LEAVES AND THEIR WORK 



121 



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 lacking. The under surface of an oak leaf of ordinary size 

 contains about 2,000,000. Under the upper epidermis is a layer 

 of green cells closely packed together (called collectively the pali- 

 sade 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 parenchyma. If we hap- 

 pen 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 i I[MI nl 



a continuation of the tubes of 

 the stem out into the blade 

 or the leaf. 



Starch made by a Green 

 Leaf. If we examine the 

 palisade layer of the leaf, we 

 find cells which are almost 

 cylindrical in form. In the 

 protoplasm of such cells are 

 found a number of little green 

 colored bodies, which are known 

 as chloroplasts or chlorophyll 

 bodies. If we place the leaf in 

 wood alcohol, we find that the 

 bodies still remain, but that 

 the color is extracted, going 

 into the alcohol and giving to 

 it a beautiful green color. The chloroplasts are, indeed, simply 

 , part of the protoplasm of the cell colored green. If the plant 

 1 is kept in the sun, the chloroplasts keep then* green color, but in 

 the dark this color is gradually lost. These bodies are of the 

 greatest importance directly to plants and indirectly to animals. 



A hydrangea plant, upon the leaves of 

 which disks of cork have been pinned in 

 order to exclude sunlight from the leaf. 



