48 BULLETIN 1134, U. S, DEPARTMENT OF AGRICULTURE. 



CONCLUSIONS REGARDING SELECTIVE FERTILIZATION. 



The occurrence of selective fertilization in favor of the Pima 

 pollen when this and upland pollen are present in approximately 

 equal quantity on the Pima stigmas is interesting in view of the 

 evidence given on preceding pages that the upland pollen, when 

 applied separately, germinates at least equally well and penetrates 

 the ovary with a rapidity equal to that of the Pima pollen. To ac- 

 count for selective fertilization it seems necessary, therefore, to as- 

 sume that there is a partial inhibition of the germination or subse- 

 quent development of the foreign pollen when pollen of the same 

 variety is also present on the same stigmatic area. 



Darwin (13, pp. 391, 392), although concluding that pollen from 

 another plant of the same variety or from another variety of the 

 same species " is often or generally prepotent over that from the 

 same flower," obtained evidence that the reverse is true in the case 

 of different (but presumably compatible) species. 



If pollen from a distinct species be placed on the stigma of a castrated 

 flower, and then after several hours pollen from the same species be placed on 

 the stigma, the effects of the former are wholly obliterated, excepting in some 

 rare cases. 



The results here described are not in accordance with Darwin's 

 findings, for in the experiments in which foreign pollen was applied 

 to unemasculated Pima flowers, pollen " from another variety of the 

 same species" (Gila) was not prepotent over the self pollen, and 

 pollen of a distinct species (Acala) fertilized a considerable per- 

 centage of the ovules although applied several hours after self- 

 pollination had begun. Acala pollen was also able to effect fer- 

 tilization when applied simultaneously with self pollen to Pima 

 flowers which had been emasculated. 



Interesting results as to pollen competition in maize have been 

 reported recently by Jones (2h, 25, 26), who found that in the great 

 majority of the combinations tested more of the ovules were fertilized 

 by self pollen than by foreign pollen when the two kinds were in 

 direct competition and that the " handicap placed upon the foreign 

 pollen is proportional to the germinal unlikeness " (25, p. 283) , not- 

 withstanding the fact that the weight of the seeds resulting from 

 cross-fertilization and the vigor of the plants grown from such seeds 

 increased with the wideness of the cross. " In proportion as the 

 cross-fertilization benefits the immediate progeny in its development 

 the less effective is that pollen in accomplishing the union " (25, 

 p. 271). 



The results of the experiments with cotton described in this bulle- 

 tin agree only partially with those obtained by Jones with maize. 

 It is true that on the stigmas of emasculated flowers of Egyptian 

 cotton, pollen of the same variety was found to be prepotent over 

 pollen of a very distinct type (upland cotton) when the two kinds 

 were in direct competition. On the other hand, when applied 

 to the stigmas of unemasculated Pima flowers, pollen of another 

 variety of the same type (Gila, Egyptian) was not more successful 

 than pollen of another type (upland) in competing with the self 

 pollen. Yet heterosis is far more pronounced in the cross Pima 

 X upland than in the cross Pima X Gila. Furthermore, McLachlan's 

 results (Table 25) indicate that on the stigmas of upland cottons 



