In the spring, a wheat-field is covered with small green plants. These grow taller and taller until they attain many times the size which they had when they first appeared; and they produce the heads of flowers which eventually change into ears of corn.
In so far as this is a process of growth, accompanied by the assumption of a definite form, it might be compared with the growth of a crystal of salt in brine: but, on closer examination, it turns out to be something very different. For the crystal of salt grows by taking to itself the salt contained in the brine, which is added to its exterior; whereas the plant grows by addition to its interior: and there is not a trace of the characteristic compounds of the plant’s body, albumin, gluten, starch, or cellulose, or fat, in the soil, or in the water, or in the air.
Yet the plant creates nothing (§ 50) and, therefore, the matter of the proteids and amyloids and fats which it contains must be supplied to it, and simply manufactured, or combined in new fashions, in the body of the plant.
It is easy to see, in a general way, what the raw materials are which the plant works up, for the plant gets nothing but the materials supplied to it by the atmosphere and by the soil. The atmosphere contains oxygen and nitrogen, a little carbonic acid gas, a minute quantity of ammoniacal salts, and a variable proportion of water. The soil contains clay and sand (silica), lime, iron, potash, phosphorus, sulphur, ammoniacal salts, and other matters which are of no importance. Thus, between them, the soil and the atmosphere contain all the elementary bodies which we find in the plant: but the plant has to separate them and join them together afresh.
Moreover, the new matter, by the addition of which the plant grows, is not applied to its outer surface, but is manufactured in its interior; and the new molecules are diffused among the old ones.
62. =The Living Plant, after it has grown up, detaches part of its Substance, which has the Power of developing into a similar Plant, as a Seed.=
The grain of wheat is a part of the flower of the wheat plant, which, when it becomes ripe, is easily separated. It contains a minute and rudimentary plant; and, when it is sown, this gradually grows, or becomes =developed= into, the perfect plant, with its stem, roots, leaves and flowers, which again give rise to similar seeds. No mineral body runs through a regular series of changes of form and size and then gives off parts of its substance which take the same course. Mineral bodies present no such =development= and give off no seeds or =germs=. They do not reproduce their kind.