28 A TEXTBOOK OF GENERAL BOTANY 
type the veins are parallel (Fig. 16), while in the other they 
form a network (Fig. 16) and are said to be netted. These two 
types of venation are characteristic of the two great divisions of 
flowering plants, monocotyledons and dicotyledons. Monocoty- 
ledons are plants with one seed leaf, while dicotyledons have two 
seed leaves. Parallel veins are characteristic of monocotyledons, 
and netted veins of dicotyledons, although some monocoty le- 
donous leaves have netted veins. 
MICROSCOPIC ANATOMY OF LEAVES 
Epidermis. The leaf is covered on both surfaces by a single 
layer of cells, known as the epidermis (Fig. 17). The outer walls 
of the epidermis are usually thickened and impregnated with a 
waxy substance called cutin. 
Walls that are impregnated 
with cutin are said to be 
cutinized. Not only do the 
outer walls of the epi- 
dermis contain cutin, but 
their outer surfaces are fre- 
quently covered by a layer 
of cutin which is known 
as a cuticle. Water passes 
readily through cellulose, 
and so, if the outer walls 
of the epidermis were thin 
cellulose walls, their outer 
surfaces would be wet and 
Fig. 17. Cross section of a portion of the water would evaporate from 
blade of a leaf (Hibiscus). (x 285) them in large quantities. 
The evaporation of water 
from plants is called transpiration. As the outer walls of the 
epidermis are thick and waxy, water does not pass through them 
readily, and the transpiration of water from the surface of the 
epidermal cells is greatly reduced, only very small quantities 

