PROBLEM I. 



Upper surface of leaf 



Spongy 



cells 



The Part Leaves Play in Making Food 



Upper epidermis 



139 



Side view of stoma 



ein fconducf- 

 ing cells of leaf) 



Lower epidermis 

 Chlorophyll bodies 



Fig. 164 Lookhjg into a leaf. Which two layers 

 lie between upper and lower epideriuis? What 

 is in the "empty" spaces between the spongy 

 cells? 



In most plants the two cells enclosing 

 the stoma, called guard cells, ordinarily 

 take the shape of a half doughnut during 

 the daytime, making the opening larger. 

 At night they straighten out, making the 

 stoma smaller. The opening is never shut 

 completely. The stomata are extremely 

 numerous. A medium-sized cabbage leaf 

 probably has about 1 1 ,000,000 stomata, 

 and a sunflower leaf may have up to 

 about 1 3,000,000. In most land plants the 

 stomata are more numerous on the lower 

 side; in floating leaves they are more 

 numerous on the upper side; none occur 

 on leaves that grow submerged in water. 

 To determine the number of stomata, do 

 Exercise 2. 



What makes the leaf green? As you 

 study a fresh section of a leaf under the 

 microscope, what strikes you most is 

 the bright green color, particularly of 

 the palisade cells. There is usually less 

 green color in the spongy cells and the 

 epidermis has faint traces of green in the 

 guard cells only. This color is caused by 

 the presence of the tiny green bodies 



Stoma Guard cell Epidermal cell 



Fig. 165 A tiny piece of lower epidermis. How 

 many stomata do yon see? Are they open or 

 closed? What are the cells on each side of a 

 sterna called? 



within the cytoplasm, the chloroplasts. 

 They are often oval in shape. They are 

 made of protoplasm containing several 

 coloring matters, one of which is bright 

 green in color. This is the chlorophyll. 

 Chloroplasts are found not only in leaf 

 cells but in all parts of the plant which 

 look green. Fruits are green before they 

 ripen and stems always have chlorophyll 

 when they are young; sometimes they 

 keep their green color throughout the 

 life of the plant. You probably saw 

 chloroplasts in Elodea cells when study- 

 ing the preceding unit. 



(Optional) Chloroplasts, In most green 

 cells the chloroplasts are small globular 

 bodies as indicated in Figure 164, but in 

 some cells they are large and of unusual 

 shapes. In the alga Spirogyra, the chloro- 

 plasts are spiral bands. There may be one 

 or several in each cell. In certain other 

 algae the chloroplasts are star shaped. 

 But a chloroplast is always a living struc- 

 ture which under certain conditions be- 

 comes very active. The chlorophyll 

 itself is a mixture of two compounds. 



