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 green parts of plants exposed to light are respir- 

 ing; for, carbonic acid is being absorbed much more rapidly 



