lOO 



The Journal of Heredity 



to difference in the number of insects. 

 The acreage has been so large during 

 the last two years that near the center 

 of the cotton growing district there 

 have not been enough insects to "go 

 around," whereas farther out towards 

 the desert there are more bees and 

 wasps in proportion to the number 

 of cotton flowers. 



GREATER NUMBER OF SEEDS FROM IN- 

 SECT POLLINATED FLOWERS 



In order to determ ne whether the 

 less thoroughly pollinated flowers pro- 

 duce fewer seeds than flowers which are 

 completely pollinated a simple experi- 

 ment was planned by the writer and 

 was carried out last summer under the 

 immediate supervision of Mr. W. F. 

 Gilpin, ass sted by Messrs. R. D. Mar- 

 tin, C. J. K ng and G. J Harrison. 

 Several hundred flowers were marked 

 by tags and left to natural pol ination. 

 In an equal number of flowers the upper 

 part of the pistil received a thorough 

 applicat on of pollen from other Pima 

 flowers. The experiment was per- 

 formed both at Sacaton and in a field 

 near Phoen x in the Salt River Valley 

 where it had been observed that the 

 flowers were not being thoroughly 

 pollinated by natural means. All bolls 

 which developed from both lots of 

 flowers were harvested in the fall and 

 the number of seeds in each boll was 

 counted. From these data it was easy 

 to compute how many seeds were ob- 

 tained from each lot of flowers. The 

 average numbers of seeds per 100 

 flowers were as follows: 



Phoenix Sacaton 

 Naturally pollinated 



llowtTs 1157+26 1520+15 



Hantl-pollinated flowers 1526±19 1496+U) 



It is evident that the naturally 

 pollinated flowers yielded a much 

 larger number of seeds at Sacaton, 

 where bees and wasps were abundant 

 in the cotton fields, than in the field 

 at Phoenix wliere such insecls were 

 much less numerous. The better polli- 

 nation under natural conditions at 

 Sacaton not only resulted in a higlur 

 average number of seeds per boll I)iii 



allowed a higher percentage of bolls to 

 set than was the case at Phoenix. It is 

 therefore not surprising that hand 

 pollinating the flowers greatly increased 

 the seed production at Phoenix, but 

 had practically no effect at Sacaton, 

 where hand pollination was a mere 

 formality, the pistils having been al- 

 ready well covered with pollen by their 

 insect visitors. The hand-pollinated 

 flowers at Phoenix yielded practically 

 the same number of seeds as did the 

 naturally pollinated flowers at Sacaton. 



THOROUGH POLLINATION ALSO 

 INCREASES YIELD OF FIBER 



These results leave little room for 

 doubt that more thorough pollination 

 results in a larger yield of seed. But, 

 the reader will ask, is the yield of fiber 

 likewise greater when more pollen 

 reaches the pistils? A record was not 

 kept of the quantity of fiber produced 

 in the experiment just described but 

 evidence from another source is at 

 hand. 



Twelve samples of seed cotton, each 

 consisting of 100 bolls from as many 

 plants and each obtained in a different 

 field, were collected in the Salt River 

 Valley in 1920. All the samples were 

 kept for several weeks under uniform 

 conditions and were then carefully 

 weighed and ginned. The number of 

 seeds in each sample was determined 

 and the total weight of fiber was cal- 

 culated by subtracting the weight of 

 the seeds from the weight of the seed 

 cotton. It was then a simple matter to 

 compute for each sample the lint index 

 or average weight of fiber per 100 seeds, 

 which expresses the relative abundance 

 of the fiber on the individual seeds. 



A study of the data thus obtained 

 showed that there was no correlation 

 between the average weight of fiber per 

 seed and the aAcrage number of seeds 

 per boll. In other words, there was no 

 evidetice of a tendency for the fiber to 

 be more abundant on the seeds in bolls 

 containing few seeds than in bolls con- 

 taining many seeds. On the other 

 hand, the weight of fiber per boll was 

 foimd to be very closely correlated with 

 (he abundance of the fiber on the 



