CARBON DIOXIDE FORMED DURING RESPIRATION. 397 



Like the respiration of animals, that of plants also consists in chemical 

 exchanges between the oxygen taken up and the organic compounds of the living 

 body, so that finally carbon dioxide and water are produced at the expense of these 

 compounds. The formation of water at the expense of the organic substance can 

 only be demonstrated indirectly by chemical analysis, as we shall see : on the other 

 hand, it is one of the easiest experiments in the province of vegetable physiology to 

 demonstrate the exhalation of carbon dioxide. In general, the evolution of carbon 

 dioxide is the more energetic the more vigorous the vital activity generally of the 

 organ observed ; and since the latter increases as a rule as the temperature ascends 

 to a certain optimum (say 2 50-3 o« C), so also the respiration and formation of 

 carbon dioxide increase in a like manner. It is particularly in the processes connected 

 with energetic utilisation of material in the growth of seedlings, unfolding buds, and 

 especially in flowers, that the evolution of carbon dioxide takes place most vigorously 

 and can be observed most certainly. The observer only meets with difficulties in 

 determining the changes in the composition of the air surrounding the plant which 

 are produced by the respiratory process when he is concerned with organs abounding 

 in chlorophyll, which are at the same time exposed to the influence of the light, 

 because in this case carbon dioxide is taken up and oxygen evolved in the process of 

 assimilation, and thus an exchange of gases takes place which aff"ects the surrounding 

 air in a manner exactly opposite to that due to respiration. Nevertheless there is no 

 doubt, cither theoretically or with reference to Garreau's experimental results, that 

 even green organs occupied in assimilation continually respire in the ordinary 

 manner like all other living organs. That green leaves, when not assimilating — 

 e.g. in a feeble light, or in the dark— respire somewhat energetically is established 

 with as much ease as in the case of organs containing no chlorophyll. 



If it is required to show that plants or entire organs convert a definite quantity 

 of the oxygen of the surrounding atmosphere into carbon dioxide, the simplest 

 experimental methods suff.ce. It is only necessary, for instance, to shut off with 

 wetted mercury an ordinary graduated absorption-tube, into which a Bean, Pea, 

 Acorn, etc. which is commencing to germinate has been placed, to convince 

 •oneself, by the absorption with potash of the carbon dioxide found in the tube after 

 a few days, of the fact that the whole of the oxygen has been converted into carbon 

 dioxide. It is at the same time observed in this simple experiment that the seedling 

 saturated with water, at first, so long as oxygen is still present in the absorption- 

 tube, goes on growing, but that after the oxygen is completely used up growth 

 ceases. 



An optical demonstration of the formation of carbon dioxide by respiration may 

 be provided still more easily in the following manner. A glass cylinder of about 2-3 

 litres capacity and furnished with a well-fitting stopper is filled with about 400-500 

 germinating peas in layers alternating with damp filter paper ; or in the same way 

 several hundreds of unfolding flower-buds of Camellia or of some fruit-tree, or the 

 unfolding leaf-buds of any plant, may be employed. After carefully closing the 

 cylinder, it is allowed to stand for 10-20 hours. On then carefully and slowly 

 raising the stopper, and slowly lowering a burning taper fastened to a wire into the 

 cylinder, the flame and the incandescent wick are extinguished, exactly as if the 

 '-vessel had been filled with pure carbon dioxide. Carbon dioxide is, as is well known. 



