348 THE FLOWER 



coherent so as to form a closed bag-like structure, the 

 ovary (Figs. 57, 59, A). The free end of the leaf forms 

 a structure, which, when mature, lacks the characteristic 

 shoot epidermis, bearing on its surface papillae or hairs 

 which secrete a sugary solution. This is called 

 the stigma (Fig. 57), and is the organ which receives the 

 pollen grains. The stigma is often raised above 

 the ovary on a more or less hollow stalk, the style. On 

 the thickened margins of the carpellary leaf (placenta) 

 inside the cavity of the ovary are the ovules (Fig. 59, A). 

 Each ovule (Fig. 59, B) is a megasporangium covered 

 by two coats, each composed of one or more layers of 

 cells, and with a body (nucellus) consisting of an ovoid 

 mass of cells, of which one, the megaspore, early becomes 

 much larger than the rest, and when mature fills much 

 of the space within the nucellus, one end lying close 

 to the surface of the free end of the nucellus, just below 

 a pore (micropyle) left by the incomplete closure of the 

 coats of the ovule. During development the ovule 

 generally turns completely round upon itself, i.e. through 

 an angle of 180 degrees, so that the free end comes to 

 point towards the placenta on which the ovule is inserted. 

 In a few cases the ovule does not turn, and the free end 

 points away from the placenta. 



Development of Gametes. The pollen grain or 

 microspore when ripe consists of a cell with a thick 

 cutinised outer wall, often covered with projections 

 so that the surface is rough, and a thinner internal 

 wall consisting of cellulose. One or more interruptions 

 in the thick outer wall leave thin places covered only 

 by the thin inner wall. The cell is densely filled with 

 cytoplasm and contains a large conspicuous nucleus. 

 This nucleus divides into two : one daughter nucleus 

 is the nucleus of the ripe grain ; the other is somewhat 



