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BULLETIN 1134, U. S. DEPARTMENT OF AGRICULTURE. 



Table 9. — Measurements of the aperture of the corolla of Pima cotton at suc- 

 cessive half-hour intervals during three periods of five days each in 1919. 





Daily mean diameter (millimeters). 



Hour of 

 measure- 

 ment. 



Daily mean diameter (millimeters). 



Hour of 

 measure- 

 ment. 



Average for 

 the 15 days. 



For the day when the 

 rate of opening was— 



Average for 



For the day when the 

 rate of opening was — 





Most rapid 

 (Aug. 18).» 



Least rapid 

 (Aug. 2f.» 



the 15 days. 



Most rapid 

 (Aug. I8).i 



Least rapid 

 (Aug. 2).« 



6.30 a. m 



7.00 a. m 



7.30 a. m 



8.00 a. m 



S.30a. m 



0.6±0.03 



1.6± .18 



4.7± .39 



10. 6± .69 



19. 6± .90 



1.3±0.13 



6.6± .48 



11. 7± .42 



21. 0± .67 



26.3± .56 



0.4±0.04 

 .7± .12 

 2.2± .38 

 7.6± .75 

 14. 3± .85 



9.00 a. m 



9.30 a. m 



10.00 a. m 



10.30 a.m....: 

 11.00 a. m 



27.3±0.60 

 30. 7± .40 

 32. 9± .47 

 33. 7± .43 



(') 



28.7±0.58 

 31. 2± .53 

 33. 2± .55 

 33. 2± .55 

 33. 2± .55 



23.3±0.66 

 26. 7± .63 

 28. 8± .55 

 30. 3± .65 

 31. 5± .71 



1 Sky clear at and after sunrise. 



' Sky partly cloudy at and after sunrise. 



• Omitted in the general average because not determined on several days. 



The time of sunrise in Arizona in 1919 was 5.11 on July 29 and 

 5.42 on September 15, the mean for the period having been 5.26. 

 The data given in Table 9 show that as a rule expansion of the 

 corolla had barely commenced at 6.30, or about one hour after sun- 

 rise. It is evident that in general the opening of the corolla pro- 

 ceeded most rapidly during the hour 8 to 9, the average increase in 

 aperture during this hour having amounted to one-half of the mean 

 diameter when the corolla ceased to open farther. As the period 

 during which the measurements were made comprised 48 days and as 

 the time of sunrise was 31 minutes later at the end than at the begin- 

 ning of this period, a progressive retardation of the opening of the 

 corolla might have been expected. In fact, however, the average 

 rate of opening was practically the same during each of the five-day 

 periods. 



Records were kept for each morning of the experiment of the 

 shade temperature, relative humidity, and degree of cloudiness at 

 hourly intervals beginning at 6.30 a. m., the object having been to 

 ascertain whether differences in the rate of opening of the corolla 

 on different days bore any relation to these meteorological factors. 

 No evidence of a general correlation was detected, except that on 

 cloudy mornings the rate of opening was somewhat slower and more 

 gradual, the curve showing a less abrupt rise between the hours 7 to 

 9 than on mornings of full sunshine. 



Simultaneous observations of the rate of opening of the corolla in 

 the Pima variety of Egyptian cotton and in the Acala variety of 

 upland cotton on several mornings in July and August, 1920, indi- 

 cated that as a rule the opening begins a few minutes earlier and 

 proceeds somewhat more rapidly in Acala than in Pima, notwith- 

 standing the fact that the Pima flowers, which are borne on longer 

 fruiting branches, are more exposed than the Acala flowers to 

 the early rays of the sun. 



Observations made in 1921 afforded data as to the relative earli- 

 ness of opening of the corollas of Pima and of upland varieties, the 

 first appearance of an aperture having been taken as the criterion. 

 On August 11, 50 flowers of Pima and 24 flowers of King (upland) 

 were examined. Of these flowers 60 per cent showed an aperture as 

 early as 6.30 in the case of King, but not until 7.15 in the case of 



