- 



