REPRODUCTION IN FLOWERING PLANTS 225 



occurs when the pollen grain of a given flower lights on the sticky- 

 surface of the stigma of a flower of the same kind. 



If we cut the pistil of a large flower (as a lily) lengthwise, we 

 notice that the style appears to be composed of rather spongy ma- 

 terial in the interior ; the ovary is hollow and is seen to contain 

 a number of rounded structures which appear to grow out from the 

 wall of the ovary. These are the o'vules. The ovules, under 

 certain conditions, will be- 



— Pollen grdin on sticky 

 -.stigma has developed 

 ....a pollen tube 



ivhich has groyrn 



down ibe 



style totheoYdry 



come seeds. An explana- 

 tion of these conditions 

 may be found if we ex- 

 amine, under the micro- 

 scope, a very thin section 

 of a pistil, on which pollen 

 has begun to germinate. 

 The central part of the 

 style is found to be either 

 hollow or composed of a 

 soft tissue through which 

 the pollen tube can easily 

 grow. Upon germination, 

 the pollen tube, carrying 

 the sperm nuclei with it, 

 grows downward in the 

 spongy center of the style, 

 follows the path of least 

 resistance to the space 

 within the ovary, and there 

 enters the ovule. It is be- 

 lieved that some chemical influence attracts the pollen tube. One 

 of the sperm nuclei penetrates an ovule by making its way through 

 a little hole called the mi'cropyle, and then toward an area of pro- 

 toplasm known as the em'bryo sac. The embryo sac is an ovoid 

 space, microscopic in size, filled with semi-fluid protoplasm con- 

 taining several nuclei. (See figure.) One of the nuclei, with 

 the protoplasm closely surrounding it, becomes the egg cell or 

 female gamete. It is to this cell that one of the sperm nuclei of 

 the pollen tube grows, ultimately uniting with it. The union of 



Orule 

 Sperm cell 



Embryo SBC mth 

 several nuclei; 

 one, the dark, 

 is egg cell 



Fertilization of the ovule. 



