illating motions) freely execute three different kinds of
movement, the very delicate investing coat of cellulose not impeding the
action of the living protoplasm within. Even when this coat is firmer
and hardened with a siliceous deposit, such crescent-shaped or
boat-shaped one-celled plants as _Closterium_ or _Naricula_ are able in
some way to move along from place to place in the water.

[Illustration: Fig. 489. A few cells of a leaf of Naias flexilis, highly
magnified: the arrows indicate the courses of the circulating currents.]

461. =Movements in Cells=, =or Cell-circulation=, sometimes called
_Cyclosis_, has been detected in so many plants, especially in
comparatively transparent aquatic plants and in hairs on the surface of
land plants (where it is easiest to observe), that it may be inferred to
take place in all cells during the most active part of their life. This
motion is commonly a streaming movement of threads of protoplasm,
carrying along solid granules by which the action may be observed and
the rate measured, or in some cases it is a rotation of the whole
protoplasmic contents of the cell. A comparatively low magnifying power
will show it in the cells of Nitella and Chara (which are cryptogamous
plants); and under a moderate power it is well seen in the Tape Grass of
fresh water, Vallisneria, and in Naias flexilis (Fig. 489). Minute
particles and larger greenish globules are seen to be carried along, as
if in a current, around the cell, passing up one side, across the end,
down the other and across the bottom, completing the circuit sometimes
within a minute or less when well warmed. To see it well in the cell,
which like a string of beads form the hairs on the stamens of
Spiderwort, a high magnifying power is needed.

462. =Transference of Liquid from Cell to Cell=, and so from place to
place in the plant, the absorption of water by the rootlets, and the
exhalation of the greater part of it from the foliage,--these and
similar operations are governed by the physical laws which regulate the
diffusion of fluids, but are controlled by the action of living
protoplasm. Equally under vital control are the various chemical
transformations which attend assimilation and growth, and which involve
not only molecular movements but conveyance. Growth itself, which is the
formation and shaping of new parts, implies the direction of internal
activities to definite ends.

463. =Movements of Organs.= The living pro