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 
Fic. 230.— A much enlarged surface view 
of the lower epidermis of a Bean leaf. u, of a number of layers of 
epidermal cells; s, stomata; g, guard cells; cells which surround the 
i, slit-like opening between the guard cells smaller conductive 
through which gases pass. 
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. 
