CHAPTER XVI 



ABSORPTION OF CARBONIC ACID BY THE 

 GREEN PLANT 



If a few green leaves, or a green branch, be placed in a closed, 

 moistened, air-tight bottle and exposed to the light, the 

 carbonic acid contained in the air within the bottle will dis- 

 appear and an equal volume of oxygen will replace it. This 

 shows that green leaves, or green parts of a plant, can obtain 

 carbon from the carbonic acid of the air. If, however, the 

 bottle and the contained leaves be placed in darkness, the 

 carbonic acid will not disappear. This experiment illustrates 

 the fact that light is required in order to enable the green parts 

 of plants to obtain carbon from carbonic acid. Again, if the 

 leaves be killed (by steamin'g, freezing, drying, or poisoning by 

 chloroform) before they are placed in the bottle, the carbonic 

 acid will not disappear even in the presence of light. This 

 demonstrates that the green parts must be living if they are to 

 obtain carbon from carbonic acid of the air. But if, instead of 

 placing green parts of plants in the bottle, we put parts with- 

 out the green colouring-matter, such as petals, roots, pieces of 

 fungus, the carbonic acid does not vanish. This serves to 

 show that only parts containing green colouring-matter can obtain 

 carbon from carbonic add in the atmosphere. 



We have not yet seen how the green parts obtain the carbon 

 from carbonic acid, or how it is that an equal volume of 

 oxygen appears in the atmosphere in place of the carbonic 

 acid. This is most easily shown by experiments on green 

 submerged water-plants. If we cut across the shoot of 

 such a water-plant, and leave it in the water exposed to light, 

 a stream of bubbles will arise from the cut end of the stem 

 (fig. 238). These bubbles consist of oxygen which is being 

 exhaled by the plant. No bubbles of oxygen will be given off 



193 N 



