FERTILIZATION IX PIMA COTTON. 29 



7 hours were required for penetration of the ovary by the pollen 

 tubes. Considering for the moment only the apically pollinated 

 flowers, it is shown that at 5 a. m., or 16 hours after the pollen was 

 deposited, the tubes had reached the ovaries of two-thirds of the 

 flowers in number sufficient to fertilize on the average more than half 

 of the mean number of ovules, which is 21; A slower rate of develop- 

 ment of some of the tubes is indicated by the much more nearly com- 

 plete fertilization of flowers in which the pistils were not excised 

 until 9 a. m. 



Some of the tubes doubtless had penetrated the ovary earlier than 

 5 a. m., but in estimating the mean rate of growth it may be assumed 

 that the period of 16 hours represents the average length of time 

 required. The further assumption is made, although proof is lack- 

 ing, that germination began as soon as the pollen reached the stigmas. 

 In the case of pollen applied at or near the apex of the stigmas, 

 which average in Pima cotton one-third the length of the pistil ex- 

 clusive of the ovary, it may be assumed that most of the grains 

 germinated within 2 millimeters of the apex of the stigmas, or 28 

 millimeters above the base of the style, the average total length of 

 stigmas and style being 30 millimeters. A growth of 28 millimeters 

 in 16 hours indicates a mean rate of 1.75 millimeters per hour. b Self 

 pollen automatically deposited at or near the base of the stigmas 

 would be located on the average about 6 millimeters nearer the ovary, 

 and tubes starting from this locus might be expected to penetrate 

 the ovary 3^ hours in advance of the tubes from grains of foreign 

 pollen starting near the apex of the stigmas. This would seem to 

 give self pollen a decided advantage over foreign pollen, provided 

 the conditions at both loci are equally favorable for the germination 

 and development of the pollen. 



Comparison of the rates of fertilization by apically and by basally 

 deposited pollen, as stated in Table 15, indicates, however, that the 

 base of the stigmas affords less favorable conditions for pollen de- 

 velopment than the apex. For each interval after pollination, 

 fertilization, as measured by the mean number of seeds per boll, was 

 significantly less complete in the basally than in the apically 

 pollinated flowers, and in the flowers excised at the latest hour^ 1 

 p. m., the mean difference in favor of apical pollination amounted 

 to 1.7 ±0.22. The mean number of seeds per boll from basally 

 pollinated flowers which had the pistils excised at 1 p. m. (24 hours 

 after the pollen was deposited) was not significantly greater than 

 the mean number from apically pollinated flowers which had the 

 pistils excised at 7 a. m. (only 18 hours after the pollen was de- 

 posited). 



Another experiment performed in 1921 yielded additional indica- 

 tions that pollen germinates and develops under relatively unfavor- 

 able conditions when deposited at the base of the stigmas. Pima 



"In a similar experiment performed in 1920, however (see Table 24), a few bolls ma- 

 tured from Pima flowers of which the stigmas and style were excised 7i hours after 

 pollination, #nd these bolls contained, relatively large numbers of seeds. Additional 

 experiments performed in 1922, the complete data of which were not available in time 

 for inclusion in this paper, gave convincing evidence that in Pima cotton within 8 hours 

 after deposition of pollen on the stigmas the tubes can penetrate the ovary in number 

 sufficient to fertilize more than half of the ovules. 



6 The fact that, in experiments performed in 1922, a few but comparatively well- 

 fertilized bolls developed from apically pollinated flowers of Pima cotton of which the 

 stigmas and style had been excised 8 hours after deposition of the pollen, indicates that 

 in exceptional cases the average hourly growth rate may attain 3.5 millimeters. 



