48 
PLANT MORPHOLOGY. 
secreting. Secreting hairs possess a head-like apex, 
consisting of one or more cells, and they secrete 
oil, mucilage and other substances. (For the various 
forms of these hairs, see plates). 
Stomata. — Epidermal cells have still another modifi- 
cation in the development of a special apparatus for 
regulating transpiration, in particular: From a single 
epidermal cell two cells are formed, which are known 
as guard cells, the whole being known as a stoma. The 
adjoining walls of the guard cells are alike in trans- 
verse section, but vary in shape in different plants ; 
they are more or less elastic, and when the cells are 
turgescent, as when there is an abundance of water 
and root pressure is strongest, the contiguous radial 
walls of the guard cells recede from each other, form- 
ing an opening between the cells, thus permitting the 
easy passage of both air and water. 
The guard cells may be slightly raised above or 
sunk below the surrounding epidermal cells, the 
number of the latter being characteristic for certain 
plants. 
In surface view the stomata may be elliptical or 
circular. They occur in the largest numbers on the 
blades of foliage leaves, being more numerous on the 
under surface, except in aquatic plants where they 
occur only upon the upper surface. 
Water Pores. — Near the margin of the leaf and 
directly over the ends of conducting cells, not infre- 
quently occur stomata, in which the function of open- 
ing and closing is wanting, and which contain in the 
cavity below the opening water and not air, as in true 
stomata. These are known as water pores, and they 
give off water in the liquid form, the drops being visi- 
ble on the edges of the leaves of nasturtiums, fuchsias, 
roses, etc., at certain times. 
