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—