FERTILIZATION IX PIMA COTTON. 65 



of the upper portion of the stigmas of bagged flowers fertilization 

 equal to that of similarly pollinated but uninclosed flowers was 

 attained. This proved that the limitation of pollen deposition to the 

 lower half of the stigmas and not the environment created by 

 the inclosure of the flower is responsible for the relatively poor fer- 

 tilization of bagged flowers. 



Proportions of boll shedding in Pima cotton ranging from 3 to 25 

 per cent have been recorded at Sacaton for different years and for 

 different lots of plants. It is improbable that deficient pollination 

 and fertilization are primarily responsible for boll shedding at this 

 locality, evidence having been obtained that the pollen of Pima 

 cotton is highly viable and perfectly self-compatible, that the flow- 

 ers almost invariably receive pollen far in excess of the probable 

 requirement for complete fertilization, and that only a few of the 

 ovules need be fertilized in order that the boll may be retained and 

 matured. 



Inbreeding Pima cotton by controlled self-fertilization during five 

 successive generations resulted in no diminution in the viability of 

 the pollen or in the number of ovules. Similar inbreeding dur- 

 ing seven generations brought about no reduction in the daily 

 rate of flowering, in the percentage of bolls retained, in the size, 

 weight, and seed content of the bolls, in the weight and viability 

 of the seeds, or in the abundance of the fiber, as compared with 

 those of the continuously open-pollinated stock. It is concluded that 

 deleterious factors had been eliminated in the ancestry of the Pima 

 variety and that its present state of fertility is due to segregation 

 rather than to heterosis. 



