246 



LEAVES 



thin film called cuticle on the outer surface of the epidermis. 

 Being a waxy substance and impervious to water, it makes the 

 epidermis more protective against the loss of water. Sometimes, 

 as in Cabbage, a waxy substance that can be easily rubbed off is 

 deposited on the outside of the epidermis. Frequently, as the 

 common Mullein and some Thistles illustrate, the epidermis 

 develops hairs, which are sometimes so long and dense as to give 

 the leaf a white woolly appearance. Some leaves, as those of the 

 Mints illustrate, have glands that secrete fluids to which the odor 

 of the plant is due. Some plants are cultivated on account of 

 the commercial value of their glandular secretions. In many 

 cases the epidermal secretions of leaves, if not unpleasant to 

 the sense of smell, are to the taste, and therefore may protect 

 plants against being eaten by stock. In fact all of the epidermal 



modifications are sup- 

 posed to be related to the 

 protection of the plant 

 in one way or another. 



Mesophyll. The 

 mesophyll, as the term 

 suggests, occupies the 

 middle region of the leaf 

 and its distinctive fea- 

 ture is its green color 

 upon which the power 

 to manufacture food de- 

 pends. It is soft spongy 

 tissue and is composed 

 of a number of layers of 

 cells which surround the 

 smaller conductive 

 tracts and fill the spaces 

 between. It is so delicate in structure and so closely joined 

 to the epidermis that in most leaves it is difficult to remove the 

 epidermis without tearing away some of the mesophyll. 



Cellular Structure of Leaves 



To learn the finer structural features of leaves, a microscope 

 must be employed, so that the cells of the different leaf tissues 

 may be studied. 



FIG. 230. A much enlarged surface view 

 of the lower epidermis of a Bean leaf, show- 

 ing three stomata. Notice each stoma con- 

 sists of two crescent-shaped cells (guard 

 cells) containing chloroplasts and so fitted 

 together as to enclose a slit-like opening. 

 After Charlotte King. 



