6 PLANT LIFE 



influence of sunlight and the work of the cell fluid, called protoplasm, 

 if chlorophyll is there, starch will be formed. Oxygen is given off 

 in the process of making starch (Fig. 3). Starch is the staple food 

 of the plant. Thus the leaves acquire from the air and develop 

 much of the substance of which plant tissues are formed. 



Breathing. The work of acquiring the materials and manu- 

 facturing starch is related to digestion. Leaves also perform a 

 function which is related to breathing in animals. The breathing 

 process is carried on through the surface of the leaves both day 

 and night. It is exactly analogous to the breathing in animals. 

 In this process the plant uses the oxygen from the air and gives 

 off carbon dioxide to the air. But in the daytime, when the sun 

 is shining, the leaves are carrying on the digestive process more 

 rapidly than they are breathing. The net result of the two 

 lines of work is that the plants really give off more oxygen than 

 they take up and they use up more carbon dioxide than they 

 exhale. In the dark the starch-forming process ceases, but breath- 

 ing continues. Those plants in the house breathe out carbon 

 dioxide at night just as people do. 



The object of the breathing in plants is similar to that in 

 animals. Because of this work of the leaves they may properly be 

 called the "lungs" of the plant. 



Leaf Structure. A fresh green leaf ma*y not be very thick, 

 but it is composed of a number of layers of cells. Those in- 

 side are very soft with very thin walls. These contain the live 

 cell fluid, called protoplasm, in which is the green chlorophyll. 

 Among these soft cells, called parenchyma, there are spaces for 

 air (Fig. 4). 



Outside of the soft cells, that is on the upper and lower surface 

 of the leaf, there are layers of cells called epidermis, corresponding 

 to skin. These epidermal cells do not contain any green coloring 

 matter. They are transparent, and the green chlorophyll from 

 the inner walls shows through, giving the leaf a green appearance. 

 Sunlight shines through the epidermis during the starch-making 

 process. 



Stomates. Numerous small openings are found in the epi- 

 dermis of the leaves. In many kinds of leaves they are more 

 abundant on the under surface. It is estimated that about 150,000 

 to the square inch exist on the under side of an apple leaf. These 

 openings, called stomates or mouths, are for the purpose of breath- 

 ing and the taking in of carbon dioxide for the making of starch. 



