42 ELEMENTS OF BOTANY 
water supply on either side grow toward the moister soil. 
Roots are very tough, because they need to resist strong 
pulls, but not as stiff as stems and branches of the same 
size, because they do not need to withstand sidewise pres- 
sure, acting from one side only. The corky layer which 
covers the outsides of roots 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. 
55. Propagation by Means of Roots. — Some familar 
plants, such as rose bushes, are usually grown from roots 
or root-cuttings. 
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. 
56. Review Summary of Roots. 
Kinds of roots as regards origin 
Kinds as regards medium in which they grow . 
aie 
Structure of root of a tree. 
f materials. 
mtorage InerOOtse sae Bo me oe Boe bk e- location. 
uses. 
{ apparatus 
| amount. 
- nature. 
Movements of roots. . . . . 2... causes. 
uses. 
Absorption of water by roots 
