Biology of Higher Plants — Anatomy and Physiology 225 



sists of a stalklike filament and an enlarged anther at its free end. The 

 hollow anther contains microspores (pollen grains). The pollen grains 

 develop into male gametophytes. . In the center of the flower are whorls 

 of small, green, pointed, oval carpels. Each carpel consists of ( 1 ) a 

 basal saclike ovary, which contains a female ovule; (2) a hooklike, 

 pointed, distal end, known as the stigma to receive pollen; and (3) a 

 style, which connects the stigma with the ovary. The stigma, style, and 

 ovary constitute the pistil. The ovule of the flower is attached to the 

 ovary and consists of a megasporangium (nucellus). The megasporan- 

 gium forms four megaspores, only one of which develops into a female 

 gametophyte. The union of a male gametophyte with the female game- 

 tophyte within the ovary constitutes fertilization, which results eventually 

 in the development of a seed. 



Pollen oraiTi 



ToUen "tube 



(J^ilaTneTti- 

 In'neT IriteaumeTit— 



Duter iTtequmerit 

 ^GCjaqaTn^ophyte 



Stiana 

 Style 



Ovary 



Petal 

 Ovule 



Sepal 



FuNiculus 

 Micropyle 

 Receptacle 



Fig. 71. — Diagram of a flower (in section) to show parts and fertilization. 

 The anther contains pollen grains which escape to the stigma. The stamens are 

 also known as microsporophylls. The pistil consists of the stigma, style, and 

 ovary. The pistil may consist of a single, basal carpel (megasporophyll, or ovule- 

 bearing "leaf") or of two or more fused carpels. The micropyle permits the en- 

 trance of the pollen tube into the female megagametophyte (egg sac). Ovules 

 later become seeds. The pollen tube and its contained pollen grains constitute 

 the male microgametophyte. The tube nucleus (of the pollen grain) germinates 

 and forms a pollen tube through which the generative nucleus (of the pollen) 

 follows to the ovule where fertilization occurs. The generative nucleus divides 

 into two sperm (male gametes); one sperm unites with the egg to form the 

 embryo; the other sperm fuses with the two polar nuclei (shown at top of mega- 

 gametophyte) to form the food endosperm. 



