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 syenn 



Three stages in the germination of the pollen UUCieus. 



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

 the sperm nuclei. Drawn under the com- 

 pound microscope. Flower. — It 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, 



the pollen tube grows downward Fertilization of the ovule. A flower 

 , , ^ cut down lengthwise (only one 



through the spongy center of the side shown). The pollen tube is 



style, follows the path of least resist- ^f^"" entering the ovule, a, an- 



. , 1 . , , . , , ther ; /, filament ; pg, pollen gram; 



ance to the space Wlthm the ovary, s, stigmatic surface ; pt, poUen 



and there enters the ovule. It is tube ; sf, style ; o, ovary ; m, micro- 



. . . ^ pyle; sp, space withm ovary; 



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



