SECT. III.
Theory of Vegetation.
3d. Of air, and its agency in vegetation:
A seed deprived of air will not germinate; and a plant placed under an exhausted receiver, will soon perish. Even in a close and badly ventilated garden, vegetables indicate their situation; they are sickly in appearance, and vapid in taste.—These facts sufficiently shew the general utility of air to vegetation: but this air is not now the simple and elementary body, that the ancient chymist described it to be. Priestly first,[1] and Lavoisier after him, analyzed it, and found, that when pure, it consisted of about 70 parts of azote, 27 of oxygen, and 2 of carbonic acid. In its ordinary (or impure) state, it is loaded with foreign and light bodies; such as mineral, animal and vegetable vapours, the seeds of plants, and the eggs of insects, &c. Is it to this aggregate, that vegetation owes the services rendered to it by air? And if not, to how many, and to which, of its regular constituents, are we to ascribe them? This inquiry will form the subject of the present article.
All vegetables in a state of decomposition, give azote; and some of them (cabbages, radishes, &c.) give it in great quantity. This abundance, combined with the fact, that vegetation is always vigorous in the neighbourhood of dead animal matter, led to the opinion, that azote contributed largely to the growth of plants: but experiments, more exactly made and often repeated, disprove this opinion, and shew that in any quantity it is unnecessary, and that in a certain proportion it is fatal to vegetation.
In hydrogen gas, plants are found to be variously affected, according to their local situation; if inhabitants of mountains, they soon perish—if of plains, they shew a constant debility—but if of marshy grounds, their growth is not impeded.
Carbonic acid is formed and given out during the process of fermentation, putrefaction, respiration, &c. and makes 28 parts out of 100 of atmospheric air. It is composed (according to Davy) of oxygen and carbon, in the proportion of 34 of the former to 13 of the latter. It combines freely with many different bodies; animals and vegetables are almost entirely composed of it; for the coal which they give, on combustion, is but carbon united to a little oxygen, &c.—Priestly was the first to discover, that plants absorbed carbonic acid; and Ingenhouse, Sennebier, and De Sausure have proved, that it is their principal aliment. Indeed the great consumption made of it, cannot be explained by any natural process, excepting that of vegetation. On this head, we cannot do better than digest the experiments of these chymists into a few distinct propositions:[2]
1. In pure carbonic acid gas, seeds will swell, but not germinate. 2. United with water, this gas hastens vegetation. 3. Air containing more than one twelfth part of its volume of carbonic acid, is most favourable to vegetation. 4. Turf, or other carbonaceous earth, which contains much carbonic acid, is unfavourable to vegetation until it has been exposed to the action of atmospheric air, or of lime, &c. 5. If slacked lime be applied to a plant, its growth will be impaired, until the lime shall have recovered the carbonic acid it lost by calcination. 6. Plants kept in an artificial atmosphere, and charged with carbonic acid, yield, on combustion, more of that acid than plants of the same kind and weight growing in atmospheric air. 7. When plants are exposed to air and sunshine, the carbonic acid of the atmosphere is consumed, and a portion of oxygen left in its place. If new supplies of carbonic acid be given to the air, the same result follows; whence it has been concluded, that air furnishes carbonic acid to the plant, and the plant furnishes oxygen to the air.—This double function of absorption and respiration, is performed by the green leaves of plants.[3] 8. Carbon is to vegetation, what oxygen is to animal life; it gives support by purifying the liquids, and rendering the solids more compact.
4th. Of light, heat, and electricity, and their agency in vegetation:
When deprived of light, plants are pale, lax and dropsical; restored to it, they recover their colour, consistency and odour. If a plant be placed in a cellar, into which is admitted a small portion of light through a window or cranny, thither the plant directs its growth, and even acquires an unnatural length in its attempt to reach it.[4] These facts admitted, no one can doubt the agency of light in vegetation; but in relation to this agency, various opinions exist; one, that light enters vegetable matter, and combines with it; another, that it makes no part either of the vegetable or of its aliment, but directly influences substances which are alimentary;[5] and a third, that besides the last effect, it stimulates the organs of plants to the exercise of their natural functions.[6]
Without doing more than state these opinions, we proceed to offer the results of many experiments on this subject. 1st. That in the dark, no oxygen is produced, nor any carbonic acid absorbed; on the contrary, oxygen is absorbed and carbonic acid produced. 2d. That plants exposed to light, produce oxygen gas in water. 3d. That light is essential to vegetable transpiration; as this process never takes place during the night, but is copious during the day; and, 4th. That plants raised in the dark, abound in watery and saccharine juices—but are deficient in woody fibre, oil, and resins; whence it is concluded, that saccharine compounds are formed in the night, and oil, resins, &c. in the day.
When the weather is at or below the freezing point, the sap of plants remains suspended and hardened in the albumum;[7] but on the application of heat, whether naturally or artificially excited, this sap is rendered fluid, is put into motion, and the buds begin to swell. Under the same impulse, through the medium of the earth, the roots open their pores, receive nutritive juices, and carry them to the heart of the plant. The leaves, being now developed, begin and continue the exercise of their functions, till winter again, in the economy of nature, suspends the operations of the machine. Nor is its action confined to the circulation of vegetable juices. Without vapour (its legitimate offspring) the fountain and the shower would be unknown—nor would the great processes of animal and vegetable fermentation and decomposition go on. Without rain or other means ameliorating the soil, what would be the aspect of the globe? what the state of vegetation? what the situation of man?
The diffusion of electrical matter, found in the air and in all other substances, furnishes a presumption, that it is an efficient agent in vegetation. Nollet and others have thought that, artificially employed, it favoured the germination of seeds and the growth of plants; and Mr. Davy "found, that corn sprouted more rapidly in water, positively electrified by the voltaic battery, than in water negatively electrified."[8] These opinions have not escaped contradiction, and we do not profess to decide where doctors disagree.
5th. Of stable yard manures, lime, marl, and gypsum, and their agency in vegetation:
We have already said, that vegetables in the last stage of decomposition, yield a black or brown powder, which Mr. Davy calls "a peculiar extractive matter, of fertilizing quality," and which the chymists of France have denominated terreau. This vegetable residuum is the simple mean employed by nature to re-establish that principle of fertility in the soil, which the wants of man and other animals are constantly drawing from it. It was first analyzed by Hassenfratz, who found it to contain an oily, extractive and carbonaceous matter, charged with hydrogen; the acetates and benzoates of potash, lime and ammoniac; the sulphates and muriates of potash, and a soupy substance, previously noticed by Bergman.—Among other properties (and which shows its combustible character) is that of absorbing, from atmospheric air, its oxygen, and leaving it only azote. This was discovered by Ingenhouse, who, with De Sausure and Bracconnet, pursued the subject by many new and interesting experiments, the result of which is—
1. That the oxygen thus absorbed, deprives the terreau of part of its carbon, which it renders soluble and converts into mucilage; and
2. That the carbonic acid, formed in the process, combines with the mucilage, and with it is absorbed by the roots of plants.
If we put a plant and a quantity of slacked lime under the same receiver, the plant will perish; because the lime will take from the atmospheric air all the carbonic acid it contains, and thus starve the plant. Vegetables, placed near heaps of lime in the open air, suffer from the same cause and in the same way; but though lime, in large quantities, destroys vegetation, in small quantities it renders vegetation more vigorous. Its action is of two kinds—mechanical and chymical; the first is a mere division of the soil by an interposition between its parts; the second, the faculty of rendering soluble vegetable matter, and reducing it to the condition of terreau.
The mechanical agency ascribed to lime, belongs also to marle and to ashes, and in an equal degree—but their chymical operation, though similar, is less.[9]
Gypsum is composed of lime and sulphuric acid. Mayer was the first to present to the public a series of experiments upon it, in its relation to agriculture. Many chymists have followed him, and a great variety of opinion yet exists with regard to its mode of operation. Yvart thinks that the action of gypsum is exclusively the effect of the sulphuric acid, which enters into its composition; and founds this opinion upon the fact, that the ashes of turf, which contain sulphate of iron and sulphate of alumina, have the same action upon vegetation as gypsum. Laysterie, observing that plants, whose roots were nearest the surface of the soil, were most acted upon by plaster, concludes, that gypsum takes from the atmosphere the elements of vegetable life and transmits them directly to plants. Bose intimates, that the septic quality of gypsum (which he takes for granted) best explains its action on vegetation; but this opinion is subverted by the experiments of Mr. Davy, who found, that of two parcels of minced veal, the one mixed with gypsum, the other left by itself, and both exposed to the action of the sun, the latter was the first to exhibit symptoms of putrefaction. Mr. Davy's own belief on this subject is, that it makes part of the food of vegetables, is received into the plant and combined with it. The last opinion we shall offer on this head, is that of the celebrated Chaptal. "Of all substances, gypsum is that of whose action we know the least. The prodigious effect it has on the whole race of trefoils, (clover, &c.) cannot be explained by any mechanical agency—the quantity applied being so small—or by any stimulating power—since gypsum, raw or roasted, has nearly the same effect; nor by any absorbent quality, as it only acts when applied to the leaves. If permitted to conjecture its mode of operation, we should say, that its effect being greatest when applied to the wet leaves of vegetables, it may have the faculty of absorbing and giving out water and carbonic acid, little by little, to the growing plant. It may also be considered as an aliment in itself—an idea much supported by Mr. Davy's experiments, which shew, that the ashes of clover yield gypsum, though the clover be raised on soils not naturally containing that substance."
[Alb. Argus.
(To be continued.)