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 hy means of Boots. — 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 watcli the development of sprouts from adventitious buds. 

 One sweet potato will produce several such crops of sprouts, and every 

 sprout may he 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 hy 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 



