FERTILIZATION IN PIMA COTTON. 13 



third of the total length of the pistil exclusive of the ovary. 10 At 

 the same hour the short uppermost stamens are found to extend on 

 the average 2.5 millimeters above the summit of the staminal sheath. 

 Consequently, at the time of opening of the corolla, approximately 

 one-fourth of the total length of the stigmas is surrounded by the 

 uppermost stamens. Owing to the density of the mass of surround- 

 ing stamens, this part of the stigmas probably is screened effectively 

 against the access of foreign pollen. The erect or semierect position 

 of the filaments of the upper stamens brings their anthers into close 

 contact with the base of the stigmas, and automatic self-pollination 

 is thus effected. 



Trelease (47, p. 322), whose observations doubtless were made 

 upon upland cottons, states : 



The reproductive organs are so placed that on the expansion of the corolla 

 pollen has usually been deposited on the stigmas, self-fertilization being thus 

 secured. 



Robson (4-1) observes that " fertilization in the majority of cotton 

 flowers is effected from the section of the stigma nearest the ovary." 



The adaptation of the cotton flower both to self-fertilization and 

 to cross-fertilization is described as follows by Kottur (3$, pp. 52, 53) : 



The entire surface of the style that projects beyond the staminal column is 

 stigmatic; and this has been proved by cutting the stigma and fertilizing it 

 only at the base. Again, in the majority of flowers the filaments of the upper 

 anthers are sufficiently long to touch the base of the stigma. All these condi- 

 tions are quite favorable for self-fertilization. The anthers are in contact with 

 the stigma and they shed their pollen as soon as the flower opens. But, on 

 the other hand, we have in most cottons a very attractive corolla. The quan- 

 tity of honey and pollen in the flower is profuse and invites the insects that 

 roam in search of them. All these favor natural crossings. We have thus one 

 set of conditions favoring self-fertilization and another set favoring cross- 

 fertilization ; but the former occurs as a rule and the latter as an exception in 

 all varieties of Indian cotton under observation at Dharwar. 



The stamens of Pima cotton change their position very slightly, 



if at all, during the day. Observation as late as 3 p. m., when the 



•corolla was beginning to wilt, showed the filaments of the uppermost 



stamens to be still erect and the anthers as though glued to the 



stigmas by the masses of extruded pollen. 



In sea-island cotton (Gossypium barbadense) the positional rela- 

 tions of the reproductive organs (PL II, Fig. 2) are much the same 

 as in Egyptian cotton, but the anthers do not form as dense a girdle 

 around the base of the stigmas, which is probably somewhat more 

 accessible to foreign pollen. 



Most varieties of upland cotton (Gossypium hirsutum) are char- 

 acterized by much shorter stigmas and by longer filaments of the 

 stamens than in Egyptian cotton (PL I, Fig. 2) . Measurements made 

 in 1918 upon fully open flowers of the Pima (Egyptian) and Holdon 

 (upland) varieties gave the means stated in Table 7, which show 

 that in a typical upland cotton the stamens are much longer and the 

 stigmas are much shorter, both absolutely and relatively, than in 

 the Egyptian type as represented by the Pima variety. 



10 Measurement of 100 Pima cotton flowers at 8 a. m. showed the mean length of the 

 pistil from the summit of the ovary to be 30.5 ±0.33 millimeters and the mean length 

 of the stigmas (portion outside the staminal sheath) to be 10.2 ±0.35 millimeters. The 

 mean projection of the stamens above the summit of the sheath in the same flowers was 

 2.5 ±0.16 millimeters. 



