108 
PLANT STUDIES 
cesses of the lower groups that it is regarded as the only 
true root. It is quite uniform in structure, consisting of a 
tough and fibrous central axis surrounded by a region of 
more spongy structure. The tough axis is mostly made 
up of vessels, so called because they 
conduct material, and is called the 
vascular axis. The outer more spongy 
region is the cortex, which covers 
the vascular axis like a thick skin 
(see Fig. 105). 
One of the peculiarities of the 
root, in which it differs from the 
stem, is that the branches come from 
the vascular axis and burrow through 
the cortex, so that when the latter 
is peeled off the branches are left 
attached to the axis, and the cortex 
shows the holes through which they 
passed. It is evident that when such 
a root is absorbing, the absorbed ma- 
terial (water with various materials 
in solution) is received into the 
of the cortex the epi- , , . , 
dermis (e) which disap- cortex, through which it must pass 
to the vascular axis to be conducted 
to the stem. 
Another peculiarity of the root 
is that it elongates only by growth at the tip, while the 
stem usually continues to elongate some distance behind 
its growing tip. In the soil this delicate growing tip is 
protected by a little cap of cells, known as the root-cap 
(see Fig. 105). 
FIG. 105. A longitudinal 
section through the root 
tip of shepherd's purse, 
showing the central vas- 
cular axis (p ! \ surrounded 
by the cortex (p), outside 
pears in the older parts of 
the root, and the promi- 
nent root-cap (c). 
