36 



FLOWERS AND THEIR WORK 



be found to have germinated, a long, threadlike mass of protoplasm 

 growing from it into the sugar solution. The presence of this 

 sugar solution was sufficient to induce growth. When the pollen 

 grain germinates, one of the nuclei enters the threadlike growth 

 (this growth is called the pollen tube ; see Figure). The cell 

 which grows into the 'pollen tube is known as the sperm cell. 



Fertilization of the Flower. — 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 

 material in the interior; 

 the ovary is hollow and is 

 seen to contain a num- 

 ber of rounded structures 

 which appear to grow out 

 from the wall of the ovary. 

 These are the ovules. The 

 ovules, under certain con- 

 ditions, become seeds. An 

 explanation of these con- 

 ditions may be had if we 

 examine, 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 grows downward through the spongy center of the style, fol- 

 lows the path of least resistance to the space within the ovary, and 

 there enters the ovule. It is believed that some chemical influence 

 thus attracts the pollen tube. When it reaches the ovary, the 

 sperm cell penetrates an ovule by making its way through a little 

 hole called the micropyle. It then grows toward a clear bit of proto- 

 plasm known as the embryo sac. The embryo sac is an ovoid space, 

 microscopic in size, filled with semifluid protoplasm containing sev- 

 eral nuclei. (See Figure.) One of the nuclei, with the protoplasm 

 immediately surrounding it, is called the egg cell. It is this cell that 

 the sperm cell of the pollen tube grows toward; ultimately the 

 sperm cell reaches the egg cell and unites with it. The two cells, 



Three stages in the germination of the pollen 

 grains ; one of the nuclei in (3) is the sperm 

 cell nucleus. Drawn under the compound 

 microscope. 



