Leaves and Their Structures 



29 



of many grasses to curl up during a period of drought. Some- 

 times these colorless mesophyll cells have heavy walls and con- 

 tribute to the stiffening of the leaf. 



The chloroplasts. Of the several structures found within the 

 mesophyll cells, the most important in the primary process of 

 food manufacture are the chloroplasts. These are round or lens- 

 shaped plastids which contain a green coloring matter called 

 chlorophyll They are living organized bodies" of protoplasm and 

 multiply by division as the cells grow or new cells are formed. 

 Cells may contain many or only a few chloroplasts, and these may 

 be located deep within the leaf or near its surface. Since the 

 chloroplasts are the special apparatus which manufactures food, 

 the amount of food produced by a plant under any given condi- 

 tions is roughly proportional to their number. 



The chlorophyll. Chlorophyll is formed in the chloroplasts 

 and colors them green. It can be taken out of the chloroplasts 

 by puttmg the leaf in alcohol. After the chlorophyll is removed, 

 the chloroplasts remain in the cell, but they are then colorless and 

 the leaf is white or yellowish instead of green. 



For the development of chlorophyll, light is usually necessary. 

 The white sprouts on potatoes in a dark cellar, the blanching of 

 celery when the lower part of the leaves is covered, and the whit- 

 ening of young growing grass under a board are familiar evidences 

 of this fact. In the inner tissues of plants and in the underground 

 parts the plastids are usually colorless, but in many plants these 



Fig. 19. Illustrating terms used in describing the attachment of leaves to stems: A, with 

 margined petiole; B, petiole clasping; C, sessile; D, perfoliate; E, connate perfoliate. 



