FERTILIZATION IN PIMA COTTON. 63 



ginning of anthesis in fewer than 10 per cent of the flowers tested, the 

 indication being that the stigmas do not remain receptive much longer 

 than this even when the flowers are protected by inclosure in bags. 



Self pollen being automatically deposited in Pima cotton upon 

 that part of the stigmas which is nearest the ovary and foreign pollen 

 being excluded from this zone by the dense girdle of stamens, the 

 self pollen would seem to have a decided advantage in the distance 

 to be traversed bv the tubes in reaching the ovary. Computation 

 from rather unsatisfactory data as to the average rate of growth of 

 the pollen tubes suggests that, other things being equal, pollen tubes 

 from self grains might reach the ovary 3J hours before penetration 

 could be effected by foreign pollen deposited higher on the stigmas. 

 Evidence has been obtained, however, that the conditions for pollen 

 development are less favorable in the basal than in the apical region 

 of the stigmas, and it is therefore to be doubted that the locus of 

 pollen deposition is an important factor in the preponderance of 

 self-fertilization in Pima cotton. The doubt is increased by the fact 

 that in upland cottons, in which also self-fertilization preponderates, 

 the whole stigmatic surface is accessible to both self pollen and 

 foreign pollen. 



The automatic deposition of self pollen upon the stigmas of Pima 

 cotton does not begin long in advance of the first arrival of insect- 

 carried pollen, and in upland cottons the first arrival of pollen from 

 both sources seems as a rule to be virtually simultaneous. It may be 

 concluded, therefore, that the time of arrival of the pollen does not 

 determine the relative frequency of self-fertilization and of cross- 

 fertilization. 



By controlling conditions so as to prevent the access of foreign 

 pollen not readily distinguishable from the self pollen, but so as 

 not to interfere with natural cross-pollination, it was demonstrated 

 that a large proportion of the pollen transferred to the stigmas by 

 insects is derived from the anthers of the same flower. As much 

 self pollen is also deposited automatically, preponderance of self- 

 pollination would seem to be an important, if not the principal, fac- 

 tor in the preponderance of self-fertilization. 



When the two types are growing side by side, the quantity of 

 Pima pollen deposited upon the stigmas of upland cottons exceeds 

 the quantity of upland pollen deposited upon the Pima stigmas, a 

 fact which helps to explain the greater frequency of upland X Pima 

 than of Pima X upland vicinists. The habits of the pollinating in- 

 sects and the relative rates of opening of the corolla and anthers 

 in the two kinds of cotton seem to account for this difference in cross- 

 pollination. Many of the Pima flowers are entered by honeybees just 

 as the petals begin to unfold and at a time when little upland pollen is 

 available for transfer, the anthers of the upland flowers being for the 

 most part still closed. In entering the Pima flower in this early stage of 

 expansion the insect comes into contact with the stigmas and stamens, 

 depositing and taking up pollen. Later in the morning, when up- 

 land pollen is available in quantity for transfer, the Pima corollas 

 are open to a degree which allows the insects to enter and leave the 

 flower without touching the reproductive organs. Insects coming 

 from Pima flowers which have just begun to open are often loaded 

 with pollen, the Pima anthers usually being well open when the ex- 



