300 INTRODUCTION TO BOTANY 



occurred, two cells, and not one, are to be seen within the wall 

 of the pollen grain. The pollen grains must be placed upon 

 the tip of the stigma before further development takes place. 

 The process of proper placing of the pollen grain is known 

 as pollination, to which a chapter has already been given 

 (Chapter X). 



285. The pistil. The pistil, or carpel, consists of three parts : 

 the enlarged base, which is called the ovary, in which the 

 ovules or developing seeds are borne ; the elongated portion 

 above the ovary (the style) ; the tip of the style, usually more 

 or less expanded (the stigma). The stigma, when ripe, is ordi- 

 narily covered by a sticky fluid which causes the pollen to 

 adhere to it. Through the style the pollen tube grows to the 

 ovules. The ovules may be borne singly or many together in 

 the ovary. They may be attached on the bottom, at the sides, 

 at the top, or on a central axis of the ovary. While the ovules 

 differ so much in their position, there is a general uniformity 

 in their individual structure. The surface of the ovule consists 

 of one or two integuments, which at the tip do not quite cover 

 the inner tissue of the ovule. This open tip is the micropyle, 

 which means " little gate." Similar structures of the ovules of 

 gymnosperms have already been mentioned (sect. 274). Within 

 the tissue of the ovule is the embryo sac (fig. 231). When fully 

 formed the embryo sac incloses seven cells. At the micropy-. 

 lar end of the sac are three cells, the central one of which is 

 the egg, while those at the sides of the egg are the helper cells, 

 or synergids. These helper cells may nourish the egg or pos- 

 sibly may assist the development of the entering pollen tube. 

 In the end of the sac opposite the micropyle are the antipodal 

 cells, which usually disappear soon after they are formed ; and 

 in the central part of the sac is the endosperm cell, which 

 later grows and produces the food or endosperm of the seed. 



286. The pollen tube and fertilization. After pollen grains 

 have fallen upon the stigma, the outer wall of the pollen grain 

 breaks, and from the inner wall the beginning of the pollen 

 tube extrudes. The tube tip enters the stigriiatic tissue and 



