656 Journal of Agricultural Research voi. xii, No. 10 



to bear normal anthers, they are practically always borne on the primary 

 flowers, while the anthers produced later have a greater tendency toward 

 normal development. The tendency toward the production of stami- 

 nodia is much greater in the early spring than later. On the other hand, 

 pistil sterility is much more frequent on the later flowers of an inflores- 

 cence and when fruits set on wild staminate clones it is practically always 

 on the first flowers of a cluster which open. 



A second type of sterility often associated with the above type but due 

 to a different cause is that which results in aborted microspores and 

 pollen grains in otherwise normal anthers. Aborted pollen has been 

 shown to be present in relatively small amounts in pure species of Fragaria, 

 but appears often in large quantities in many of the cultivated varieties. 

 This type of abortion has long been recognized in hybrids, and recently 

 Jeffrey and his students have gone so far as to consider any plant bearing 

 ever 15 or 20 per cent of this type of pollen a hybrid. 



Selfed seedlings and F^ plants of crosses between varieties of cultivated 

 strawberries are so extremely variable for many factors that it seems 

 self-evident that they are of hybrid origin, and this is to a great extent 

 confirmed by what is known of the origin of the numerous cultivated 

 varieties, many of which are the result of variety crosses, while by far 

 the larger number are chance seedlings. It thus seems evident that 

 pollen abortion in the cultivated varieties is due to the same causes 

 which produce sterility in other hybrids. 



As would be expected, there are varying degrees of sterility resulting 

 from hybridization and varying degrees of irregularities in the stages 

 which lead up to the final abortion of pollen. There appear in the 

 literature numerous instances of abortion in both male and female repro- 

 ductive organs following irregular reduction divisions. The irregular 

 divisions, especially in the pollen mother cells, result in the production 

 of more than four cells of unequal size in the tetrad. These produce 

 microspores of varying sizes, few of which ever come to maturity. Gates 

 (77, p. 98) pointed out that most of the forms studied by Wille (44) 

 showing supernumerary cells in the tetrad are either hybrids or have 

 been under cultivation for some time and are open to the suspicion of 

 being hybrids. Other plants, some of which are knowTi to be hybrids 

 while others which have been cultivated as horticultural varieties and 

 are under suspicion as hybrids, have been studied in more detail by 

 various workers. Tischler (40, 41, 42), who has done much work with 

 plants of this type, finds that in hybrids of Ribes spp. and in the sterile 

 hybrid Mirahilis jalapa X M. iubiflora pollen degeneration usually takes 

 place following normal divisions. In the hybrids Poientilla tabernae- 

 montani X P. rubens, Syringa chinensis, and Bryonia alba X B. dioica, 

 and in three varieties of banana {Musa paradisiaca) having different 

 chromosome numbers, irregular divisions are common and are always 

 followed by much pollen abortion. In these banana varieties, the 



