218 BOTANY 



of the living substance. The presence of oxygen is necessary to 



THE CHEMICAL PROCESSES TAKING PLACE WITHIN THE CELL, IN ORDER 

 TO MAINTAIN THE LIVING SUBSTANCE IN A CONDITION OF NORMAL 

 ACTIVITY. 



The absorption of oxygen and the evolution of carbonic acid by living plants 

 can be demonstrated both qualitatively and quantitatively by simple experiments. 

 From what has already been said of the contradictory nature of assimilation and 

 respiration, it will be at once apparent that these experiments must be conducted 

 either in the dark or on portions of plants devoid of chlorophyll. Coloured or 

 white flowers, roots, germinating seeds and Fungi furnish suitable objects on 

 which, at any time, the gaseous interchange occurring during respiration may 

 be observed. The more abundant the protoplasm and the more energetic its 

 vital activity, so much the more vigorous is the respiration. The best results are 

 obtained, therefore, from young portions of plants in an active state of growth. 

 It should also be mentioned that from the following experiments only the carbonic 

 acid and not the whole of the products of the respiratory activity are determined. 

 From theoretical considerations, and also from exact chemical analysis, it has been 

 definitely established that, in addition to carbonic acid, water is formed 



FROM THE ORGANIC MATTER BY RESPIRATION. 



The absorption of oxygen and the formation of carbonic acid may be clearly 

 shown by the following experiments (Fig. 191). A flask (B) filled with young mush- 

 rooms or Composite flowers is inverted with its mouth in an open vessel of mercury 

 {Q), and a few centimetres of caustic potash solution introduced within its neck. 

 In the same degree as the carbonic acid produced by respiration is absorbed by the 

 caustic potash, the volume of air in the flask will be reduced and the mercury will 

 rise in the neck. After a time, the ascent of the mercury ceases and it remains 

 stationary. If the quantity of air remaining in the flask be estimated, it will be 

 found that it has lost a fifth of its original volume ; this means, however, that the 

 whole of the oxygen (which makes up one-fifth of the atmospheric air) has been 

 absorbed. If caustic potash is not used in this experiment to absorb the ex- 

 haled carbonic acid, the mercury remains at its natural level, or, in other words, 

 the volume of air in the flask remains unchanged. From this experiment it is 

 apparent that the volume of oxygen absorbed is equal to the volume of carbonic 



CO 

 acid evolved, as expressed by the formula -j=r-?=l. This equivalence of volume 



between the oxygen absorbed and the carbonic acid exhaled exists only in cases 

 where the oxygen is used exclusively for respiration, and not where it is con- 

 sumed in transforming the contents of the cells, as is observed in the germination 

 of seeds rich in fat, and in the interchange of gases in the case of the succulents. 

 In the germination of seeds rich in fat, the fat is converted into carbohydrates 

 richer in oxygen. The oxygen consumed remains combined in the plant. On the 

 other hand, in the case of the succulents, their peculiar power of effecting oxidation 

 during the night and subsequent deoxidation in the light, modifies the gaseous 

 interchange of respiration. 



The absorption of oxygen in the respiration of plants can also be shown by the 

 fact that a flame, held in a receptacle in which plants have been kept for a time, is 

 extinguished. If a lighted taper be thrust into a glass cylinder which has been 

 partially filled with flowers or mushrooms, and then tightly covered and allowed 

 to remain for a day, it will be extinguished, as the oxygen of the air in the 

 cylinder will all have been absorbed. The carbonic acid exhaled in respiration can 



