FERTILIZATION 191 



(Hallock, 1930) the pollen tube takes 17 days to arrive at the apex 

 of the nucellus, and in Carica papaya (Foster, 1943) about 10 days; 

 fertilization occurs a few days later. In Colchicum autumnale 

 (Heimann-Winawer, 1919) there is an interval of about 10 to 11 

 days between pollination and fertilization, and in Carya illinoensis 

 (McKay, 1947) about 4 to 7 days. 



In most plants, however, the period ranges from 24 to 48 hours, 

 and in some it is still shorter. In Oryza sativa (Juliano and Aldama, 

 1937), Coffea arabica (Mendes, 1941), and Oxybaphus nyctagineus 

 (Cooper, 1949) fertilization takes place in about 12 to 14 hours after 

 pollination, and in Lactuca muralis (Dahlgren, 1920) within 6 to 7 

 hours. In L. sativa (Jones, 1927, 1929) sperm nuclei have been seen 

 in the embryo sac 3 hours after pollination and a couple of hours 

 later most of the embryo sacs were already fertilized. In Portulaca 

 oleracea (Cooper, 1940) the period between pollination and fertiliza- 

 tion is 3 to 4 hours; in Impatiens sultani (Lebon, 1929) and Hordeum 

 distichon palmella (Pope, 1937) pollen tubes arrive inside the embryo 

 sac in less than an hour after pollination ; and in Parthenium argenta- 

 tum (Dianowa et al., 1935) and Crepis capillaris (Gerassimova, 1933) 

 fertilization is completed within 60 minutes after pollination. The 

 shortest period on record is in Taraxacum kok-saghys (Poddubnaja- 

 Arnoldi and Dianowa, 1934; Warmke, 1943), where fertilization 

 occurs within 15 to 45 minutes after pollination. 



Several computations have been made of the average hourly 

 distance traversed by the pollen tube: 4 mm. in Iris versicolor 

 (Sawyer, 1917), 6.25 mm. in Zea mays (Miller, 1919), 15 mm. in 

 Crepis capillaris (Gerassimova, 1933), and 35 mm. in Taraxacum 

 kok-saghys (Poddubnaja-Arnoldi and Dianowa, 1934). It is likely, 

 however, that the actual rate of growth is still higher; for the path 

 of the pollen tube from the stigma to the ovule is not like a straight 

 line but is marked by many twists and convolutions. 



Of the factors influencing the rate of growth, temperature is the 

 most important. As early as 1861, Hofmeister had observed that 

 in Crocus vernus, in warm moist air and bright sunshine, the pollen 

 tubes can be seen in the micropyle within 24 hours after pollination, 

 while in cooler and drier weather they take twice or thrice this time. 

 Working on Monotropa uniflora, Shibata (1902) found that in the 

 first week of May the pollen tubes took 10 days to reach the embryo 



