This section is from the book "The Gardener's Monthly And Horticulturist V21", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
In Mr. Peter Henderson's article in the June number of the Gardeners' Monthly he says: "My practice, which has extended through a period of over thirty-five years, and which I believe has been as varied and extensive as that of most men in that time, has never yet shown me a single instance wherein I was certain that plants either absorbed liquid manure, or even fertilizing gases by their leaves." My experience in plants has not been as long and varied as it might have been; but in chemistry I have dab-bied more, and would modestly suggest to him or others interested, a very simple and effective test in regard to the absorption of fertilizing gases by the leaves of plants, which I believe will prove the fact to a certainty. Take a bell glass and fill it with water, to which a considerable proportion of carbonic acid gas has previously been added, and place in it a branch or an entire plant covered with leaves, and expose the whole to the rays of the sun for some hours. The air if now analyzed will be found to contain scarcely any carbonic acid gas, but it will contain a larger proportion of oxygen than before the experiment.
Now if we take a branch of a plant with the roots fixed and growing in the soil, - consequently in its normal state of vegetation, - and place it in a glass vessel, and by means of an air pump a given quantity of air is caused to circulate round it, this air which before the experiment contained from four to five ten-thousanth parts of carbonic acid gas. after the apparatus has been exposed to the sun's rays for a certain time, will not be found to contain more than from one to two. If on the contrary the experiment is made during the night it will be found that the quantity of carbonic acid gas would be increased, and at the expiration of a certain time would have risen to eight ten-thousanth parts. If we now reverse our first experiment and put a plant or leafy branch in a jar or balloon with gas which cannot be renewed and leave the whole in darkness for some ten to fifteen hours, we may assure ourselves at the end of this time that the atmospheric air contained in our vessel is no longer of the same composition as before the experiment. Carbonic acid will now be there in greater abundance and the quantity of oxygen will be less.
But if in place of leaving the plant in darkness we expose the apparatus to the sun's rays, the balloon or jar will have lost a noticeable quantity of its carbonic acid gas and will be enriched in its oxygen.
These experiments, in which there is an interchange of gas between the plant and the atmosphere, exhibit the double phenomena of absorption and exhalation, forming a true respiration. Differing from animals in not being continuous by night and day without cessation, but having two modes, one diurnal, in which the leaves absorb the carbonic acid of the air, decompose this gas and extract and give forth the oxygen whilst the carbon remains in their tissues; the other nocturnal and the reverse, in which the plant absorbs the oxygen and extracts the carbonic acid - that is to say they breathe as animals do. The carbon which plants use" during the day is indispensable to the perfect development of their organs and the consolidation of their tissues.
If further proof were needed we have only to take a given weight of pure sand and wash it clean and boil it to dissolve all soluble matter out of it, and when so prepared put a healthy strong cutting of a plant into it and grow it into a plant which will soon double, treble and quadruple its weight, even if watered with pure distilled water, in which no gases or salts remain. If at the end of a time we now weigh our sand again we find the same weight without any appreciable diminution; while our plant has gained fourfold by absorption of carbon, nitrogen, ammonia,etc, contained in the atmosphere, and that, principally through its leaves; as we had no roots to start with, they only contributing to sustain the equilibrium when formed later on. So that it may be fairly claimed that by respiration, i.e., absorption and exhalation, plants live, feed and grow, and while doing so purify the air injured by combustion and the respiration of men and animals by pouring into it large quantities of oxygen gas, as can readily be demonstrated as above.
Plants subjected to perpetual nocturnal respiration by long continued darkness undergo certain modifications in their exterior aspect, by losing a great part of their carbon, which passes into the state of carbonic acid, and they exhale large quantities of water, producing a decided elongation of the plant, a greater softness in its tissues, and the absence of green coloring. The juices are considerably modified and changed, a fact the market gardener turns largely to his profit, rendering his lettuce, sea kale, etc., succulent, sweet and delicate by carefully applying this principle of blanching.
I have only treated of carbonic acid, which supplies all plants with the carbon of their tissues, whether through the soil by the roots, or through the air by the leaves, as being the most easily demonstrated on account of the latter being the medium through which the great bulk of this food is absorbed. The three foods all plants require are first, water; second, carbonic acid; third, ammonia. According to Schleiden the greater part of water is supplied to plants in the form of vapor, the other portion in rain. The sources of ammonia are nearly the same as carbonic acid and the decomposition of nitrogenous matter.
In closing I will only add that the sap of special plants contains the metallic oxides as potassium, sodium, calcium and magnesium. How these various substances permeate through the tissues of plants is much more of a mystery than the absorption of water and carbonic acid gas is by the leaves. At all events they are not so easily demonstrated, although simple facts on which all botanists are agreed. I would have liked to touch on this as well as root action, electrolyses and the effect of heat and light on plants also, but this note is too long already, and I will leave these to abler hands.