FERTILIZATION IN PIMA COTTON. 33 



the few flowers which were still closed at this time, self pollen was 

 just beginning to be deposited in the interstamen region. Most of 

 the flowers examined on August 8 at 7.30 a. m. were open from 5 to 

 10 millimeters and had pollen present on the upper half of the 

 stigmas. Of 10 closed buds which were examined at the same hour, 

 7 had a very feAv grains of self pollen on the interstamen section of 

 the stigmas. On August 21 the opening of the corolla had been re- 

 tarded by the coolness of the early morning (minimum temperature 

 62° F.), but at 8.40 a. m., of 30 flowers which were open from 5 to 10 

 millimeters, only 3 or 4 had the upper half of the stigmas free from 

 pollen. Many flowers which were open only about 5 millimeters had 

 the stigmas well pollinated at this hour. 



Observations in 1921 indicated that bees sometimes enter Pima 

 flowers and deposit pollen upon the stigmas when the orifice of the 

 corolla is still minute and that they occasionally do so by pushing 

 aside the loosened petals before any orifice has formed. It was noted, 

 however, on August 16 that most of the flowers had not been entered 

 until the orifice had reached a diameter of 2 or 3 millimeters, in 

 which stage most of the flowers had pollen present upon the upper 

 half of the stigmas. The readiness with which insects enter unopen 

 corollas seems to be controlled in some degree by the weather, for on 

 the cloudy morning of August 18, when as late as 8 a. m. many of the 

 buds showed no distinct orifice, although the petals were well loosened 

 at the apex, most of the flowers in this stage of anthesis had more or 

 less pollen upon the upper half of the stigmas. 



It is evident that at the same locality there is considerable varia- 

 tion on different days in the earliness of the arrival of insect-conveyed 

 pollen in the flowers of Pima cotton. ^This is doubtless to be ac- 

 counted for by variations in the weather and in the number, kind, and 

 habits of the pollinating insects. The conclusion seems warranted, 

 however, that as a rule many of the flowers are entered by pollen- 

 conveying insects soon after the expansion of the corolla has begun. 



Evidence has already been presented that the automatic deposition 

 of self pollen upon the stigmas does not commence much in advance 

 of the time when the petals begin to unfold and that the quantity 

 deposited before the corolla has developed an orifice is usually very 

 small. It is probable that as a rule the interval of time between the 

 first arrival of self pollen and of foreign pollen does not exceed half 

 an hour and that frequently foreign pollen begins to arrive before 

 any considerable quantity of self pollen has been deposited upon the 

 interstamen region of the stigmas. 



OBSERVATIONS ON UPLAND COTTON. 



It has been pointed out that when the two types are growing under 

 similar conditions the corollas of upland varieties open somewhat 

 more rapidly than those of Pima cotton. On the other hand, while in 

 Pima the opening of the anthers always precedes that of the corolla, 

 the anthers of upland varieties frequently do not begin to open until 

 the expansion of the corolla has begun, while in some upland varie- 

 ties the anthers are often still closed when the corolla is partly open. 

 In the Cleveland and Dixie Triumph varieties on July 27, 1920, the 

 anthers were observed to be still closed in flowers which had opened 

 sufficiently to allow the stigmas to become well covered with foreign 



22421—23 5 



