FERTILIZATION IN PIMA COTTON. 31 



anthers with the base of the stigmas would favor the retention there 

 of a greater number of pollen grains than in the case of emasculated 

 flowers. Nevertheless, in experiments which afforded a comparison 

 of the fertilization resulting from (a) automatic self-pollination in 

 flowers that were not emasculated but in which the pollen was con- 

 fined to the lower halves of the stigmas and (b) artificial pollination 

 of emasculated flowers in which the bulk of the pollen was deposited 

 on the upper halves of the stigmas, the latter treatment gave sig- 

 nificantly better fertilization in six out of seven comparisons. On 

 the other hand, data given in Table 16, which were fully confirmed 

 by the results of an experiment performed in 1922, indicate that 

 flowers receiving pollen on the upper halves of the stigmas only are 

 fully as well fertilized as flowers receiving pollen on the whole stig- 

 matic surface. It is probable, therefore, that apart from conditions 

 affecting the adhesion of the pollen, there is a qualitative difference 

 between different parts of the stigmas and that penetration of the 

 tissues is effected more readily at the apex than at the base. 



If in Pima cotton under natural conditions the pollen germinates 

 and develops better at the apex of the stigmas than at the base, this 

 probably more than offsets any advantage which the automatically 

 deposited self pollen might derive from its nearness to the ovary. The 

 structure of the flower in other types of cotton in which self-fertiliza- 

 tion predominates increases the probability that the locus of pollen 

 deposition is not a factor of much importance in determining the pre- 

 dominance of self-fertilization. In many varieties of upland cotton 

 the uppermost stamens equal or even surpass the stigmas, so that 

 the entire length of the latter is accessible to automatically dis- 

 charged self pollen. The whole stigmatic surface is also accessible 

 to insect-carried foreign pollen, no part of it being screened by a 

 dense mass of stamens, as in the Egyptian cottons. Examination of 

 open flowers of upland cotton when growing in close proximity to 

 Pima shows the yellow pollen grains of the latter to be scattered over 

 the whole surface of the stigmas, although usually most abundant 

 near the apex. 



RELATIVE EARLINESS OF ARRIVAL OF SELF-DEPOSITED AND OF 

 INSECT-CARRIED POLLEN. 



Especial interest in connection with the problem of the relative 

 frequency of self-fertilization and of cross-fertilization attaches to 

 the question whether there is an appreciable interval of time be- 

 tween the first arrival upon the stigmas of self pollen and of foreign 

 pollen. There can be little doubt that if the stigmas greatly exceed 

 the stamens, as is the case in Pima cotton, all pollen present upon 

 the upper halves of the stigmas has been conveyed there by insects. 

 VVlien the pollen present on the stigmas is all of the same type it is 

 impossible to determine what part of it has originated in the same 

 flower and what part has been conveyed from other flowers by insects, 

 but examination of flowers exposed to cross-pollination by a differ- 

 ent type of cotton having readily distinguishable pollen showed that 

 foreign pollen was being conveyed to the stigmas of most of the 

 flowers. It may be assumed, therefore, that when pollen is present on 

 the upper halves of the stigmas some of it has originated in other 

 flowers. 



