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 manifested 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 *|, 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 4 considerable diameter, 
the roots by which a plant feeds are usually thread-like 
and often exceedingly slender. 
Spongioles.—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-Cap.—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 
