52 ELEMENTS OF PLANT ANATOMY. 
EPIDERMIS. 
This consists of a single layer of cells, the larger portion of 
which are similar in size and shape, hexagonal with somewhat 
shortened radial diameter, and with no intercellular spaces. 
This uniformity is broken by breathing pores or stomata, and 
by frequent outgrowths into trichomes. The outer or surface 
wall of the ordinary epidermal cell is generally thicker than 
the remaining ones, and is also cutinized and thereby rendered 
nearly impervious to air or water. The walls at right angles 
to this are thinner and often wavy in outline. ‘This latter 
characteristic is supposed to secure additional strength. In 
some instances all of the walls are thickened till scarcely any 
lumen remains, for example, in the epidermal cells of the pine 
leaf. 
STOMATA. 
These are special organs designed to allow and regulate the 
interchange of gases, and especially to facilitate the process of 
transpiration. The need for such an organ is seen from the 
facts that there are no intercellular spaces between the epider- 
mal cells, and that their outer walls are of such a nature as to 
admit very little air or water. The stoma consists of two 
peculiarly shaped cells, called guard cells, between which is an 
opening leading to a large air space in the parenchyma below. 
The cells lining this air space connect with the guard cells and 
may be considered a part of the stoma, though their origin is 
from a different meristem than that giving rise to the epidermis. 
The walls of these cells are thin and consist of pure cellulose, 
so that both air and water may easily pass through them. 
The guard cells originate early, being formed in the dermat- 
ogen layer itself. A cell divides into two unequal parts, the 
smaller one becoming the mother cell of the stoma. The 
1 In many instances a farther division takes place so that the epidermal 
cells of the stoma are more than the two guard cells. 
