1363. The action of the atmosphere on plants differs at different periods of their growth, and varies with the various stages of the development and decay of their organs. If a healthy seed be moistened, and exposed to air at a temperature not below 45ﾦ, it soon germinates, and shoots forth a plume, which rises upwards, and a radicle which descends. If the air be confined, it is found that in the process of germination the oxygen, or a part of it, is absorbed. The azote remains unaltered; no carbonic acid is taken away from the air; on the contrary, some is added. Seeds are incapable of germinating, except when oxygen is present. In the exhausted receiver of the air-pump, in pure azote, or in pure carbonic acid, when moistened they swell, but do not vegetate; and, if kept in these gases, lose their living powers, and undergo putrefaction. If a seed be examined before germination, it will be found more or less insipid, at least not sweet; but after germination it is always sweet. Its coagulated mucilage, or starch, is converted into sugar in the process; a substance difficult of solution is changed into one easily soluble; and the sugar carried through the cells or vessels of the cotyledons is the nourishment of the infant plant. The absorption of oxygen by the seed in germination has been compared to its absorption in producing the evolution of f£tal life in the egg; but this analogy is only remote. All animals, from the most to the least perfect classes, require a supply of oxygen. From the moment the heart begins to pulsate till it ceases to beat, the aeration of the blood is constant, and the function of respiration invariable: carbonic acid is given off in the process; but the chemical change produced in the blood is unknown; nor is there any reason to suppose the formation of any substance similar to sugar. It is evident that, in all cases of semination, the seeds should be sown so as to be fully exposed to the influence of the air; and one cause of the unproductiveness of cold clayey adhesive soils is, that the seed is coated with matter impermeable to air. In sandy soils the earth is always sufficiently penetrable by the atmosphere; but in clayey soils there can scarcely be too great a mechanical division of parts. Any seed not fully supplied with air, always produces a weak and diseased plant. We have already seen that carbon is added to plants from the air by the process of vegetation in sunshine; and oxygen is added to the atmosphere at the same time. It is worthy of remark, that the absence of light is necessary to the formation of sugar in the germination of seeds; and its presence to the production of sugar in fruits. The following is the late Dr. Murray's ingenious explanation of these remarkable facts. The seed consists chiefly of farinaceous matter, which requires oxygen to convert it into sugar. Now, living vegetables appear to absorb oxygen in the dark; unripe fruits usually contain an acid, that is, have an excess of oxygen; and light is favourable to the evolution of oxygen from living plants. (T.) Plants absorb the greater part of their carbon from the atmosphere. Chemists have found by experiments that the earth in which plants had grown did not contain one-tenth part of the carbon afterwards found in those plants, and hence were led to the supposition that the additional quantity of carbon which they contained was obtained by them from the atmosphere, and on following up this train of enquiry they found that certain substances in the soil had the power of attracting carbonic acid from the atmosphere, and presenting it to the plants in such a form and such proportions as to be readily absorbed by them. In this manner it was found that burnt clay and charcoal acted, both having the power of attracting carbonic acid from the atmosphere, and presenting it in a free state to the plants.
