THE PRIMARY ROOT. 83' 



vegetation, the former grow onward and continually renew the ten- 

 der, hygrometric tissue through which the absorption, required to 

 restore what is lost by evaporation or consumed in growth, is 

 principally effected. Hence the danger of disturbing the active 

 roots during the season of growth. In early summer, when the sap 

 is rapidly consumed by the fresh leaves, the rootlets are also in rap- 

 id action. The growth of the branches and roots being simultane- 

 ous, while new branchlets and leaves are developing, the rootlets are 

 extending at a corresponding rate, and their tender absorbing points 

 are most frequently renewed. They cannot now be removed from 

 the soil without destroying them, at the very time when their action 

 is essential to restore the liquid which is exhaled from the leaves. 

 But towards the close of summer, as the leaves grow languid and 

 the growth of the season is attained, the rootlets also cease to 

 grow, the loose tissue of their extremities, not being renewed, grad- 

 ually solidifies, and absorption at length ceases. This indicates 

 the proper period for transplanting, namely, in the autumn a'fter 

 vegetation is suspended, or in early spring before it recommences. 



122. This elongation of roots by their advancing points alone is 

 admirably adapted to the conditions in which they are placed. 

 Growing as they do in a medium of such unequal resistance as the 

 soil, if roots increased like growing stems, by the elongation of the 

 whole body, they would be thrown, whenever the elongating force 

 was insufficient to overcome the resistance, into knotted or con- 

 torted shapes, very ill adapted for the free transmission of fluid. 

 But, lengthening only at their farthest extremity, they insinuate 

 themselves with great facility into the crevices or yielding parts of 

 the soil, and afterwards by their expansion in diameter enlarge the 

 cavity ; or, when arrested by insuperable obstacles, their advan- 

 cing points follow the surface of the opposing body until they reach 

 a softer medium. In this manner, too, they readily extend from 

 place to place, as the nourishment in their immediate vicinity is 

 consumed. Hence, also, may be derived a simple explanation of 

 the fact, that roots extend most rapidly and widely in the direction 

 of the most favorable soil, without supposing any prescience on the 

 part of the vegetable, as some have imagined. 



123. The advancing extremity of the root consists of parenchy- 

 ma alone ; but bundles of vessels and woody tissue appear in the 

 forming root, soon after their appearance in the primordial stem 

 above : these form a central woody or fibrous portion, which con- 



