THE LEAF 



165 



Fig. 220. — a small 

 piece of the under epider- 

 mis of an oak leaf, highly- 

 magnified to show the 

 stomata, g, and minute 

 hairs, h. 



182. Distribution of stomata. — While stomata are gen- 

 erally more abundant on the under side of leaves, this is not 

 always the case. In vertical leaves, like those of the iris, 

 which have both sides equally exposed to the sun, they are 

 distributed equally on both sides. In plants like the water 

 lily, where the under surface lies upon the 



water, they occur only on the upper side. 

 Succulent leaves, as a general thing, have 

 very few, because they need to conserve 

 all their moisture. Submerged leaves 

 have none at all ; why ? 



183. Minute study of a leaf epidermis. 

 — Place a bit of the lower epidermis of 

 a leaf under the microscope, and examine 

 with a high power. It will appear, if a 

 monocotyl, to be composed of long, flat, 

 rectangular spaces (Fig. 221) ; if the leaf 

 of a dicotyl is used, they will be more or less irregular (Fig. 

 220), with the outlines fitting into each other like the tiling 



of a floor or the blocks of a Chinese puzzle. 

 These spaces are the cells of the epidermis, 

 and the lines are the cell walls. Can you 

 find any of the cell contents? The cell 

 sap is not often visible; do you see the 

 nuclei ? Can you give a reason why the 

 epidermal cells are so thin and flat ? Be- 

 tween some of the cells you will see two 

 kidney-shaped bodies placed with their 

 concave sides together so as to leave a 

 lenticular opening between them. This 

 is a stoma, and the kidney-shaped bodies 

 (Figs. 218, 219) are guard cells. They 

 are given this name because they open 

 or close the mouth of the stoma. If 

 you will imagine a toy balloon made in the form of a hol- 

 low ring, like the tire of a bicycle, you can easily see, from 



Fig. 221. — Under 

 epidermis of an oat leaf, 

 showing stomata. 



