nts took up and assimilated. Animals depend upon
vegetables absolutely and directly for their subsistence; also
indirectly, because

457. _Plants purify the air for animals._ In the very process by which
they create food they take from the air carbonic acid gas, injurious to
animal respiration, which is continually poured into it by the breathing
of all animals, by all decay, by the burning of fuel and all other
ordinary combustion; and they restore an equal bulk of life-sustaining
oxygen needful for the respiration of animals,--needful, also, in a
certain measure, for plants in any work they do. For in plants, as well
as in animals, work is done at a certain cost.

Sec. 6. PLANT WORK AND MOVEMENT.

458. As the organic basis and truly living material of plants is
identical with that of animals, so is the life at bottom essentially the
same; but in animals something is added at every rise from the lowest to
highest organisms. Action and work in living beings require movement.

459. Living things move; those not living are only moved. Plants move as
truly as do animals. The latter, nourished as they are upon organized
food, which has been prepared for them by plants, and is found only here
and there, must needs have the power of going after it, of collecting
it, or at least of taking it in; which requires them to make spontaneous
movements. But ordinary plants, with their wide-spread surface, always
in contact with the earth and air on which they feed,--the latter
everywhere the same, and the former very much so,--might be thought to
have no need of movement. Ordinary plants, indeed, have no locomotion;
some float, but most are rooted to the spot where they grew. Yet
probably all of them execute various movements which must be as truly
self-caused as are those of the lower grades of animals,--movements
which are overlooked only because too slow to be directly observed.
Nevertheless, the motion of the hour-hand and of the minute-hand of a
watch is not less real than that of the second-hand.

[Illustration: Fig. 488. Two individuals of an Oscillaria, magnified.]

460. =Locomotion.= Moreover, many microscopic plants living in water are
seen to move freely, if not briskly, under the microscope; and so
likewise do more conspicuous aquatic plants in their embryo-like or
seedling state. Even at maturity, species of Oscillaria (such as in Fig.
488, minute worm-shaped plants of fresh waters, taking this name from
their osc