ANATOMY OF TISSUES. 125 
Roots may also branch dichotomously in the same way that the 
stem does, for example, Selaginella, Isoetes, and other crypto- 
gam roots. 
There are one or two peculiarities in the manner of the axial 
erowth of the root which may be mentioned here. As the root 
develops normally under the soil, the stretching or growth of 
the cells axially is confined to a much shorter distance than in 
the stem. Thus growth in the length of the stem occurs in a 
number of internodes simultaneously, some of which are at a 
considerable distance from the stem apex, while in the root this 
stretching of the cells in direction of the long axis is probably 
confined to the space of a centimeter, at most, back from the 
apex and in many instances probably to a few millimeters. It 
is by this sudden lengthening of the end of the root that the tip 
is driven into the ground. Another pecularity is the shortening 
or shrinking of the roots of certain plants, particularly those 
which are herbaceous. This gives a shrunken appearance to 
the rind and a winding course to the bundles. The result in case 
of herbaceous plants is to draw the stem part closer to the 
ground while in woody plants it seems to fasten the whole more 
securely to the soil. 
Notre. —In order to simplify as much as possible, all unnecessary terms 
have been omitted in the foregoing chapters on tissues and systems. The word 
sclerenchyma is used by some authors as the name of a tissue including all 
thick-walled cells. Others limit its meaning to parenchymatic, thick-walled 
cells, like the stone cells of fruits and rinds. We have preferred to use it only 
as an adjective to describe any cell with a hard, thick wall. Hypoderma also 
is used here, not as the name of any special tissue, but of the layer immediately 
under the epidermis when the character of the cells composing this layer is 
changed from the normal type. 
