ANATOMY OF TISSUES. a9) 
the Orobanchaceae there is only one for several square milli- 
meters. 
Epithel cells are similar in form to those of the regular 
epidermis, except that they are often extended externally into 
small papillae. They lack other trichomes, and stomata seldom 
occur. The wavy outline of the walls perpendicular to the 
surface is often seen here. 
Epiblem, or the covering of roots, differs but little from that 
of leaves and stems. In general the cells are less apt to have 
a shorter radial diameter, are very uniform in shape, and have 
no stomata, except in case of large roots where the external 
covering is such as to change to periderm. 
TRICHOMES AND SIMILAR FORMATIONS. 
Those cells or tissues which grow out from the surface of 
the plant are generally known as hairs, scales, thorns, etc. Ac- 
cording to their origin they may be classed under two heads, 
those which are derived from the dermatogen, and thus belong 
strictly to the epidermal system, and those coming in part from 
the layers under the dermatogen, thus belonging to one or both 
of the other systems as well as to the epidermal. By some 
authors, the former are named trichomes, the latter outgrowths. 
Those known as hairs may be either one- or several-celled ; 
stems and leaves are often thickly covered with these, and they 
vary greatly in form as well as in function. Some of the more 
complex have the form of stem and branches, appearing when 
macnified like miniature trees. . Some have a single cell for a 
stem on which rests a spherical head cell. Scale is a term given 
to a short perpendicular stem terminated with a layer of tissue 
parallel to the surface of the organ. All these generally have 
the nature of trichomes. It they are lignified, thus becoming 
hard and brittle, they are called bristles, while the term thorn 
is generally applied to such as contain elements of vascular 
