10 



BULLETIN 1134, U. S. DEPARTMENT OF AGRICULTURE. 



There was much more variation in the percentage of hybrids 

 among the progenies of the Pima plants than among the Acala prog- 

 enies, one of the Pima progenies having yielded a somewhat higher 

 percentage than most of the Acala progenies. If the eight prog- 

 enies of each variety are taken as a single population, however, 

 it is seen that the percentage of cross- fertilized ovules was more 

 than twice as great in the upland variety as in the Egyptian variety. 

 The results of this experiment indicate that on the average 88 per 

 cent of the ovules in Pima (Egyptian) cotton and 72 per cent in 

 Acala (upland) cotton were autogamically fertilized. 



These types of cotton differ less in height of plant and rate of 

 flowering in early summer than later in the season, when the Pima 

 plants become much taller than the Acala and produce relatively 

 a greater number of flowers. In order to determine whether these 

 differences are reflected in different degrees of cross-fertilization of 

 the early and late flowers, dated tags were attached to all flowers 

 which opened on the isolated plants. The progeny of each indi- 

 vidual was planted in three sections, representing as many periods 

 during which the flowers had opened — July 1 to 21, July 22 to 

 August 11, and August 12 to September 3. The number and per- 

 centage of F t hybrids from seed produced by flowers which opened 

 during each period were determined for each variety, these data 

 being presented in Table 6. 



Table 6. — First-generation hybrids yielded in 1921 by seed representing different 

 flowering periods which icas produced by the isolated plants of Pima and 

 of Acala cotton, at Sacaton, Ariz., in 1920. 





Progenies of 8 Pima (Egyptian) 

 plants. 



Progenies of 8 Acala (upland) 

 plants. 



Period. 



Number. 



F: hybrids. 



Number. 



Fi hybrids. 





Number. 



Per cent. 



Number. 



Per cent. 



July l to 21 



311 

 793 

 815 



63 



20. 2+1. 5 



646 



462 

 706 



166 



120 

 220 



25.7±1.2 



July 22 to August 11 



136 ! 17. 2± .9 

 32 3-9+ .5 



26. 0±1. 4 



August 12 to September 3 



31.2±1.2 











The difference between the two varieties in the percentage of hy- 

 brids from seed produced by flowers of the first period probably was 

 not significant, but flowers of the second and third periods yielded 

 significantly greater percentages of hybrids in the case of Acala than 

 in the case of Pima plants. The very marked decline during the last 

 period (August 12 to September 3) in the relative cross-fertilization 

 of the flowers borne by the isolated Pima plants is probably to be 

 attributed to a diminished flower production of the Acala plants 

 which surrounded them. The isolated Acala plants, on the other 

 hand, showed a slight increase in the percentage of cross-fertiliza- 

 tion during the same period, indicating that no corresponding reduc- 

 tion had taken place in the rate of flowering of the Pima plants by 

 which they were surrounded. 



The fact that the Pima and the Acala flowers which opened during 

 the period from July 1 to 21, when both types of cotton were in full 



