FERTILIZATION 197 



faintly staining than the other. In Vallisneria (Wylie, 1923) both 

 nuclei are oval at the time of their discharge from the pollen tube 

 but the second soon becomes spherical. Very recently Kadry 

 (1946) has reported that in Cardiospermum the male nucleus fertil- 

 izing the egg is swollen and rounded in front but tapering behind, 

 and that it is about four times as long as the other male nucleus, 

 which is more or less spherical. In Vinca minor (Finn, 1928a) also, 

 the sperm cells are unequal, one with a larger and the other with a 

 shorter plasma tail, but it could not be determined which fuses with 

 the egg and which with the polar nuclei. 



Although slight differences in the size and shape of the two male 

 gametes are possible, the few examples cited above do not seem 

 adequate to justify any generalization. In the first place consider- 

 able care has to be taken to make sure that the observed differences 

 are not due to the plane of sectioning. Stages in fertilization are 

 infrequent, and in the same section one male gamete may be cut 

 across its longer diameter and the other in a plane at right angles 

 to it, so that the two appear to be of different sizes, although in fact 

 they are quite similar. Further, the male gametes often change 

 their form and, as Gerassimova (1933) has suggested, the reported 

 size differences may well be due to a disparity in the rates of their 

 transformation. 



There has been a good deal of discussion about the mechanism of 

 movement of the generative cell and the male gametes. It is well 

 known that the antherozoids of the thallophytes, bryophytes, and 

 pteridophytes are actively motile. Even among the gymnosperms, 

 the Cycadales and Ginkgoales have ciliated sperms and their pollen 

 tube serves merely as a haustorial organ. In the Coniferales, 

 Gnetales, and angiosperms, on the other hand, cilia are absent and 

 the pollen tube becomes the agent for the transport of the male 

 gametes from the pollen chamber or the stigma to the embryo sac. 

 Long ago, Strasburger (1884, 1900, 1908) put forth the view that 

 the male gametes are carried passively by the streaming movements 

 of the cytoplasm inside the pollen tube. On the other hand, Na- 

 waschin (1898) and some other workers believed that the vermiform 

 appearance of the male gametes, observed in several members of the 

 Liliaceae and Compositae, is indicative of an independent power of 

 movement. Studies on living pollen tubes (Wulff, 1933) seem to 

 support Nawaschin's view. The cytoplasm in the tube shows 

 several plasma strands running in opposite directions, while the 



