WATER-STRESS BEHAVIOR OF PIMA COTTON. 5 



half of the cotton acreage in the Salt Kiver Valley. No estimate was 

 made by the writer for the whole valley, but in several fields visited 

 it appeared that from 30 to 40 per cent of the bolls had been de- 

 stroyed. The variation in season from year to year will not permit 

 any rigid rule to be fixed for a critical date of planting, and conse- 

 quently mistakes are made in planting too early as well as planting 

 too late. The prolonged period of frosts and low temperatures oc- 

 curring in late March and early April, 1920, was responsible for very 

 heavy loss to the cotton growers of the Salt River Valley on ac- 

 count of the poor stands which resulted. 



The Pima cotton plant will continue to develop bolls during a 

 long period when environmental conditions are favorable. The 

 writer has observed flowers to be still appearing on plants which had 

 commenced flowering 150 days previously. However, the normal 

 period of flowering for the Pima variety in the Salt River Valley is 

 from 90 to 110 days. Results by Ewing (12) in Mississippi and Lloyd 

 (19) in Alabama show that the length of the flowering period in those 

 States for Upland varieties is from 40 to 80 days. The flowering 

 curves of Balls (3) show that in Egypt the flowering of Egyptian 

 varieties continues through a period of 120 to 140 days, while flower- 

 ing curves for Sea Island cotton in the West Indies as presented by 

 Harland (14) show that the duration of the principal fruiting 

 period there is from 140 to 150 days. 



In Egypt Balls (3) places considerable importance on " the time 

 of arrival " of the cotton crop ; this is not so much because of frost 

 damage as because of the injury resulting from " root asphyxiation," 

 when the water table rises in late summer. After the flood season of 

 the Nile, when the water table has been lowered, the flowering often 

 revives and continues through October and into November. Har- 

 land (14) states that in the West Indies the time of arrival of the 

 crop is not of practical importance. In Arizona the general tendency 

 of the Pima variety is to continue flowering at a somewhat reduced 

 rate throughout the month of September, but not infrequently many 

 of the bolls set during this month are damaged or destroyed by early 

 frosts. 



The percentage of bolls set in September, 1919, which reached ma- 

 turity on the heavy soil above described in which the seed was planted 

 on April 5, was as follows: Plat 1, 51; plat 2, 40; plat 3, 39; plat 

 4,41. 



The period of maturation for bolls of Pima cotton, as derived from 

 the data obtained by the writer during the season of 1919, is consid- 

 erably longer than that given by any commercial variety reported to 

 cotton literature. Ewing (12) gives the mean length of the boll- 

 development period of small-boll Upland long-staple varieties in 

 Mississippi as 51.5 days and for small-boll early varieties 48.5 days. 



