THE RESPIRATION OF PLANTS 465 



tints of leaves are clue. The former appears to be always present 

 with chlorophyll in the chloroplasts. 



Though many of these substances, both excretions and bye- 

 products, are of no value for nutrition, some of them may play a 

 very important part in the defence of plants against their 

 natural enemies, their nauseous smell or flavour preventing their 

 being eaten by animals, &c. Some odours and the nectar found 

 in flowers are doubtless of great service in attracting insects, 

 which assist in the process of cross-pollination, to be discussed 

 in a subsequent chapter. 



CHAPTER XL 



THE RESPIRATION OF PLANTS. 



One of the conditions of life of almost every organism, whether 

 animal or vegetable, is that it shall be supplied with free oxygen. 

 Not only do all the vital activities of plants as well as animals 

 depend upon this, but the very life of the protoplasm ceases if 

 this gas is withdrawn. Consequently, as long as life is main- 

 tained, the organism must be continuously absorbing oxygen. 

 This is very evident in the case of an animal, but it is not so 

 obvious in that of a gi-een plant, for the process of decomposing 

 the COo taken in as a material for food construction is, as we have 

 seen, accompanied by an evolution of oxygen. In the case of a 

 plant which has no chlorophyll the absorption of oxvgen can be 

 much more easily detected, as it is not masked by this converse 

 process of its exhalation. If a fungus, such as a mushroom, be 

 placed in a closed receiver containmg ordinary air. and be left 

 there for some hours, at the conclusion of the experiment the 

 vessel will be found to contain but little oxj^gen, which will have 

 been replaced by about an equal amount of CO.,. It is not dith- 

 cult to devise an experiment which will show that a green plant 

 has the same absorbing power. If the light be excluded from 

 one placed in a similar vessel, no evolution of oxygen will take 

 place from it, as we have already noticed in a preceding chapter, 

 and that the oxygen diminishes even to extinction can be made 

 evident just as in the case of the mushroom. We have, however, 

 evidence that this is not caused by the exclusion of the light. 

 An apparatus can be easily arranged to show the absorption of 

 oxygen even when a green plant is exposed to a bright sunlight. 

 The atmosphere in which the plant is placed can be freed fi-om 



VOL. II. II H 



