8G 



PLANTS MAKE FOOD 



microscope usually shows numbers of tiny oval openings. These 

 are called stomata (singular stoma). Two cells, usually kidney- 

 shaped, are found, one on each side of the opening. These are 

 the guard cells. By change in shape of these cells the opening 

 of the stoma is made larger or smaller. Larger irregular cells 

 form the epidermis, or outer covering of the leaf. Study of the 



leaf in cross section shows that these 

 stomata open directly into air chambers 

 which penetrate between and around 

 the loosely arranged cells composing 

 the underpart of the leaf. The upper 

 surface of leaves sometimes contains 

 stomata, but more often they are lack- 

 ing. The under surface of an oak leaf 

 of ordinary size contains about 2,000,000 

 stomata. Under the upper epidermis 

 is a layer of green cells closely packed 

 together (called collectively the palisade 

 layer). These cells are more or less 

 columnar in shape. Under these are 

 several rows of rather loosely placed 

 cells just mentioned. These are called 

 collectively the spongy tissue. If we 

 happen to have a section cut through 

 a vein, we find this composed of a 

 number of tubes made up of, and strengthened by, thick-walled 

 cells. The veins are evidently a continuation of the tubes of the 

 stem out into the blade of the leaf. 



Evaporation of Water. — During the day an enormous amount- 

 of water is taken up by the roots and passed out through the 

 leaves. So great is this excess at times that a small grass plant 

 on a summer's day evaporates more than its own weight in water. 

 This would make nearly half a ton of water delivered to the air 

 during twenty-four hours by a grass plot twenty-five by one hun- 

 dred feet, the size of the average city lot. According to Ward, 

 an oak tree may pass off two hundred and twenty-six times its 

 own weight in water during the season from June to October. 



Section through the blade of a 

 leaf as seen under the com- 

 pound microscope. S, air 

 spaces, which communicate 

 with the outside air; V, vein 

 in cross section; *S.r., breath- 

 ing hole (stoma); E, outer 

 layer of cells; P, green cells. 



