FERTILIZATION IN PIMA COTTON. 



55 



ers which receive pollen only on the lower half of the stigmas is due 

 primarily to this region being relatively unfavorable to the germina- 

 tion or development of the pollen. 



BOLL SHEDDING IN RELATION TO POLLINATION AND 

 FERTILIZATION. 



A general discussion of the phenomenon of boll shedding would be 

 out of place in this bulletin. The physiological aspects of the sub- 

 ject have been treated by Balls (8, pp. 65-75), Lloyd (37 and 38), 

 Ewing (17, pp. 21-37), and King (31, pp. 11-21). It may be well, 

 however, to consider briefly such data as have been obtained at 

 Sacaton, Ariz., concerning the relation between the shedding of bolls 

 and fertilization. The observed percentages of boll shedding in 

 Pima cotton at Sacaton, as recorded in Table 34, are in most cases 

 much lower than have been reported by investigators of sea-island 

 and upland cottons at other localities (20, p. 195; 17, p. 21; lfi). In 

 Pima cotton grown at Phoenix, Ariz., 40 miles distant from Sacaton, 

 King (31, p. 19) recorded instances of bolls shed in 1919 ranging 

 from 16.7 to 26.5 per cent. 



Table 34. — Boll shedding from flowers of Pima cotton naturally pollinated in 

 different years at Sacaton, Ariz. 



Year of experiment . 



Flowers 

 recorded. 



Flowers 

 which 

 failed to 

 develop 

 bolls (per 

 cent). 



Year of experiment. 



Flowers 

 recorded. 



Flowers 



which 



failed to 



develop 



bolls (per 



cent). 



1919.. 



200 



13. Oil. 6 

 16.0±3.0 

 3.0±1.2 

 8.1±1.8 



1920... 



377 



4.4±0.7 



1919 ! 69 



1921 



999 



10.3± .6 



1920 98 



1921 



» 4,931 



25. 1± .4 



1920... 



99 











1 Flowers tagged daily during the period from July 11 to September 15. 



It is unlikely that deficient pollination is a frequent cause of boll 

 shedding in Pima cotton. Meade (40), working with upland 

 varieties, found that " bolls failed to set unless at least 25 grains of 

 pollen were applied to the stigmas ; even with this number only one 

 or two seeds matured in each lock." The records for flowers of Pima 

 cotton which have been bagged to prevent cross-pollination show 

 that the quantity of self pollen deposited automatically upon the 

 basal portion of the stigmas is sufficient to insure, as a rule, the re- 

 tention and maturation of 80 to 90 per cent of the bolls. In flowers 

 naturally open pollinated at Sacaton, additional large quantities of 

 pollen are conveyed to the stigmas by insects. It is probable that 

 only in rare instances is the number of pollen grains which reach- 

 the stigmas fewer than 10 times the number of ovules (PL VII). 

 Even where insect pollination is deficient, as in the field at Phoenix 

 where the experiment in artificial pollination was performed, the 

 data given in Table 28 show that the additional pollen applied by 

 hand to the stigmas increased the proportion of bolls retained and 

 matured by only about 9 per cent, while increasing the mean number 

 of seeds per boll by 21 per cent. 



Evidence also has been obtained that it is not requisite that many 

 of the ovules be fertilized in order that bolls may be retained and 



