34 ELEMENTS OF BOTANY. 
have been observed penetrating horizontal tunnels in a mine 
at a depth of about fifty feet. 
The total absorbing surface of the roots of a tree must be 
enormous, since it is greatly increased by the presence of the 
root-hairs. 
48. Fitness of the Root for its Position and Work. — The 
distribution of material in the woody roots of trees and shrubs 
shows many adaptations to the conditions by which the roots 
are surrounded. The growing tip of the root, as it pushes 
its way through the soil, is exposed to bruises ; but these are 
largely warded off by the root-cap. The corky layer which 
covers the outside is remarkable for its power of preventing 
evaporation. It must be of use in retaining in the root 
the moisture which otherwise might be lost, on its way from 
the deeper rootlets (which are buried in damp soil) through 
the upper portions of the root-system, about which the soil 
is often very dry. 
49. Propagation by means of Roots. —Some familiar plants 
are usually grown from roots or root-cuttings. 
Experiment 14, — Bury a sweet potato or a dahlia root in damp 
sand, and watch the development of sprouts from adventitious buds. 
One sweet potato will produce several such crops of sprouts, and every 
sprout may be made to grow into a new plant. It is in this way that the 
crop is started wherever the sweet potato is grown for the market. 
50. Absorption of Water by Roots. — Many experiments on 
the cultivation of corn, wheat, oats, beans, peas, and other 
familiar plants in water have proved that some plants, at 
any rate, can thrive very well on ordinary lake, river, or well 
water, together with the food which they absorb from the air 
(Chapter XII). Just how much water some kinds of plants 
give off (and therefore absorb) per day will be discussed 
when the uses of the leaf are studied. For the present it 
is sufficient to state that even an annual plant during its life- 
time absorbs through the roots very many times its own 
