14 THE FLOWERING PLANT. 
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. Lzennials, 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 means of a carrot. The outside, at least 
in young examples, is covered by a thin skin, the epzdermis, com- 
posed of a single layer of flattened cells. Numerous delicate root- 
hairs, each consisting of a single cell, grow out from the epidermis 
(figs. 3 and 7, H). It may not be unnecessary to state again 
that the elements in question, like most cells, are microscopic. 
T'o the naked eye a piece of epidermis appears as a homogeneous 
membrane. <A transverse or cross section of the carrot will show 
that it is composed internally of (1) a thick orange-coloured band 
inside the epidermis, and (2) a bright yellow central portion. In 
a dry carrot, the orange part can be stripped off from the yellow 
part, which remains as a tapering rod-like body. ‘These two parts 
are called respectively the cortex and the vascular cylinder. A 
further and very important point here demands notice. Numerous 
fibres, the secondary roots, project from the main axis, which they 
resemble in structure. ‘The transverse section shows that these 
can be traced as yellow streaks through the cortex to the vascular 
cylinder, where they commence. And in peeling off the cortex 
from the dry example, the secondary roots remain behind, attached 
to the cylinder. ‘These roots then arise endogenously, 7.e., from 
the internal tissue, as 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 
