60 



DEPARTMENT OF AGRICULTURE. 



Table 39. — Pima cotton strictly inbred during seven successive generations 

 compared with a random sample of the continuously open-pollinated com- 

 mercial stock of this variety. 



Population. 



Flowers 



tagged. 



Percentage 

 of bolls 

 shed. 



Mean 



number of 



seeds per 



matured 



boll. 



Mean 

 weight of 

 100 seeds 

 (grams). 



Percentage 

 of germina- 

 tion of the 

 seeds. 1 



Inbred 



296 

 367 



11.8±1.3 

 8.4±1.0 



17.2±0.12 

 17. 1± .12 



13.6±0.04 

 13. 4± .03 



90. 8 ±0.8 





90.2± .9 







Difference 



3.4±1.6 



.1± .17 



.2± .05 



.6±1.2 











Boll dimensions. 



Boll weight and lint index. 



Population. 



Number 

 of bolls. 



Length 

 (milli- 

 meters). 



Diameter 

 (milli- 

 meters). 



1 



Number. Seed cotton 

 of bolls. | (grams). 



Lint index. 





25 

 25 



46.6±0.56 

 45. 7± .80 



26.8±0.19 

 26. 1± .19 



105 

 115 



3.22±0.21 

 3.04± .06 



4. 90 ±0.27 



Open pollinated 



5.12± .03 











.9± .97 



.7± .27 





• 18± .22 



• 22± .27 



•» 









1 Determined for 500 seeds of each population. 



Further comparison of the same populations was made in regard to 

 the length and the greatest diameter of the bolls, the weight of seed 

 cotton in the ripe boll, and the lint index. The determinations were 

 made on bolls from naturally pollinated flowers on five plants in each 

 population, individuals which occupied opposite positions in the two 

 rows having been used and the bolls having been from flowers which 

 had opened during the same period in both populations. Five bolls, 

 judged to be full grown, although not yet open, were measured on 

 each plant. The weight of seed cotton per boll and the lint index 

 were determined separately on from three to five lots of five bolls each 

 from each plant. The units from which were computed the means 

 and probable errors, as given in Table 39 (lower part) , were in all 

 cases the averages for the individual plants. The results of this 

 comparison show no significant differences between the two popula- 

 tions. 



CONCLUSIONS IN REGARD TO THE EFFECTS OF INBREEDING. 



No evidence was obtained that the fertility of Pima cotton had 

 been impaired by strict inbreeding during five or seven successive 

 generations. The inbred families were not inferior to the continu- 

 ously open-pollinated stocks in viability of the pollen; number of 

 ovules; daily flower production; percentage of bolls retained; size, 

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

 seeds ; and abundance of the fiber. 32 



The absence of superior fertility in the continuously open- 

 pollinated populations is hardly surprising in view of the evidence 



32 This comparison of the naturally pollinated commercial stock of Pima cotton, a 

 relatively uniform variety, with the closely inbred (self -fertilized) strain may be regarded 

 as a comparison of restricted or " narrow " breeding with " line "' breeding, to use Cook's 

 evolutionary terminology (10, p. 9). The evidence here presented that the closest in- 

 breeding during seven generations resulted in no diminution of fertility does not prove 

 that such effect might not be shown eventually. From Cook's point of view, " though 

 all forms of restricted descent lead ultimately to degeneration, the decline may be ex- 

 ceedingly slow and gradual if methods of line breeding are followed" (10, p. 38). 



