32 BOTANY. [CHAP. i. 



having the ovules concealed in an ovary, hence fertiliza- 

 tion is indirect ; the ovary terminates in a structure of 

 variable form, called the stigma, the function of which 

 is to receive the pollen, which then germinates and finds 

 its way indirectly to the ovules for effecting fertilization. 



The cells, or ultimate structures of plants, have been 

 incidentally alluded to, but being of primary importance 

 and indispensable to a clear comprehension of the work- 

 ings of plant life, a more detailed account of the most 

 prominent features of the cell and its modifications is 

 necessary. 



The cells of plants are, as a rule, very minute and 

 invisible to the unaided eye, and when fully developed 

 present the following structures, proceeding from with- 

 out : 



(1) An external continuous pellicle or protective 

 layer, known as the cell-wall, which although showing 

 no visible perforations is pervious to liquids and gases. 

 The cell- wall is composed of a substance called cellulose. 



(2) Closely applied to the inner surface of the cell- 

 wall is a somewhat irregular layer of protoplasm the 

 living portion of the cell. 



(3) The nucleus, a small, differentiated portion of the 

 protoplasm in which it is embedded. 



(4) The sap-cavity or central cavity bounded by the 

 layer of protoplasm. 



The cell- wall is at first very thin, and of equal thick- 

 ness throughout, but this condition rarely continues 

 throughout the entire existence of the cell ; in many 

 pollen grains, also the spores of many cryptogams, the 

 outside of the cell- wall becomes ornamented with warts, 

 spines, or ridges that often anastomose to form a net- 



