EPIDERMIS 245 



The veins run through the mesophyll of the leaf and form a 

 frame-work, which with its numerous fine branches, known as 

 vemlets, 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 charactef 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 

 1-egions, 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 example. 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 



