THE TRUE OR PRIMARY ROOT. 125 
Section 2. Tur Root or DeEscEeNDING AXIS. 
Tue root is defined as that part of the axis which at its first 
development in the embryo takes an opposite direction to the 
stem, avoiding the light and air, and hence called the descend- 
ing axis, and fixing the plant to the soil or to the substance upon 
which it grows, or suspended in the water when the plant is 
placed on the surface of, or in, that medium. That part of the 
root which joins the stem is called the base, and the opposite 
. extremity the apex. 
We distinguish two varieties of roots, namely, the Tvrwe or 
Primary, and the Adventitious or Secondary. 
RTE VEEL 
Fig. 247. Root apex (Polygonum Fagopyrum), median longitudinal section, 
v. Rudiment of a vessel. pc, Pericambium—the outside boundary of the 
plerome or procambium. e, Dermatogen, between which and pe is the 
periblem. h, Root-cap or pileorhiza. «a. Apical cells, After De Bary. 
1. TRvuE oR Primary Root.—The true root, which, except in 
rare cases, can only exist in Dicotyledons (page 134), is formed 
at first by additions made within the extremity of the radicle 
(fig. 248, a) of the embryo; and the mode in which it takes place 
may be thus stated :—Growth commences by the multiplication - 
of cells by division just within the apex of the radicle; the mass 
of cells thus formed becomes gradually differentiated into three 
layers, an outer, inner, and intermediate. From the inner layer, 
