34 



BULLETIN 1134, U. S. DEPABTMENT OF AGRICULTURE. 



pollen. In most of the 20 upland varieties upon which observations 

 were made in 1920, it appeared, however, that the first arrival upon 

 the pistil of self pollen and of foreign pollen was virtually 

 simultaneous. 



In 1921 several instances were recorded of the occurrence of for- 

 eign pollen upon the stigmas of partly open upland flowers in which 

 self-pollination had not yet taken place, the anthers being still closed. 

 The presence of foreign pollen upon the stigmas was readily deter- 

 mined in these cases, as the upland varieties in 1920 and 1921 were 

 grown in close proximity to Pima cotton, the bright-yellow pollen 

 grains of which are easily distinguished from the whitish grains of 

 the upland pollen. It may be concluded, therefore, that in upland 

 cottons the interval between the beginning of automatic self-pollina- 

 tion and that of pollination by insects is at most a very brief one 

 and that not infrequently foreign pollen reaches the stigmas in ad- 

 vance of self pollen. 



DEPOSITION OF SELF POLLEN AND OF FOREIGN POLLEN BY 



INSECTS. 



Evidence of the degree in which cross-pollination occurs in the two 

 types of cotton was afforded in 1920 by examination of flowers borne 

 by isolated plants of Pima (Egyptian) cotton distributed through a 

 plat of Acala (upland) and of flowers borne by isolated Acala plants 

 distributed through a plat of Pima in an experiment described on a 

 preceding page, the object of which was to determine the percentages 

 of natural hybrids or vicinists which would be produced under these 

 conditions (Table 5). The isolated plants were separated from each 

 other by several plants of the other variety, and only one flower was 

 allowed to open daily on each isolated plant. It is therefore certain 

 that most, if not all, of the pollen from other flowers which reached 

 the stigmas of these plants was of the other type ; hence, readily dis- 

 tinguishable from the self pollen. Observations on several days 

 (July 30 to August 3) during a period when both types were pro- 

 ducing flowers in approximately equal numbers gave the results 

 stated in Table 18. 



Table 18. — Relative proportions of self pollen and of foreign pollen present 

 on the upper portion of the stigmas of Pima and of Acala plants isolated 

 among plants of the other type. 



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



Nature of the pollen present. 



On the stigmas of— 



Pima 



(Egyptian) 



Acala 

 (upland) - 



Self pollen only 



Self pollen predominating 



Approximately half-and-half . 

 Foreign pollen predominating 

 Foreign pollen only 



Total flowers examined 



Since in Pima cotton the pollination of the upper portion of the 

 stigmas is effected by insects, the data in Table 18 point strongly 



