GROWTH. 55 
Darwin has shown that the joints of grass stems continue 
to exhibit movements on a small scale for a long period. 
Supposing the stem to be “laid,” such movements would 
clearly aid the upward tendency above described, and 
facilitate the uprising of the stem. (Darwin, Power of 
Movement, p. 503). 
Movements of Leaves.—The leaves of plants exhibit 
several-kinds of motion ; some periodic, as in the case of 
the so-called sleep of leaves, some due to the stimulus of 
light or its removal, some the consequence of contact, as 
in the case of the sensitive plant; but those to which 
mention is here made are the result of the same causes as 
those before alluded to in the case of stems and roots. 
The growth movements of leaves are observable in the 
stalk, or in the blade, or in both, and are chiefly exerted 
in a vertical direction, so that the leaf rises or falls ; but 
as the ascent is never quite in the same line as the de- 
scent, some side to side motion must also take place. It 
is noticed that the rise occurs generally in the evening, 
the fall on the following morning. These movements 
are probably due to the intensity of growth being greater 
first on one side, then on the other. 
Growth-movements of the kind indicated have now 
been shewn to exist in the roots, in the stems, and in the 
leaves. The probability is that they occur more or less 
wherever growth is going on actively. In accordance 
with this, it may be mentioned that seedling plants mani- 
. fest these movements to a remarkable degree. Thus all 
the parts of seedling cabbages, the radicle, the caulicle 
above the radicle supporting the seed leaves or cotyle- 
dons, as well as these latter organs, were observed by 
Darwin to exhibit growth: movements facilitating the 
downward passage of the root and the upward progress 
of the caulicles, 
