SEXUAL REPRODUCTION. 



283 



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

 finally penetrate the megasporangium. The sperm nucleus 

 then fuses with the ^^j^g nucleus (see •" 369). 



Fig. 317. 



Fig. 31S 



Fig. 317. — Gametoph\'te of the pinesap (Mojiotropa Hy/-opitys\. a. microspore show- 

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

 wall of the spermary, undivided. /■. the same, 6 hours later, shoi.\-ing the pollen tube 

 de\"eloped from the larger cell. The smaller one has become disorganized and its 

 nucleus (still undi\-ided into spermsi and that of the larger cell have migrated into the 

 tube. Magnihed 600 diam.^After Strasburger. 



Fig. 318. — One stage in the fertilization of the i:%% of an orchid (Orchis latifolia'). The 

 pollen tube, /, has entered the narrow micropyle, ju, of an o\'ule. and reached the 

 megaspore e, the upper lialf of which only is shown ^^^th three eggs (two imperfectt. 

 In the pollen tube, just above and below the entrance of the micropvle, 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, y, placental scale ; ,t,', vascular bundles : ;/. megasporangium ; 

 /, integument ; c'. female gametoph\'te inside megaspore whose limit is sho\\n by oval 

 line ; a, ov^ry;/, pollen-tube. Magnified 14 diam, — After Strasburger. 



The growth of the spermary as a tube within wliich the 

 sperms may migrate to the egg is necessary because the female 

 gametophyte is forced to de\'elop 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 



