54 



FUNCTIONS OF LIVING THINGS 



The presence of this sugar 

 solution was sufficient to 

 induce growth. When the 

 pollen grain germinates, 

 the nuclei enter the thread- 

 like growth (this growth is 

 called the pollen tube ; see 

 Figure). One of the nuclei 

 which grows into the pollen 

 tube is known as the sperm 



Three stages in the germination of the pollen nucleus. 



grain. Th? nuclei in the tube in (3) are Fertilization of the 



the sperm nuclei. Drawn under the com- T 



pound microscope. 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 in- 

 terior ; 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 ovules. The ovules, under 

 certain conditions, will become seeds. 

 An explanation of these conditions 

 may be had if we examine, under 

 the microscope, a very thin section 

 of a pistil, on which pollen has be- 

 gun 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, 

 th^ pollen tube grows downward 

 through the spongy center of the 

 style, follows the path of least resist- 

 ance to the space within the ovary, 

 and there enters the ovule. It is 





side shown). The pollen tube is 

 seen entering the ovule, a, an- 

 ther ; /, filament ; pg, pollen grain; 

 s, stigmatic surface pt, pollen ; 

 tube ; st, style ; o, ovary ; m, micro- 



pyle ; sp, space within ovary ; 



believed that some chemical influ- e, egg cell ; P, petal ; s, sepal. 



