32 



BULLETIN 1134, U. S. DEPARTMENT OF AGRICULTURE. 



OBSERVATIONS OX PIMA COTTON. 



Examination in 1916 of numerous flowers, the corollas of which 

 were open only 1 or 2 millimeters, showed none of them to have 

 pollen present on the upper half of the stigmas. Flowers were ex- 

 amined in 1919 at intervals on July 27, beginning about two hours 

 after sunrise, and on August 3. beginning about an hour after sun- 

 rise. Ten flowers were inspected at each interval on both dates. An 

 endeavor was made to select in all eases flowers which had been fully 

 exposed to the direct rays of the sun at and after sunrise and which 

 should therefore have been favorably situated for the earliest pos- 

 sible opening of the corolla. Table 17 shows for each interval the 

 percentage of the total number of flowers (10 in each case) in which 

 an appreciable quantity of pollen was found upon the stigmas at a 

 height sufficient to justify the conclusion that it must have been con- 

 veyed by insects. 



Table 17. — Rate of deposition of pollen on the upper portion of the stigmas in 

 op en- pollinated floicers of Pima cotton in 1919. 



Hour of observation. 



Flowers having the 

 upper portion of 

 the stisrmas pol- 

 linated (per cent). 



Hour of observation. 



Flowers having the 

 .upper portion of 

 the stigmas pol- 

 linated(per cent). 





July 27. 



Aug 



3. 



July 27. 



Aug. 3. 





6' 



10 

 50 

 60 

 70 















'26" 



40 



10.30 a. m 





50 



7.30 a. m 



12.00 m 



1.00 p. m 





40 



8.00 a. m. . 



50 



S.30 a. m 



2.00 p. m 





70 



9.30 a. m 



3.00 p.m 





90 



10.00 a. m 









On both dates, at the earliest hour of observation, the anthers oi 

 the flowers were discharging pollen, some of which presumably was 

 reaching the lower half of the stigmas. Reference to Table 9 

 (p. 18) shows that at 8 a. m.. the earliest hour when pollen was found 

 in appreciable quantity upon the upper half of the stigmas, the av- 

 erage aperture of the corolla in Pima cotton is about 10 millimeters. 

 A difference in the rapidity of pollination of the upper half of the 

 stigmas on the two dates is indicated by the data in Table 17. Cross- 

 pollination of 70 per cent of the flowers had taken place at 10 a. m. 

 on July 27, and not until 2 p. m. on August 3. Yet the conditions 

 would seem to have been more favorable to early pollination by in- 

 sects on August 3. a clear sunny day. than on July 27 when the sky 

 was overcast during the morning. The probable explanation is that 

 on August 3 there was a marked scarcity of bees and other active 

 pollen carriers in the cotton field. 



Observations in 1920 indicated an earlier arrival of insect-carried 

 pollen. On July 25. at 7.40 a. in.. Pima flowers which were open 

 about 10 millimeters were found to have numerous grains of pollen 

 on the upper half of the stigmas, and in one flower, which had a 

 corolla aperture of only 2.5 millimeters, much upland pollen was 

 present. Most of the flowers examined on July 30, 1920. between 

 7. 45 and 8 a. m. were already open from 5 to 20 millimeters and had 

 n unerous pollen grains on the upper half of the stigma, while in 



