WATER-STRESS BEHAVIOR OF PIMA COTTON. 



15 



THE PERIOD BETWEEN FLOWERING AND SHEDDING. 



The interval between the opening of the flower and the completion 

 of the abscission process is of some importance in identifying the 

 stimulus which produces shedding. From Table VI it is apparent 

 that there is little or no correlation between the length of this period 

 and the moisture content of the soil at the time of flowering. 



Table VI. — Period of retention of shed bolls developing from flowers of Pima 

 cotton blooming on successive weeks on four plats compared with the avail- 

 able moisture present in the soil during the season of 1919. 



Date. 



Plat 1. 



Plat 2. 



Plat 3. 



Plat 4. 



July 16 



da vs.. 



9.0 



8.2 



7.7 



8.1 



22 



do.... 



10.8 



10.6 



6.0 



10.7 



30 



do.... 



7.8 



13.8 



12.5 



8.5 



Aug. 6 



do.... 



7.5 



8.1 



7.7 



5.6 



13 



do.... 



9.2 



8.0 



12.0 



10.1 



20 



do.... 



7.1 



9.2 



8.9 



10.7 



27 



do.... 



9.4 



9.0 



10.4 



11.8 



Sept. 3 



do.... 



16.1 



16.5 



15.1 



12.0 



10 



do.... 



18.3 



13.8 



8.8 



10.4 



16 



do.... 



10.6 



9.5 



6.1 



10.0 



23 



do.... 



16.2 



10.2 



11.3 



21.2 





11.1 

 5.0 



10.6 

 3.0 



8.8 

 2.5 



10.8 



Mean available moisture in top 4 feet of soil. . . . 



percent.. 



2.8 



Ewing (12) states that " the period between flowering and shed- 

 ding in general is longest when shedding first begins and shortest 

 at the close of the season." Lloyd (18) shows ttiat " there is little 

 evidence that the mean of age frequencies of shed bolls was reduced 

 during the season, but that there is definite evidence that the mean 

 may recede or advance as a result of the additive or subtract! ve 

 effects of stimuli, such as the additive effect of rain upon that of the 

 soil moisture which resulted in a recession of the mearn of three 

 days." It appears from the results shown in Table VI that in Pima 

 cotton there is a tendency for the period between flowering and 

 abscission to increase as the season advances, under the conditions 

 which obtained under this experiment, but the fluctuations were great 

 even near the end of the season. 



A study of the results in Table VI will show that there is only one 

 weekly mean which exhibits clearly the effect of some stimulus oper- 

 ating generally on all the plats. This is the mean for the week 

 ended August 6 when there was a recession of the mean on all the 

 plats. It so happens that the only rain of any importance during the 

 fruiting season fell on August 1 (fig. 2), so that there seems to be 

 little doubt that rainfall was the responsible factor for the relatively 

 large shedding of bolls during the week specified. 



VARIATIONS OF PERIODICITY OF DEVELOPMENT. 



The cotton plant, like many other plants, exhibits pronounced peri- 

 odic variations in its developmental activity. Among these is the 



