54 



FUNCTIONS OF LIVING THINGS 



Three stages in the germination of 'the pollen 

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

 the sperm nuclei. Drawn under the com- 

 pound microscope. 



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 thfe pollen tube ; see 

 Figure) . One of the nuclei 

 which grows into the pollen 

 tube is known as the sperm 

 nucleus. 



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 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 



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 



believed that some chemical influ- 



Fertilization of the ovule. A flower 

 cut down lengthwise (only one 

 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; 

 e, egg cell ; P, petal ; S, sepal. 



