126 GROWTH OF THE ROOT. 
which is termed the plerome or procambium (fig. 247), is subse- 
quently developed the fibro-vascular portion of the root, 1, 
the cortical layers being formed from the intermediate layer or 
periblem, whilst the outer single layer of cells, known as the 
dermatogen, e, in addition to giving rise to the epidermis, forms 
the cap-shaped mass of tissue called the root-cap or pileorhiza, 
h, by which the growing apex of the root is always clothed. 
All roots (fig. 248, a) and the branches of a root grow in length 
in a similar manner to the radicle as above described; hence 
roots do not grow throughout their entire length like stems, but 
only within their extremities, which are continually pushed 
forward and renewed. Thus the apex of the root is always 
clothed by a layer of denser tissue which is commonly known 
Fre. 249, 
eee 
NG 
= 
i: 
4 
Fig. 248. Young root of the Maple, magnified. a.The part where growth 
is taking place. 6. The original extremity. c,c. Fibrils or root-hairs. 
After Gray. Fig. 249. Highly magnified vertical section of an Orchis 
root. sp. The so-called spongiole, c, c, Parenchymatous cells. fv. Wood- 
cells and vessels. 
as the root-cap (fig. 248, 6). All the branches of a root are 
likewise terminated by a similar cap (fig. 250, h, h). This cap 
forms in fact a sort of protecting shield to the young extremities 
of the root; and its external cells are commonly thrown off as new 
cells are formed within them. (See also Development of Roots, 
in Physiological Botany.) These cap-like coverings at the extre- 
mities of the root were formerly regarded as special organs, and 
called spongioles or spongelets (fig. 249, sp), under the idea that 
they absorbed fluid for the use of the plant, in the same manner 
as a sponge sucks up water. But it will be seen from the above 
description of the growth of roots that such structures have no 
existence. Roots increase in diameter by the formation of annual 
layers of wood, much in the same manner as stems. 
At first the elongating growing extremities of the root con- 
