FUNCTIONS OF LIVING THINGS 



49 



pear to be everywhere the same. 



The leaf is much thinner and 



more delicate in some parts 



than in others. Holding the 



flat, expanded blade away from 



the branch is a little stalk, 



which extends into the blade of 



the leaf. Here it splits up into 



a network of tiny " veins ' 



which evidently form a frame- 

 work for the flat blade some- 

 what as the sticks of a kite 



hold the paper in place. If we 



examine under the compound 



microscope a thin section cut 



across the leaf, we shall find 



that the veins as well as the 



other parts are made up of many tiny boxlike units of various 



sizes and shapes. These smallest units of building material of the 



plant or animal disclosed by the 

 compound microscope are called 

 cells. The organs of a plant or 

 animal are built of these tiny 

 structures. 



Tissues. 1 - The cells which 

 form certain parts of the veins, 

 the flat blade, or other portions 



Section through the blade of a leaf, e, 

 cells of the upper surface ; d, cells of the 

 lower surface ; i, air spaces in the leaf ; 

 v, vein in cross sections ; p, green cells. 



cs. 



Several cells of Elodea, a w^ater plant. 



V V* t * 1 - V>VjAAO \Jt. J-J l/_/*.*/ "^-j C4J ^"- _ ,, -| 



chi., chlorophyll bodies; c.s., cell sap; of the plant, are otten tound in 



groups or collections, the cells 

 of which are more or less alike 



c.w., cell wall ; n., nucleus ; p. proto- 

 plasm. The arrows show the direc- 

 tion of the protoplasmic movement. 



1 To the Teacher. - - Any simple plant or animal tissue can be used to demon- 

 strate the cell. Epidermal cells may be stripped from the body of the frog or 

 obtained by scraping the inside of one's mouth. The thin skin from an onion 

 stained with tincture of iodine shows well, as do thin sections of a young stem, as 

 the bean or pea. One of the best places to study a tissue and the cells of which 

 it is composed is in the leaf of a green water plant, Elodea. In this plant the cells 

 are large, and not only their outline, but the movement of the living matter within 

 the cells, may easily be seen, and the parts described in the next paragraph can 

 be demonstrated. 



HUNTER, CIV. BI. - -4 



