THE VEGETATIVE ORGANS OF PLANTS. 235 



action of light upon them, however, appears to be unfavor- 

 able to their functions. 



The Growth of Roots occurs mostly by lengthening, 

 and very little or very slowly by increase of thickness. 

 The lengthening is chiefly na.anifested toward the outer 

 extremities of the roots, as was neatly demonstrated by 

 Wigand, who divided the young root of a sprouted pea 

 into four equal parts by ink-marks. After three days, the 

 first two divisions next the seed had scarcely lengthened 

 at all, while the third was double, and the fourth eight 

 times its previous length. Ohlerts made precisely similar 

 observations on the roots of various kinds of plants. The 

 growth is confined to a space of about 'I, of an inch from 

 the tip. {Linnea, 1837, pp. 609-631.) This peculiarity 

 adapts the roots to extend through the soil in all direc- 

 tions, and to occupy its smallest pores, or rifts. It is 

 likewise the reason that a root, which has been cut off in 

 transplanting or otherwise, never afterwards extends in 

 length. 



Although the older parts of the roots of trees and of 

 the so-called root-crops acquire a considerable diameter, 

 the roots by which a plant feeds are usually thread-like 

 and often exceedingly slender. 



SpongioIeSi — The tips of the rootlets have been termed 

 spongioles, or spongelets,. from the idea that their texture 

 adapts them especially to collect food for the plant, and 

 that the absorption of matters from the soil goes on exclu- 

 sively through them. In this sense, spongioles do not 

 exist. The real living apex of the root is not, in fact, the 

 outmost extremity, but is situated a little within that 

 point. 



Root-Capi — The extreme end of the root usually consists 

 of cells that have become loosened and in part detached 

 from the proper cell-tissue of the root, which, therefore, 

 shortly perish, and serve merely as an elastic cushion or 



Digitized by Microsoft® 



