LEAVES IN THEIR RELATION TO LIGHT 7 



Guard-cells. These little openings in the epidermis of the 

 leaves are guarded by special cells, called ' 'guard-cells. " They 

 differ from the other cells in appearance by being of a different 

 shape and containing a little chlorophyll. They are long and 

 slender, but curve so that two of them surround each opening. 

 The pair may be said to form the lips of the little mouths in the 

 leaves. They are sensitive to moisture and heat. They are called 

 guard-cells because they control the size of the openings. When 

 the weather is favorable they leave the mouths open, but when 

 the weather is dry and hot they close the mouths to help prevent 

 the evaporation of water from the inner parts of the leaf (Fig. 4,S) . 



FIG. 4. A section of green leaf under the microscope, showing air spaces (A, A), a breath- 

 ing pore at S, epidermal (skin) cells at the top and bottom, palisade cells below the upper skin, 

 and the loose cells just below these. Chlorophyll grains are seen in all but the epidermis. 



Leaves in Their Relation to Light. Probably the chief office 

 of the leaves of the plant is to manufacture starch from carbon 

 dioxide and water in the presence of sunlight. The leaves on a 

 plant are therefore arranged in such a way as to gather as much 

 light as possible. Many different plans or devices to accomplish 

 this end are found among plants. The leaves of the corn plant 

 turn their broad surface toward the light and curve over to catch 

 as much as possible. The under leaves of many plants may wither 

 and drop off when younger leaves from above have completely 

 shaded them. Leaves on vines climbing on a wall often spread out 



