20 BULLETIN 1134, U. S. DEPARTMENT OF AGRICULTURE. 



still closed or is open at the apex to the extent of not more than 

 1 or 2 millimeters, as is usually the case in July and August up to 

 about 7 a. m., the anthers are found to be partly open, so as to expose 

 the pollen. The rapidity with which both corolla and anthers open 

 depends to some extent upon the position of the flower on the plant, 

 which determines how early it is exposed to the rays of the sun. 

 The condition of the sky also is doubtless a factor in the earliness of 

 opening. 



On July 25, 1920, during the half hour from 5.30 to 6 a. m. 25 

 Pima flowers which had the corolla still closed, although in some 

 cases the petals were beginning to loosen at the tip, showed the 

 anthers to be partly open in all of the flowers. Pollen grains in 

 greater or less number were already present on the interstamen 

 region of the stigmas in all but one flower, but some of this pollen 

 may have been deposited in the act of opening the bud. That this 

 was probably the case is indicated by further observations during 

 the same summer, in which extreme care was taken to avoid the dis- 

 charge of pollen upon the stigmas while opening the bud. Ten 

 flowers were examined at about 7 a. m. on each of six days during the 

 period from August 3 to August 14. At the time of observation the 

 corolla was closed or was open to an extent of not more than 1 or 2 

 millimeters, while the anthers were at least half open and were 

 extruding pollen. Pollen was found upon the interstamen region of 

 the stigmas in about half of the flowers, but the number of grains 

 there present was very small, frequently not exceeding one or two. 



When the natural opening of the corolla is delayed the discharge 

 of pollen also is retarded. 14 On July 25, 1917, 10 closed or nearly 

 closed flowers were examined at 8 a. m. (hence nearly three hours 

 after sunrise), and four of these had no pollen on the stigmas, even 

 on the portion surrounded by the uppermost anthers. Examina- 

 tion, on July 30, 1920, of a few buds which were still closed at 8 

 a. m. showed that self pollen was just beginning to be deposited 

 upon the stigmas in appreciable quantity. On August 19, 1921, fully 

 90 per cent of the flowers were open sufficiently between 7.30 and 

 8 a. m. to admit insects. Of the belated buds, which were either 

 tightly closed or were just beginning to loosen at the tip, 20 were 

 opened during this half hour, taking every precaution to avoid 

 further discharge of pollen, and the number of grains present on the 

 stigmas was determined as accurately as could be done without touch- 

 ing the anthers. The counts showed numbers of grains present on the 

 lower half of the stigmas as follows : In 8 flowers, 6 grains or fewer ; 

 in 5 flowers, 6 to 12 grains; in 7 flowers, more than 12 grains. Of 

 the buds examined 65 per cent had no more than a dozen grains of 

 pollen present on the stigmas. 



In upland cottons the opening of the anthers may or may not 

 precede that of the corolla. Five flowers each of some 20 upland 

 varieties were examined at Sacaton, Ariz., in 1920. In six varieties 

 some of the flowers had the anthers still closed after the corolla had 

 begun to expand. In the other varieties the opening of the anthers 

 was keeping pace with the expansion of the corolla in most of the 

 flowers. Examination of 10 closed or barely opening flower buds 



14 Cook (11, p. 204) states that "in cool moist weather the anthers sometimes fail to 

 open, so that no pollen is available." An instance of complete failure to set bolls from 

 this cause was observed in Guatemala. 



