FERTILIZATION IN PIMA COTTON. 



35 



to the conclusion that much of the pollen conveyed to the stigmas 

 by this agency originates in the same flower and that the prepon- 

 derance of self-pollination is an important factor in the preponder- 

 ance of self-fertilization in cotton. It would appear also that under 

 like conditions as to climate and insect fauna upland cotton is 

 more subject than Egyptian to cross-pollination, self pollen having 

 predominated in 73 per cent of the Egyptian flowers and in only 

 48 per cent of the upland flowers. The latter conclusion is sup- 

 ported by the results of an examination at Sacaton, Ariz., in 1920, 

 of flowers of Pima and of upland cotton which were growing in 

 adjacent rows. There was much more Pima pollen on the stigmas 

 of the upland flowers than of upland pollen on the Pima stigmas. 



Further observations were made in 1921. The flowers examined 

 were taken from adjacent rows of Pima and of upland, in every case 

 from the side of the plant which faced a plant of the other type. 

 They were collected in pairs, one flower of each type from opposite 

 or nearly opposite plants. Pima flowers were preferred -which were 

 borne at approximately the same height above the ground as the 

 upland flowers. The Pima plants were flowering somewhat more 

 freely than the upland, but the difference in this respect was not 

 great. The results are stated in Table 19. 



Table 19. 



-Flowers of Pima and of upland cottons having different quantities 

 of pollen of the other type present on the stigmas. 



[The figures indicate the number of flowers belonging to each category.] 



Type of cotton. 



Pima (Egyptian). 

 Upland 



Flowers 

 examined. 



100 

 100 



Pollen of other type on stigmas. 



None. 



Fewer 

 than 10 

 grains. 



Ten or 

 more 



grains, 

 but less 

 than half 

 of total 

 pollen 

 present. 



Half or 

 more 

 than half 

 of total 

 pollen 

 present. 



Of the total number of Pima flowers examined 31 per cent had no 

 upland pollen present on the stigmas and 88 per cent had fewer than 

 10 grains of upland pollen. Of the total number of upland flowers 

 only 10 per cent had no Pima pollen on the stigmas and only 57 per 

 cent had fewer than 10 grains of Pima pollen. These percentages 

 do not, however, fully indicate the difference between the two types, 

 for of the flowers which were classed as having fewer than 10 grains 

 of pollen of the other type on the stigmas, the number which had 

 received very few grains (1 to 3) was much greater in the case of 

 Pima than in the case of upland. There can be no doubt, there- 

 fore, that when both types of cotton are growing side by side and are 

 flowering at approximately the same rate Pima pollen is conveyed 

 to the upland stigmas in greater quantity than upland pollen to the 

 Pima stigmas. This must be a very important factor in the observed 

 greater prevalence of cross-fertilization in upland than in Pima 

 cotton. 



