456. The protoplasm and other products containing nitrogen (gluten, legumine, etc.), and which are most accumulated in grains and seeds (for the nourishment of their embryos when they germinate), compose the most nutritious vegetable food consumed by animals; they form their proper flesh and sinews, while the earthy constituents of the plant form the earthy matter of the bones, etc. At length decomposed, in the secretions and excretions, these nitrogenous constituents are through successive changes finally resolved into mineral matter, into carbonic acid, water, and ammonia or some nitrates,—into exactly or essentially the same materials which the plants 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.
[§ 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.
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 oscillating 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.