SENSITIVENESS, 67 
nected with growth, gravitation, or the action of light, 
which are manifested only during active growth, there 
are others which occur in the fully-developed leaf, as the 
periodical night and day movements, the movements af- 
fected by light and temperature, and lastly, those which 
are caused by mechanical contact, as by the impact of 
certain nitrogenous substances, as in the so-called car- 
nivorous leaves before referred to, and those caused by a 
touch or other mechanical effect, as in the leaves of the 
sensitive plant. Chloroform and ether arrest these move- 
ments, while they have no effect upon the movements 
that are due to light and heat. The cause of the move- 
ments in question is attributed to the sudden contraction 
of the protoplasm, the expulsion of the watery contents 
of the cells forming the lower portion of the swelling 
which leaves endowed with this property possess at the 
base of their stalks. The cells so emptied become flaccid, 
and the leaf in consequence falls. The water expelled 
from the interior of the cells passes into the spaces 
between them and into the stem, as in the case of frozen 
leaves (p. 65), aud is re-absorbed when the irritation 
ceases. The balance being restored, the leaf resumes its 
horizontal position. 
The Action of Gravity on Stems.—The cause of the 
upward growth of stems, though so familiar, is not 
understood. It is in general exerted in opposition to the 
direction of gravitation. Jf a stem be bent downwards, 
growth takes place much more rapidly on the lower sur- 
face, tending to make it convex on the lower surface, and 
consequently to raise its free end (see p. 54). It is this 
tendency, which, as has been previously stated, permits 
the stalks of the wheat when laid to recover their erect 
position. Some stem or portions of stem are, however, 
directly influenced by gravitation, as in the case of under- 
ground stems and branches, which burrow in the ground 
