14 THE FLOWERING PLANT. 
and amount of branching; (3) the thickness. The branching is 
always monopodial, that is to say, the branches are not formed 
by forking, but arise as outgrowths from the side of a pre-existing 
axis. "The thickness partly depends upon the duration of life. In 
annuals, which live for one year only, the parts of the root system 
are usually of no great thickness. vennials, which live for two 
years, develop in many cases a greatly dilated tap-root, which 
contains a store of reserve material that is used up the second 
year, when flowering takes place, as in the carrot, turnip, and 
radish. Perennials, which live for more than two years, fre- 
quently have thickened roots, e.g., dahlia, where there is a bunch 
_ of secondary roots swollen into spindle-shaped bodies. 
The structure of roots is somewhat complex, and can only be 
very briefly dealt with here. Just as (see p. 8) a leaf is divisible 
into three systems of tissue, so also is a root. ‘This may be con- 
veniently illustrated by the main root of a young bean-seedling 
grown in damp sawdust. The younger part of this, ze. the part 
near the tip, will be covered by a thin ill-defined skin, the 
epidermis, composed of a single layer of flattened cells, from 
which numerous delicate unicellular root-hairs grow out (¢/. fig. 
7, H). Such hairs are seen much better in the case of mustard- 
seedlings (fig. 3). If the root is cleanly cut across and examined 
with a lens, an outer spongy-looking portion can be distinguished 
from an inner denser portion. These correspond, respectively, 
to a sort of external jacket, the cortex, and an internal firm 
vascular cylinder (cf. fig. 2). In roots which, like those of the 
bean, increase in thickness, the epidermis and cortex are early 
thrown off, being replaced by a brown layer of cork formed in 
the outer part of the vascular cylinder. This may easily be 
made out in the roots of an old bean-plant. Suitable cross- 
sections through the young root show that the secondary roots 
run in to the vascular cylinder. If the cortex is peeled off, which 
can be readily done, the secondary roots remain behind, attached 
to, the cylinder. These roots then arise endogenously, t.e., from 
the internal tissue, and break their way through the cortex to 
the exterior (figs. 5, B, and 7, H). This way of origin is charac- 
teristic of all roots, primary, secondary, and adventitious. 
The cortex is composed of the second kind of tissue, which has 
received the name of fundamental or ground tissue. This term 
is a very broad and general one, and it must by no means be sup- 
posed that all the component cells are of the same shape or nature. 
On the contrary, several varieties of tissue may be included under 
this system. The commonest and most important is parenchyma 
(fig. 7, H), which is made up of cells that are fairly equal in 
length, breadth, and thickness. The cells in question are not 
