. Elementary botany . Botany. 2l6 RESPIRATION. Fig. 246. gas due to the absorption of the minute amount of carbonic acid present in the air at the commencement of the experiment would be imperceptible.) But we find that, on the contrary, the liquid rises in that arm of the tube which is in direct communication with the air inside the bottle (see ^ fig. 246). This proves that the seeds are absorbing gas from the contained air. The absorbed gas is oxygen. The evolution of carbonic acid implies that the organic substance of the plant is being split up into simpler con- stituents, one of which is
. Elementary botany . Botany. 2l6 RESPIRATION. Fig. 246. gas due to the absorption of the minute amount of carbonic acid present in the air at the commencement of the experiment would be imperceptible.) But we find that, on the contrary, the liquid rises in that arm of the tube which is in direct communication with the air inside the bottle (see ^ fig. 246). This proves that the seeds are absorbing gas from the contained air. The absorbed gas is oxygen. The evolution of carbonic acid implies that the organic substance of the plant is being split up into simpler con- stituents, one of which is carbonic acid. Though it is not capable of simple proof, water is also produced as a result of this decomposition. The fact that the plant loses some of its solid substance by this process is well illustrated by the following experiment. Seeds or potato- tubers are germinated in darkness, being suppHed only with air and pure (distilled) water. Growth continues for a time, but the plants are growing at the expense of the reserve- foods contained in their substance, for they are receiving no food capable of building up organic matter. After they have grown for a time we examine them chemically, and find (by calculation) that they contain less organic substance than they did in the seed- or tuber- condition. They have lost substance, particularly carbon, because of the evolution of carbonic acid gas derived from their organic substance. All the facts above cited prove that in actively-living plants there is a process going on by which the organic matter of the plant is being broken down and carbonic acid is being evolved, and that the decomposition is accompanied by an absorption of oxygen. This process is termed respiration. It will be noted that the process of respiration involves an excretion of carbonic acid and an absorption of oxygen. The assimilation of carbon by green parts exposed to light involves processes which are just the reverse. Hence it is not easy to prove that g
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