﻿FERTILIZATION IN PIMA COTTON. 27 



pollen-bearing flowers as early as 8 a. m., but it is believed that in 

 every case the quantity applied was sufficient to have insured 

 thorough pollination under normal conditions. 



The resulting data, as given in Table 14, show that the stigmas 

 had retained their receptivity in only a few of the flowers the pollin- 

 ation of which was deferred until the second day. Those which re- 

 mained receptive, however, were as well fertilized as the flowers 

 pollinated on the day of anthesis. 16 



Table 14, — Results obtained by pollinating Pima cotton flowers on the day of 

 anthesis and on the following day, showing the percentage of bolls matured 

 and the mean number of seeds per boll. 



Time of pollination. 



1 p. m. of the day of anthesis 



8 a. m. of the day following anthesis. 



Flowers 

 treated. 



Percentage 



of bolls 



matured. 



90.0±2.86 

 8.3±2.70 



Mean num- 

 ber of seeds 

 per boll. 



13. 7 ±0.32 

 i 13. 2± 3. 05 



i Omitting one of the four bolls matured which contained only a single seed, the mean number of seeds 

 n the remaining bolls was 17. 3± 1.79. 



LOCUS OF POLLEN DEPOSITION IN RELATION TO SELF- 

 FERTILIZATION AND CROSS-FERTILIZATION. 



Numerous examinations of flowers of Pima cotton when the corolla 

 is still closed or is just beginning to expand have shown that if the 

 flower is opened with sufficient care pollen is rarely found upon the 

 stigmas at a height of more than 2 millimeters above the upper- 

 most anthers, to which height the grains probably can be thrown 

 by automatic discharge. A similar condition is found in flowers 

 which have been bagged to prevent cross-pollination, even when the 

 corolla has opened to a degree which in unbagged flowers would 

 permit the ready access of pollen-carrying insects. As about 2.5 

 millimeters of the length of the stigmas is surrounded by the upper- 

 most stamens, the portion upon which self pollen is automatically 

 discharged therefore does not exceed, as a rule, 5 millimeters, or 

 about half the average total length of the stigmas. The girdle of 

 self-pollen grains deposited upon the stigmas at the height of the 

 uppermost anthers, hence just above the summit of the staminal 

 sheath, is shown in Plate VI. It has been pointed out that this zone, 

 on which is lodged the great bulk of the automatically discharged 

 self pollen, is so closely screened by the uppermost stamens with 

 their short filaments as to be practically inaccessible to foreign 

 pollen. 



On the other hand, in flowers which open under natural conditions 

 in a locality like Sacaton where pollinating insects are abundant, 

 the entire surface of the stigmas usually becomes covered with pollen 

 during the morning (PI. VII). There can be little doubt that in 

 this type of cotton, pollen which is found upon the upper half of the 



18 In Pima cotton the style normally drops off within 36 hours after the beginning of 

 anthesis. ■ A much longer duration of receptivity of the stigmas has been noted in other 

 plants. Dorsey states (/J, p. 116) that in the plum the " stigma remains receptive for a 

 maximum period of about one week." It is reported by Anthony and Harlan (5, p. 528 > 

 that the stigmas of barley retained their receptivity during five days following emas- 

 culation, although the degree of fertilization effected diminished rapidly after the 

 second day. 



