Section Ii.—of the Growth and Substance of Plants.

In this way the perfect plant derives its food from the soil and from the air; but perfect plants arise from seeds; and the study of the entire life—the career, so to speak—of a plant, presents many interesting and instructive subjects of consideration.

When a portion of flour is made into dough, and this dough is kneaded with the hand under a stream of water upon a fine sieve, as long as the water passes through milky, there will remain on the sieve a glutinous sticky substance resembling birdlime, while the milky water will gradually deposit a pure white powder. This powder is starch, that which remains on the sieve is gluten. Both of these substances exist, therefore, in the flour; they both also exist in the grain. The starch consists of carbon, hydrogen, and oxygen only; the gluten, in addition to these, contains also nitrogen.

When ground into flour, these substances serve for food to man; in the unbruised grain they are intended to feed the future plant in its earliest infancy.

When a seed is committed to the earth, if the warmth and moisture are favourable, it begins to sprout. It pushes a shoot upwards, it thrusts a root downwards, but, until the leaf expand, and the root has fairly entered the soil, the young plant derives no nourishment other than water, either from the earth or from the air. It lives on the starch and gluten contained in the seed. But these substances, though capable of being separated from each other by means of water, as above stated, yet are neither of them soluble in water. Hence, they cannot, without undergoing a previous change, be taken up by the sap, and conveyed along the pores of the young shoot they are destined to feed. But it is so arranged that, when the seed first shoots, there is produced at the base of the germ, from a portion of the gluten, a small quantity of a substance (diastase) which has so powerful an effect upon the starch as immediately to render it soluble in the sap, which is thus enabled to take it up and convey it by degrees, just as it is wanted, to the shoot or to the root. As the sap ascends, it becomes sweet,—the starch thus dissolved changes into sugar. When the shoot first becomes tipped with green, the sugar is again changed into the woody fibre, of which the stem of perfect plants chiefly consists. By the time that the food contained in the seed is exhausted,—often, as in the potato, long before,—the plant is able to live by its own exertions, at the expense of the air and the soil.

In malting barley, it is made to sprout a certain length, and the growth is then arrested by heating and drying it. Mashed barley, before sprouting, will not dissolve in water, but when sprouted, the whole of the starch (the flour) it contains dissolves readily by a gentle heat. The diastase formed during the germination effects this. By further heating in the brewer’s wort, this starch is converted into sugar as it is in the growing plant.

This change of the sugar of the sap into woody fibre is observable more or less in all plants. When they are shooting fastest the sugar is most abundant; not, however, in those parts which are growing, but in those which convey the sap to the growing parts. Thus the sugar of the ascending sap of the maple and the alder disappears in the leaf and in the extremities of the twig; thus the sugar-cane sweetens only a certain distance above the ground, up to where the new growth is proceeding; and thus also the young beet and turnip abound most in sugar, while in all these plants the sweet principle diminishes as the year’s growth draws nearer to a close.

In the ripening of the ear also, the sweet taste, at first so perceptible, gradually diminishes and finally disappears; the sugar of the sap is here changed into the starch of the grain, which, as above described, is afterwards destined, when the grain begins to sprout, to be reconverted into sugar for the nourishment of the rising germ.

In the ripening of fruits a different series of changes presents itself. The fruit is first tasteless, then becomes sour, and at last sweet. In this case the acid of the unripe is changed into the sugar of the ripened fruit.

The substance of plants,—their solid parts that is—consist chiefly of woody fibre, the name given to the fibrous substance, of which wood evidently consists. It is interesting to inquire how this substance can be formed from the compounds, carbonic acid and water, of which the food of plants in great measure consists. Nor is it difficult to find an answer.

It will be recollected that the leaf drinks in carbonic acid from the air, and delivers back its oxygen, retaining only its carbon. It is also known that water abounds in the sap. Hence carbon and water are thus abundantly present in the pores or vessels of the green leaf. Now, woody fibre consists only of carbon and water chemically combined together,—100 lbs. of dry woody fibre consisting of 50 lbs. of carbon and 50 lbs. of water. It is easy, therefore, to see how, when the carbon and water meet in the leaf, woody fibre may be produced by their mutual combination.

If, again, we inquire how this important principle of plants may be formed from the other substances, which enter by their roots, from the ulmic acid, for example, the answer is equally ready. This acid also consists of carbon and water only, 50 lbs. of carbon with 37½ of water forming ulmic acid, so that when it is introduced into the sap of the plant, all the materials are present from which the woody fibre may be produced.

Nor is it more difficult to see how starch may be converted into sugar, and this again into woody fibre; or how, again, sugar may be converted into starch in the ear of corn, or woody fibre into sugar during the ripening of the winter pear after its removal from the tree. Any one of these substances may be represented by carbon and water only. Thus,—

50 lbs. of carbon with 50 of water, make 100 of woody fibre.

50 lbs. 37½ 87½ of ulmic acid.

{ of cane sugar, 50 lbs. 72½ 122½ { of starch, or { of gum.

50 lbs. 56 106 of vinegar.

In the interior of the plant, therefore, it is obvious that, whichever of these substances be present in the sap, the elements are at hand out of which any of the others may be produced. In what way they really are produced, the one from the other, and by what circumstances these transformations are favoured, it would lead into too great detail to attempt here to explain.

We cannot help admiring to what varied purposes in nature the same elements are applied, and from how few and simple materials, substances, the most varied in their properties, are in the living vegetable daily produced.

For fuller and more precise explanations on these interesting topics, see the Author’s LECTURES on Agricultural Chemistry and Geology, Part I.