62 BULLETIN 1134, U. S. DEPARTMENT OE AGRICULTURE. . 



of the ovules being fertilized by pollen from other plants of the same 

 variety, but even when single plants of Egyptian or of upland cotton 

 were scattered through a field of the other type and the conditions 

 were so controlled as to make it highly probably that all seeds not 

 derived from self-fertilization had resulted from cross-fertilization 

 by pollen of the other type, the maximum number of vicinists in popu- 

 lations grown from these plants did not exceed 35 per cent. The 

 average percentage of cross-fertilized ovules under these conditions 

 was 12 in Pima (Egyptian) and 28 in Acala (upland). 



The cotton flower is large and showy, and the reproductive organs 

 are readily accessible to pollen-carrying insects attracted in large 

 numbers by the abundant production of nectar and of pollen. One 

 might therefore expect cross-fertilization to predominate over self- 

 fertilization. The structure of the flower, however, is well adapted 

 not only to cross-pollination by insects but to automatic self-pollina- 

 tion from the uppermost anthers. These are in contact with the base 

 of the stigmas in Egyptian cotton and with practically the whole 

 stigmatic surface in many varieties of upland cotton. 



In the Egyptian type only the extrastaminal portion of the stig- 

 mas is accessible to foreign pollen, the basal region being effectively 

 screened by the upper anthers. Experimental evidence was obtained 

 that in the Pima variety cross-fertilization does not occur when the 

 extrastaminal portion of the stigmas is excised. In upland cottons, 

 on the other hand, the entire length of the stigmas is accessible to 

 foreign pollen, and excision did not prevent cross-fertilization. 



The corolla of Pima cotton commences to open about an hour after 

 sunrise and continues to expand during the next four hours, the 

 maximum aperture usually being attained about 10.30 a. m. In 

 upland cottons an aperture first appears at about the same time as in 

 Pima, but the subsequent expansion is more rapid. In both types 

 the flower is of brief duration, the petals beginning to wilt and the 

 pistils to lose their turgor before sunset on the day of anthesis. 



The opening of the anthers and release of pollen in Pima cotton 

 begin somewhat earlier than the expansion of the corolla. In upland 

 cottons, as a rule, corolla and anthers commence to open almost simul- 

 taneously, although not infrequently the upland anthers are found 

 to be still closed when the corolla is open sufficiently to admit insects. 

 Even in Pima cotton, however, very little self pollen is deposited au- 

 tomatically upon the stigmas before the petals have begun to unfold. 



The viability of the pollen of Pima .(Egyptian) and of Durango 

 (upland) cotton, as measured by the rate of ejection of the cell con- 

 tents when the grains are immersed in a sugar solution, is low during 

 the early hours of the morning, begins to increase rapidly between 

 8 and 9 a. m., and shows a gradual decline after midday. Inclosure 

 of the flower in a paper bag greatly prolongs the period of ^ viability. 



Pollen extracted from the anthers and applied to the stigmas the 

 evening before anthesis showed a very limited ability to effect fertili- 

 zation. On the other hand, the viability of pollen collected from 

 bagged flowers 26 hours after the beginning of anthesis, as indicated 

 by the ability to effect fertilization, was still considerable, although 

 greatly inferior to that of fresh pollen. 



The style of the Pima cotton flower normally is lost by abscission 

 within 36 hours after the beginning of anthesis. In bagged flowers it 

 was found that fertilization could be effected 24 hours after the be- 



