FERTILIZATION 203 



by a pollen tube. Another ovule with a single embryo sac had also 

 received two pollen tubes, but only one of them had entered the sac, 

 the other remaining behind in the nucellus. He concluded that 

 there is a quantitative relation between embryo sacs and pollen 

 tubes, two embryo sacs secreting enough chemotactic material to 

 attract two pollen tubes, while one can attract only one tube. 



Nemec (1931) considered that there are mechanical contrivances 

 which exclude other pollen tubes from entering an ovule after the 

 first had done so. He found that in Gagea lutea the micropyle is 

 originally in close contact with the glandular conducting tissue of 

 the placenta, but that after a pollen tube has entered the micropyle, 

 there is a slight elongation of the funiculus causing the ovule to 

 retract from its original position and thus make it difficult for other 

 tubes to gain entrance. 



Beside the position effect noted by Nemec, there must no doubt 

 be some other factors also which bring about a similar result. In 

 the pistils of Phaseolus vulgaris (Weinstein, 1926) there are many 

 more pollen tubes than ovules, yet only one tube enters each ovule; 

 the superfluous tubes grow down to the basal end of the ovary and 

 eventually disintegrate. In Scurrula atropurpurea, Rauch (1936) 

 frequently saw several pollen tubes attached to the wall of the 

 embryo sac, but only one entered it, and as soon as this had been 

 accomplished the wall of the sac seemed to become firmer and more 

 resistant so as to exclude the others. Cooper (1938) states that in 

 Pisum sativum he frequently saw two or more pollen tubes at the 

 entrance to the micropyle, but only one actually entered it. More 

 recently, Pope (1946) saw an ovule of Hordeum with one pollen 

 tube inside the micropyle and four at its mouth, but how the embryo 

 sac admitted the first and excluded the others could not be deter- 

 mined. 



Although one pollen tube to an embryo sac may thus be considered 

 as the usual condition, the entry of accessory tubes is not unknown. 

 To quote a few examples, two pollen tubes have been recorded in 

 Elodea (Wylie, 1904), Ulmus (Shattuck, 1905), Juglans (Langdon, 

 1934), Xyris (Weinzieher, 1914), Oenothera (Ishikawa, 1918), 

 Boerhaavia (Maheshwari, 1929), Beta (Artschwager and Starrett, 

 1933), Acacia (Newman, 1934), Fagopyrum (Mahony, 1935), Sagit- 

 taria (Johri, 19366), Cephalanthera, Platanihera (Hagerup, 1947), 



