ANGIOSPERMS 



381 



Fig. 375- 

 Nearly mature 



which floats in the tube cell but at first is separated by a thin 

 arched cell wall (fig. 375). This is the generative cell or body 

 cell, and was formed on germination of the young 

 microspore by the division of the single nucleus of 

 the microspore before the pollen grain was ripe. 

 After the pollen grain has reached the stigma of 

 the pistil (see Pollination, Chapter XVIII) it germi- 

 nates, producing a tube which travels down the 

 pollen sj^^jjj- st yj e ( or m fa e s tyl a r canal in some plants) , enters 

 five cen he genera " the ovary, and passing through the micropyle * 

 bores its way through the nucellus until it reaches 

 the embryo sac. The tube nucleus has followed along with the 

 advance of the tube. Soon after the pollen grain germinates the 

 generative cell divides into two cells, the sperm cells, and these 

 move into the tube, if the generative cell was not already there 

 before division. The stigma of the pistil is covered by a moist 

 somewhat viscous secretion which holds the pollen grains. The 

 moisture and certain saccharine or other secretions favor or 

 stimulate the germination of the pollen grain. 



546. The female gametophyte. The female gametophyte 

 is developed from a special cell in the nucellus of the ovule, 

 The ovule is an outgrowth usually 

 from some part of the ovary and 

 enclosed by it (the main part of 

 the carpel or macrosporophyll which 

 in development folds in such a way 

 as to form a case). It is the 

 macrosporangium (or megasporan- 

 gium). It consists of the nucellus 

 surrounded by two coats which 

 do not quite cover up the nucellus SA $ 

 at one end, thus leaving a minute 3S&2 

 opening (the micropyle). In the pollen grain ' 

 lily a subepidermal cell in the nucellus, near the micro- 

 pyle, becomes larger than its neighbors, with a large nu- 



* In a few plants, as in some of the ament-bearing plants, the pollen tube 

 enters the ovule at the chalaza end. 



G erminating spores 

 (pr>llen grains) of pel- 

 t.mdra; generative nu- 

 cleus in on- undivided, 



