76 



HOW GREEN PLANTS MAKE FOOD 



time drops of moisture are seen to 

 gather on the inside of the jar. If after 

 a few hours we weigh the potted plant 

 again, we find it weighs less than be- 

 fore. Obviously the loss comes from 

 the water vapor which has escaped from 

 stem, or leaves, or both. 



The Structure of a Leaf. — In the ex- 

 periment with the red ink and young 

 shoots we shall find that the fluid has 

 gone out into the skeleton or framework 

 of the leaf. Let us now examine a leaf 

 more carefully. It shows usually (1) a 

 flat, broad Made, which may take almost 

 any conceivable shape ; (2) a stalk, o:- 

 pefiole, which spreads out into veins in 

 the blade ; (3) stip'ules, a pair of out- 

 growths from the petiole at its base. 

 In many leaves the stipules fall off early. 

 Some leaves are compound, that is, each 

 of the little leaflike parts is in reality a 

 section of the leaf blade which is so 



deeply indented that it is cut away to the midrib or central vein, 



as in the rose leaf shown in the figure below. 



The Cell Structure of a Leaf. — The outer covering of a leaf, on 



both the upper and the lower surfaces, is called the epidermis, and 



is composed of large (in 



dicotyledons, irregular) cells. 



The under surface of most 



leaves, as seen through a micro- 

 scope, shows many tiny oval 



openings, called sto'mata (sing. 



sto'ma). Two guard cells, 



usually kidney-shaped, are 



found, one on each side of a 



stoma. By a change in the 



shape of these cells the stoma 



is made larger or smaller. 



Experiment to prove that 

 water vapor is given off from a 

 green plant. 



Compound leaf of rose, showing stipules si. 



