192 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



sac, but in June only 6 days were necessary. He further observed 

 that tube growth is adversely affected by abnormally low tempera- 

 tures (8 to 10°C.) as well as abnormally high ones (above 31°C.) and 

 he was able to lengthen or shorten the interval between pollination 

 and fertilization merely by altering the experimental conditions. 

 In plums (Dorsey, 1919) a low temperature (4.5 to 10.5°C.) renders 

 fertilization uncertain because of a retardation of pollen tube growth. 



The first detailed and direct study of the problem was made by 

 Buchholz and Blakeslee (1927) who found that in Datura stramo- 

 nium the rate of growth of the pollen tube steadily increased when 

 the temperature was raised from 11 to 33 °C. At 33 °C. it was four 

 and a half times that at 11°C. 



Since then a somewhat similar range has been reported in several 

 other plants. In Lycopersicum esculentum (Smith and Cochran, 

 1935) the maximum growth rate occurs at 21 °C, gradually declining 

 at both lower and higher temperatures. At 38°C. germination was 

 extremely poor; 84 hours after pollination only 3.9 per cent of the 

 pollen grains had germinated, none of the pollen tubes had grown 

 more than 2 mm. long, and even these had become abnormally 

 swollen and bulbous at the tips. In Hordeum vulgar e var. pallidum 

 (Pope, 1943) the optimum temperature is about 23°C. At this 

 temperature the male gametes were found to reach the embryo sac in 

 about 20 minutes after pollination, while at 5°C. 140 minutes were 

 required for the attainment of this stage. 



It may be concluded that according to present records (based 

 mostly on observations made in temperate regions) pollen germina- 

 tion and tube growth are definitely inhibited at temperatures below 

 5°C, but occur freely above 10°C. and reach an optimum at about 

 25 to 30°C. Although the pollen grains themselves can withstand 

 higher as well as lower temperatures and escape serious damage, 

 such extremes are definitely harmful to the delicate tissues of the 

 stigma and style. This may be the reason why many European 

 vegetables fail to set seed in the tropics. 



Another factor which markedly affects the rate of pollen tube 

 growth is the degree of compatibility between the male gametophyte 

 and the sporophytic tissues of the pistil. When their reciprocal 

 relations are correct, the pollen tubes travel down the full length 

 of the style and fertilization is accomplished before the formation 

 of an abscission layer at the base of the flower. In incompatible 



