FERTILIZATION IN PIMA COTTON. 



51 



SEASONAL VARIATIONS IN THE RELATIVE COMPLETENESS OF 



FERTILIZATION. 



Comparison of the fertilization attained in different seasons can 

 be made most effectively on the basis of the mean number of seeds 

 per boll expressed as a percentage of the mean number of ovules. 

 Counts made in 1921 on 250 3-celled and on 25 4-celled ovaries of 

 Pima cotton showed the mean numbers of ovules to be 20.6d=0.09 and 

 25.3dz0.34, respectively. Since at Sacaton approximately 5 per cent 

 of the Pima ovaries are 4-celled and practically all the others are 

 3-celled, the mean number of ovules in a random sample of ovaries 

 should be 20.8. The frequency distributions for the number of 

 ovules in the 3-celled and the 4-celled ovaries are given in Table 29. 



Tarle 29. 



-Frequency distributions of the number of ovules in 3-celled and in 

 4-celled ovaries of Pima cotton. 





Ovaries. 



Number of ovules. 





15 



16 



17 



18 



19 

 24 



20 



21 



22 



23 



24 



25 



26 



27 



28 



29 ! 30 



3-celled 



5 



8 



11 



16 



41 



52 

 1 



44 

 3 



35 

 3 



14 

 4 















3 



2 



4 



1 



3 1 

















Bolls from various lots of naturally pollinated flowers at Sacaton 

 have been found to contain mean numbers of developed seeds as given 

 in Table 30, which also states the mean numbers of seeds as per- 

 centages of the mean number of ovules when taken as 21. 



Table 30. — Mean numbers of seeds per boll from different lots of naturally 

 pollinated flowers of Pima cotton at Sacaton, Ariz. 



[The mean numbers of seeds are stated also as percentages of the mean number of ovules.] 





Plants. 



1 



; Mean number of seeds. 



Year. 



Bolls. 



[ Actual. 



As percent- 

 age of the 

 mean num- 

 ber of 

 ovules. 



1915 



53 

 81 



82 



85 



200 



530 i 16. 0± 0.12 



810 ! 16. 9± .08 



820 17. 7± .07 



850 16. 5± .07 



200 16. 5± .13 



95 i 14. 2± .21 



91 15. 0± .20 



360 15. 9± .11 



» 100 18. 6± .13 



560 , 17.3± .09 



76 



1916 



80 



1917 



84 



1918 



78 



1919 



78 



1920 



67 



1920 



71 



1920 





75 



1921... . ..... 



100 

 30 



89 



1921 



82 







1 3-locked bolls only. 



The mean percentage of ovules fertilized, as stated in Table 30, 

 varied from 67 to 89. The fact that both lots of flowers in 1921 gave 

 a significantly higher mean number of seeds than any of the three 

 lots in 1920 indicates that there is considerable variation from year 

 to year in the conditions for fertilization. Reference to Table 28 

 shows that the conditions vary also during the same season. Con- 



