The Trees of Texas 7 
absorb such great quantities of water as we have already seen 
are thrown off by the leaves in transpiration. But the extent 
of the root system is much greater than we ordinarily suspect, 
usually equaling or even surpassing in extent the spread of the 
branches. The roots of tree frequently penetrate the soil to great 
depths. This is particularly true of trees which grow in very 
dry soil. The roots of the mesquite are known to attain depths 
of sixty feet when growing under arid conditions. 
The work of absorbing water is carried on by delicate hair-like 
outgrowths known as root hairs, which form near the tip of the 
root. These root hairs are fine and delicate, and attach them- 
selves closely to the water-bearing particles of the soil. 
Roots of trees are made up largely of woody tissue like the stem, 
and this enables them to serve as an avenue for the conduction 
of water as well as to give the trees a firm anchor in the soil. 
Roots are sensitive to the pull of gravity, and are also attracted 
by moisture. The main central or tap root, if growing in soil 
equally moist on all sides and not hindered by any mechanical ob- 
struction, takes a course straight downward, while the lateral or 
secondary roots branch out in all directions. Roots usually grow 
toward moisture. This may be observed along ditches or irri- 
gation canals as well as on the sides of vertical cliffs. The sensi- 
tiveness of roots to water, and their ability to grow toward it 
has been of great advantage to plants. Trees which develop 
long tap roots are transplanted only with great difficulty. The 
pecan is a noteworthy example. To offset this difficulty, the tap 
root on young trees is sometimes cut by nurserymen and a mass 
of much branched roots of about equal size develops to take its 
place. The root system must be able to supply the water tran- 
spired by the leaves or the tree will die. Accordingly, in trans- 
planting young trees the top should be cut back sufficiently to give 
the roots time to become established and develop their root hairs. 
Roots are very useful in many places because of the binding 
action which they exert on the soil, thus preventing erosion. 
Such trees as willows growing along water courses, or the trees 
of mountain sides and hill slopes, furnish examples of this prin- 
ciple. 
The stem may be defined as that part of the tree which bears 
the roots and leaves and serves as a connection between them. 
