toplasm, in all but the lowest
grade of plants, is enclosed and to common appearance isolated in
separate cells, the walls of which can only in their earliest state be
said to be alive. Still plants are able to cause the protoplasm of
adjacent cells to act in concert, and by their combined action to effect
movements in roots, stems, or leaves, some of them very slow and
gradual, some manifest and striking. Such movements are brought about
through individually minute changes in the form or tension in the
protoplasm of the innumerable cells which make up the structure of the
organ. Some of the slower movements are effected during growth, and may
be explained by inequality of growth on the two sides of the bending
organ. But the more rapid changes of position, and some of the slow
ones, cannot be so explained.

464. =Root-movements.= In its growth a root turns or bends away from
the light and toward the centre of the earth, so that in lengthening it
buries itself in the soil where it is to live and act. Every one must
have observed this in the germination of seeds. Careful observations
have shown that the tip of a growing root also makes little sweeps or
short movements from side to side. By this means it more readily
insinuates itself into yielding portions of the soil. The root-tips will
also turn toward moisture, and so secure the most favorable positions in
the soil.

465. =Stem-movements.= The root end of the caulicle or first joint of
stem (that below the cotyledons) acts like the root, in turning downward
in germination (making a complete bend to do so if it happens to point
upward as the seed lies in the ground), while the other end turns or
points skyward. These opposite positions are taken in complete darkness
as readily as in the light, in dryness as much as in moisture:
therefore, so far as these movements are physical, the two portions of
the same internode appear to be oppositely affected by gravitation or
other influences.

466. Rising into the air, the stem and green shoots generally, while
young and pliable, bend or direct themselves toward the light, or toward
the stronger light when unequally illuminated; while roots turn toward
the darkness.

467. Many growing stems have also a movement of _Nutation_, that is, of
nodding successively in different directions. This is brought about by a
temporary increase of turgidity of the cells along one side, thus bowing
the stem over to the opposite side; and th