ITS STRUCTURE AND GROWTH. 81 



extended farther into the soil. The absorbing surface of the new 

 parts of roots is greatly increased by the 



134. Root-hairs, or delicate fibrils which they bear. These are 

 often discernible by the naked eye, as in the young seedling Maple 

 (Fig. 106), and may almost always be plainly shown by a moderate 

 magnifying power, as in Fig. 135; while a higher power distinctly 

 reveals their nature, as prolongations of some of the supei-flcial cells 

 from a certain point into slender tubes (Fig. 136, 137), thus largely 

 increasing the absorbing surface. As fast as the superficial cells are 

 converted into epidermis, the root-hairs die away, fresh ones taking 

 their place on the newer parts. 



135. The advancing extremity of the root consists of parenchyma 

 alone ; but vessels and woody tissue appear in the forming root 

 soon after their appearance in the radicle or stemlet above. The 

 arrangement of the woody matter is generally the same as in the 

 stem, except that the root seldom exhibits a distinct pith. The root 

 increases in diameter in the same manner as the stem. (Chap. IV. 

 Sect. IV., V.) 



136. The growth of the root and its branches keeps pace with 

 the development of the stem. As the latter shoots upward and 

 expands its leaves, from which water is copiously exhaled during 

 vigorous vegetation, the former grow onward and continually renew 

 the tender 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, while new branchlets and 

 leaves are developing, and when the sap is rapidly consumed by the 

 fresh foliage, the rootlets are also in rapid action, are extending at a 

 corresponding rate, and their tender absorbing points are most fre- 

 quently renewed. They cannot now be removed from the soil with- 

 out injury, at the very time when their action is essential to restore 

 the liquid which is continually exhaled from the leaves. But at the 

 close of summer, as the leaves become inactive and the growth of 

 the season is attained, the rootlets also cease to grow, the epidermis 

 forms a comparatively firm covering quite down to the tip, and ab- 

 sorption at length ceases. This indicates the proper period for 

 transplanting, namely, in the autumn after vegetation is suspended, 

 or in early spring before it recommences. 



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

 admirably adapted to the conditions in which they are placed. 



