16 



BULLETIN 1134, U. S. DEPARTMENT OF AGRICULTURE. 



Table 8. — First-generation hybrids in progenies groion at Sacaton, Ariz., in 

 1920, from seed produced by flowers the stigmas of which had been excised 

 in the bud at the level of the uppermost stamens. 



Variety of which seed was produced by excised flowers. 



Total 

 plants. 



Fi hybrids. 



Number. Per cent. 



Pima (Egyptian).. 

 Lone Star (upland) 

 Holdon (upland) . . 



172 

 19 

 54 







47.3±7.7 

 1.8±1.2 



Reference to Table 1 shows that a population grown from seeds 

 produced by unmutilated naturally pollinated flowers of the Pima 

 plants used in the present experiment contained approximately 3 per 

 cent of hybrids, while the data given in Table 8 show that excision 

 of the extrastaminal portion of the stigmas had prevented cross- 

 fertilization of the Pima flowers. This might have been explained 

 on the ground that the removal of a portion of the corolla in the 

 process of excising the stigmas had rendered the flowers unattractive 

 to insects, were it not for the fact that cross- fertilization occurred in 

 similarly treated flowers of the two upland varieties. It seems 

 probable, therefore, that in Pima cotton the basal portion of the 

 stigmas is effectively screened by the surrounding stamens against 

 the access of foreign pollen, whereas in upland cottons no portion of 

 the stigmas is inaccessible to such pollen. 



An anomalous result of the experiment is the much higher per- 

 centage of hybrids in the population derived from treated flowers 

 of the Lone Star variety than in the corresponding Holdon popu- 

 lation, whereas in the vicinism experiment, involving untreated 

 naturally pollinated flowers on the same plants (Table 1), Holdon 

 yielded more than twice as high a percentage of hybrids as Lone 

 Star, and the difference was five times its probable error. 



ONTOGENY OF THE FLOWER IN RELATION TO POLLINATION. 



Only the last stages in the ontogeny of the flower are of importance 

 in relation to pollination. The time and rate of opening of corolla 

 and anthers, the condition of the pollen from shortly before the 

 flower opens until it has begun to wilt, and the duration of recep- 

 tivity of the stigmas will be considered in this connection. The 

 final stages in the development of the flower are illustrated in 

 Plate III. 



OPENING OF THE COROLLA. 



The bud remains tightly closed during the night preceding anthe- 

 sis, the petals beginning to separate at the apex usually about an hour 

 after sunrise. During the next hour or so the opening of the corolla 

 proceeds slowly, but thereafter the aperture widens rapidly, with 

 a slowing down in the rate shortly before the maximum diameter is 

 attained. Accurate data as to the rate of opening were obtained 

 from an experiment performed in 1919. During three periods of 

 five days each (July 29 to August 2, August 18 to August 22, and 

 September 11 to September 15) 20 flowers were tagged daily, and 

 the aperture of the corolla was measured at half -hour intervals. The 

 mean diameter of the aperture for each half hour is stated in Table 



