AKGIOSPEEMS 



325 



of the ovule were seen in the gymnosperms. Within the cen- 

 tral tissue of the ovule a megaspore is formed. Upon its for- 

 mation this megaspore at once proceeds to grow to produce 

 the female gametophyte. The wall enlarges and elongates to 

 form the embryo sac, and the nu- 

 cleus divides. One new cell passes 

 to each end of the developing sac 

 and soon divides again, thus pro- 

 ducing two cells at each end, or 

 four in all. Each of these divides, 

 thus producing eight cells in all. 

 One passes from each end toward 

 the center (Fig. 265), and these 

 two unite to produce the endosperm 

 cell from which the endosperm of 

 the seed develops later. The female 

 gametophyte at this time consists 

 of seven cells inclosed within the 

 old megaspore wall or embryo sac. 

 In the micropylar end of the game- 

 tophyte are three cells, the central 

 one of which is the egg, and on 

 each side of the egg is a cell which 

 resembles the egg. These are called 

 the synergids or helper cells. They 

 may nourish the egg, or possibly 

 may assist in directing the pollen 

 tube when it enters. In the oppo- 

 site end of the embryo sac are three 

 antipodal cells, which usually dis- 

 appear soon after they are formed, 

 and near the middle of the embryo sac is the endosperm cell. 

 299. Fertilization. After pollen grains or microspores have 

 fallen upon the stigma the outer spore wall breaks, and from 

 the inner wall there extrudes the beginning of the pollen tube. 

 The tube tip enters the stigmatic tissue and forces its way 



FIG. 266. Germinating pollen 

 grains 



The pollen grains (g) have been 

 deposited upon the stigma. The 

 roughened surface of the stigma 

 is made by cell extensions or pa- 

 pillae (p). Pollen tubes (f) grow 

 from the grains through the tis- 

 sue or along the central canal (c) 

 until they reach the ovule. Only 

 a small part of the stigma and 

 style are shown in this cut 



