28 



BULLETIN 1134, U. S. DEPARTMENT OF AGRICULTURE. 



stigmas has been conveyed there by insects, whether it originated in 

 the same or in other flowers, and that pollen which is lodged upon the 

 basal quarter of the stigmas has been self-deposited. 



The question suggests itself whether the rate of growth of the 

 tubes from pollen grains deposited at different loci on the stigmas 

 may be an important factor in determining the relative frequency of 

 self-fertilization and of cross-fertilization. There is a consider- 

 able difference in the distance to the ovary to be traversed by the 

 tubes from self-pollen grains automatically deposited near the base 

 of the stigmas and by the tubes from insect-carried pollen grains 

 deposited higher on the stigmas, the maximum difference, corre- 

 sponding to the average length of the stigmas, being about 10 milli- 

 meters. This might be expected to give the self pollen a decided 

 advantage in the time required for the pollen tubes to reach the 

 ovary and to account, at least in part, for the observed preponderance 

 of self-fertilization in Egyptian cotton. 



Fertilization in Gossypium, according to Balls (8, p. 12), "is 

 normally completed within 30 hours after the first opening of the 

 flower, i. e.; by the afternoon of the following day/' An experi- 

 ment was performed at Sacaton, Ariz., in 1921, in an endeavor to 

 determine the length of time required to effect fertilization, or rather 

 penetration of the ovary by the pollen tubes. Flower buds of Pima 

 cotton were emasculated in the evening before anthesis and were 

 pollinated at 1 p. m. the following day with pollen of the same 

 variety. The pollen was deposited in some of the flowers at the 

 apex and in others at the base of the stigmas. The pistils of approxi- 

 mately equal number of these flowers were then excised at the summit 

 of the ovary at 8 p. m. of the day of pollination and at 5, 7, 9, and 

 11 a. m., and 1 p. m. of the following day.° A record was kept of 

 the number of bolls which matured and of the number of seeds per 

 boll, from which were computed the data given in Table 15. 



Table 15. — Degrees of fertilization attained in Pima cotton flowers pollinated at 

 the apex and at the oase of the stigmas, the pistils having oeen excised at 

 various intervals following pollination. 



Pistil excision. 



Apical pollination. 



Basal pollination. 



Hour excised. 



After pol- 

 lination 

 (hours.) 



Flowers 

 treated. 



Percentage 



of bolls 

 matured. 



Mean num- 

 ber of seeds 

 per boll. 



Flowers 

 treated. 



Percentage 



of bolls 



matured. 



Mean Clum- 

 ber of seeds 

 per boll. 





7 

 16 



45 

 45 

 45 

 45 

 45 

 37 







66.7±4.7 

 77.8±4.2 

 80.0±4.0 

 S2.1±3.9 

 83.8±4.1 





45 

 45 

 45 



t 

 40 







46. 7 ±5.0 

 46.7±5.0 

 68.9±4.7 

 84.4±3.6 

 87.6±3.5 





5 a. ra 



11.4±0.49 

 14.7± .34 

 16. 9± .23 

 16. 5± .19 

 16. 8± .IS 



8.1 ±6. 63 



7 a. ni 



18 

 20 

 22 



24 



12. 8± .45 



9 a. m 



11 a. m 



1 p. m 



12. 7± .31 

 13. 6± .28 

 15.1± .13 



Fertilization did not occur in either the apically or the basally 

 pollinated flowers of which the pistils were excised at 8 p. m. on the 

 day of pollination. It is therefore evident that in this case more than 



a Heribert-Nilsson (22) describes results obtained by this method of excising the style 

 in computing- the rate of pollen-tube development in Oenothera, which he found to 

 average 4.5 millimeters per hour in mid-July. Fertilization did not occur in flowers of 

 which the styles were excised earlier than 19 hours after pollination. This investigator 

 also obtained evidence " that the pollen tubes of O. gifjas grew slower in the styles of 

 0. lamarcMana than O. lamarclciaua's own pollen tubes," 



