FERTILIZATION 195 



two fusions — one between the egg and the first male gamete, and 

 the other between the polar nuclei and the second male gamete. 



In form, the male gametes may be spherical as in Erigeron (Land, 

 1900), ellipsoidal as in Levisticum (Hakansson, 1923), rod-shaped 

 as in Urtica (Strasburger, 1910), or vermiform as in Lilium (Guig- 

 nard, 1899) and Fritillaria (Sax, 1916). Frequently they may 

 change their shape after their discharge into the embryo sac. Na- 

 waschin (1898) reported that in Fritillaria the sperms lose their 

 worm -like form just before fertilization. In Silphium (Land, 1900),. 

 Monotropa (Shibata, 1902), Taraxacum (Poddubnaja-Arnoldi and 

 Dianowa, 1934), Lacluca (Jones, 1927), and Nicotiana (Goodspeed, 

 1947) they are at first elongated or oval, but gradually become 

 shorter and more spherical as they approach the female nuclei. In 

 Juglans (Nawaschin, 1900), on the other hand, they are spherical 

 in the beginning but become curved afterward. The male cells 

 of Vallisneria (Wylie, 1923, 1941) are originally only slightly 

 longer than broad (Fig. 1 13 A-D) but become considerably elongated 

 as the pollen tube enters the ovarian cavity (Fig. USE, F). Fi- 

 nally they once again present a contracted appearance at the time 

 of their discharge into the embryo sac (Fig. 11SG-J). Gerassimova 

 (1933) found that the changes undergone by the male gametes of 

 Crepis are so rapid that it is difficult to follow them satisfactorily. 

 Eventually the sperms become more or less band-shaped and appear 

 to consist of two definite halves, folded along their entire length. 

 Sometimes they roll up into a ball-shaped body, but the two longi- 

 tudinal halves still remain distinguishable. 



There are a few reports of differences in the size and shape of the 

 two male nuclei discharged by a pollen tube. Guignard (1899) 

 stated that in Scilla nonscripta the male nucleus destined to fertilize 

 the egg is smaller than the one fusing with the polar nuclei, and 

 Hoare (1934) has confirmed this although admitting that the size 

 difference is not a constant feature. In Lilium auratum (Blackman 

 and Welsford, 1913), Iris versicolor (Sawyer, 1917), Fritillaria 

 pudica (Sax, 1916, 1918), Trillium grandiflorum (Nothnagel, 1918), 

 Acacia bailey ana (Newman, 1934), and Camassia leichtlinii (Smith, 

 1942) also, the male gamete entering the egg is said to be somewhat 

 smaller than the one fusing with the polar nuclei and is sometimes 

 also not so vermiform. 



In some other plants the reverse condition has been reported. 



