120 ELEMENTS OF PLANT ANATOMY. 
Regarding the urmeristem, or the cell or cells from which 
the primary meristems are derived, it has been found here as in 
the stem tip, that the roots of most vascular cryptogams have a 
single apical or urmeristem cell. In some roots of this class, 
however, there has been found an urmeristem consisting of a 
few definitely arranged initial cells. There is the same un- 
certainty respecting the structure of the point of vegetation of 
phanerogamic roots, as there is in respect to that of the stems 
of this class of plants. Here, as in the stem, it has been found 
difficult to ascertain the exact manner of growth of the cells 
which give rise to the lasting tissue and what has been said of 
the Hanstein theory applies equally well to root and stem. 
The epidermal system consists of the so-called epiblem cells. 
These differ in various ways from the cells covering organs ex- 
posed to light and currents of air. They are more uniformly 
isodiametric, contain fewer stomata and have only one kind of 
trichome, which is known as the root hair. From a physiological 
standpoint these root hairs hold a very important position in the 
economy of the plant, as they are the organs by which it obtains 
food from the soil. They are simple in structure, consisting of 
a single epiblem cell prolonged into a thin-walled hair. They 
begin to develop a short distance from the root tip and extend 
entirely around the root, covering a portion several centimeters 
in length. As the root grows on from the tip, the hairs are 
constantly renewed, the oldest dying away in about the same 
proportion that the new hairs are added. 
Beginning now with the root of the vascular cryptogams, it 
may be added that in these the epiblem is not replaced by a 
periderm, although a well developed hypoderma frequently 
takes the place of the epiblem layer, which dies and disappears. 
The ground system consists generally of parenchymatic starch- 
holding cells, which are bounded outside by the hypodermal 
layer, and inside by the endodermis which surrounds the central 
vascular bundle or bundles. The vascular system is composed 
