EPIDERMIS 245 
The veins run through the mesophyll of the leaf and form a 
frame-work, which with its numerous fine branches, known as 
veinlets, resembles a fine-meshed net when a leaf is held up to the 
light. The finest veinlets can be seen only with the aid of the 
microscope. It is by this profuse branching of the veins and 
veinlets that all parts of the mesophyll are brought into direct 
contact or close relation with the conductive tissues. Although 
the larger veins are often thicker than the leaf and form prominent 
ridges on its under side, they taper down to the veinlets which 
are well buried within the mesophyll. 
The character of the veining, known as venation, differs con- 
siderably in different leaves and there are two types of venation 
of some prominence. (Fig. 229.) One is the parallel-veined type, 
in which there are a number of parallel principal veins with 
obscure cross veins. This type is familiar in Corn leaves and 
is characteristic of monocotyledonous plants in general. The 
other is the net-veined type, in which there is one or only a few 
principal veins and their branches so fork and join each other 
that a quite noticeable network of veins and veinlets is formed 
as Maple or Oak leaves will illustrate. This type is characteristic 
of Dicotyledons. Many leaves have one large primary vein 
called midrib. Some leaves have a number of primary veins, 
which are then called nerves, and a leaf is described as three- 
nerved, five-nerved, or whatever the number may be. 
Epidermis. — The epidermis forms a continuous covering over 
the leaf except where it is broken by the openings of the stomata. 
The stomata, although microscopical in size, afford the openings 
necessary for the exchange of gases between the interior of the 
leaf and the outside air. The epidermis is usually one layer of 
cells in thickness, but in some leaves, especially those of dry 
regions, it is often thicker. Except in the cells of the stomata, 
the epidermis usually contains no pigments, although it may 
appear to have since the green color of the mesophyll beneath 
readily shows through it. Sometimes the epidermis contains a 
red pigment, called anthocyan, which causes a part or all of the 
leaf to be red. Red pigment is often noticeable in the leaves of 
Sorghum and is common in some greenhouse plants of which 
the Wandering Jew is a familiar exampie. The epidermis when 
smooth has the appearance of having been greased, due to the 
deposits of cutin in its outer cell walls. Cutin usually forms a 
