FERTILIZATION IN PIMA COTTON, 



indicated maximum proportion of hybrids was therefore 2.3 per 

 cent. Taken in connection with the low percentages of hybrids pro- 

 duced by seed from open-pollinated flowers of Pima cotton grown 

 adjacent to upland cottons (Tables 1, 2, and 3) these data indicate 

 a strong tendency to self-fertilization in the Pima variety. 



VICINISM NOT A COMPLETE MEASURE OP CROSS-FERTILIZATION. 



The percentage of recognizable vicinists does not afford an ade- 

 quate expression of the relative frequency of cross-fertilization as 

 compared with self-fertilization, for the plants produced by ovules 

 which have been fertilized with pollen from other plants of the 

 same variety are usually not distinguishable from the plants 

 resulting from self-fertilization. In order to determine the actual 

 percentage of ovules which have been cross-fertilized, a single indi- 

 vidual of one variety should be isolated among plants of another 

 and readily distinguishable variety, allowing only one flower to 

 open daily on the isolated mother plant. In such case only recog- 

 nizable hybrids would be produced by all seeds from ovules not 

 fertilized with pollen of the same flower. 



The conditions outlined in the preceding paragraph were met in 

 an experiment begun at Sacaton, Ariz., in 1920. In the central row 

 of a 7-row plat of Acala (upland) cotton 8 plants of Pima (Egyp- 

 tian) were so located that from 5 to 10 Acala plants intervened be- 

 tween each 2 Pima plants. Eight plants of Acala cotton were simi- 

 larly located in a plat of Pima. Only one flower was allowed to 

 open daily on each of the isolated plants, any additional flower buds 

 due to open on the same day having been removed before the corolla 

 expanded. It is believed that under these conditions all or very 

 nearly all of the ovules were either strictly self- fertilized or were 

 cross-fertilized by pollen of the other type. Consequently, the total 

 cross-fertilization which took place should be indicated by the per- 

 centages of first-generation hybrids in the progenies of these plants. 



The seed produced by each of the isolated individuals was planted 

 in 1921. No thinning was done, all seeds which germinated having 

 been allowed to develop. The percentages of hybrids were deter- 

 mined after the plants had developed sufficiently to make identifica- 

 tion certain. The results are stated in Table 5. 



Table 5. — First-generation hybrids in the progenies of Pima and of Acala plants 

 which had been grown isolated in a plat of the other variety and on which 

 only one flower had been allowed to open daily, at Sacaton, Ariz., in 1920. 





Pima (Egyptian) plants. 



Progeny. 



Acala (upland) plants. 



Progeny. 



Number. 



Fi hybrids. 



Number. 



F t hybrids. 





Number. 



Per cent. 



Number. 



Percent. 



No. 1 



263 

 340 

 211 

 259 

 157 

 234 



4 

 26 

 33 

 30 



47 

 28 



1.5±0.5 

 7.6±1.0 

 15.6±1.7 

 11.6±1.3 

 29. 9 ±2. 5 

 12.0±1.4 

 5.2±1.0 

 21.5±1.8 



No. 1 



219 

 283 

 197 

 225 

 285 

 223 

 260 

 123 



64 

 98 

 66 

 48 

 64 

 75 

 73 

 18 



29.2±2.1 



No.2 



No.2 



34.6±1.9 



No.3 



No. 3 



33.5±2.3 



No. 4 



No. 4 



21.3±1.8 



No.5 



No. 5 



22.4±1.7 



No.6 



No. 6 



33.6±2.1 



No.7 



212 



No.7 



28.1±1.9 



No.8 



242 



52 



No.8.... 



14.6±2.1 





Total 





Total 



1,918 



231 



12. 0± . 5 



1,815 



506 



27. 9± .7 









22421—23- 



