ANATOMY OF TISSUES. _ 57 
The only form of trichome from the epithel is in the.shape 
of papillae which often reach a considerable length and serve to 
give the beautiful velvety appearance to petals like those of the 
pansy. 
The epiblem produces the form of trichome known as the 
root hair, whose function is so important that it has been classed 
as one of the essential organs of the plant. These hairs consist 
of a single, long, and generally unbranched cell. They develop 
near the tip of the rootlet and always on that portion which is 
just about to cease its growth in length, in this respect dif- 
fering greatly from those of the epidermis proper, as these begin 
their development long before the organ has reached its growth. 
They are very numerous and extend over a zone averaging five 
or six millimeters long. This zone remains about the same 
length, as the older hairs are constantly dying and new ones are 
forming toward the tip. In this way the nutritive matter con- 
tained in the ground is as exhaustively covered by the growing 
rootlets as though each had an independent power of motion. 
The manner of their action in taking up this nourishment, as 
well as the performance of the other functions of hairs, belongs 
to the province of physiology. | 
The outer cuticularized wall of the iden mis is often covered 
with a thin layer of wax. This is generally absent from the 
guard cells of the stoma, and has never yet been found on 
trichomes or outgrowths. In appearance it is either that of a 
soft bloom as on plums, or of a crust as on the bay berry. Ac- 
cording to the structure of the wax these coatings may be clas- 
sified as follows: granular, rod-formed, crustaceous, and glazing. 
The granular is formed by extremely small particles which give 
the appearance of bloom. The rod-formed consists of parallel 
rods standing at right angles to the surface of the plant. This 
form is of rare occurrence, the best known examples being found 
in the culm of the sugar-cane and in the leaves of the Brazilian 
wax palm. The crustaceous resembles the granular except that 
