SEXUAL REPRODUCTION. 



283 



of the ovulary, then through the micropyle (fig. 323), and 

 finally penetrate the megasporangium. The sperm nucleus 

 then fuses with the egg nucleus (see ^[ 369). 



Fig. 317- 



Fig. 318. 



Fig. 319. 



Fig. 317. — Gametophyte of the pinesap (Monotrofia Hyfiopitys). a, microspore show- 

 ing two cells ; the smaller being the sperm cell and the larger corresponding to the 

 wall of the spermary, undivided, b, the same, 6 hours later, showing the pollen tube 

 developed from the larger cell. The smaller one has become disorganized and its 

 nucleus (still undivided into sperms) and that of the larger cell have migrated into the 

 tube. Magnified 600 diam.— After Strasburger. 



Fig. 318.— One stage in the fertilization of the egg of an orchid (Orchis latifolia). The 

 pollen tube, fi, has entered the narrow micropyle, m, of an ovule, and reached the 

 megaspore e, the upper half of which only is shown with three eggs (two imperfect). 

 In the pollen tube, just above and below the entrance of the micropyle, are the two 

 sperms, s, s' . Magnified 360 diam. — After Strasburger. 



Fig. 319.— Longitudinal section through the ovule of the larch and the placental scale 

 to which it is attached, f, placental scale ; g, vascular bundles ; ;/, megasporangium ; 

 i. integument ; e, female gametophyte inside megaspore whose limit is shown by oval 

 line ; a, ovary ; />, pollen-tube. Magnified 14 diam.— After Strasburger. 



The growth of the spermary as a tube within which the 

 sperms may migrate to the egg is necessary because the female 

 gametophyte is forced to develop within the megaspore, 

 which is not released from the sporangium. In angiosperms 

 the further enclosure of the megasporangia in the sporophyll 

 (carpel) makes it necessary for the tube to be sufficiently long 



