Breathing of Plants 87 



food which passes down the liber is in a fit state to be absorbed 

 by the plant. 



(It is generally stated that the presence of chlorophyll is 

 necessary to the decomposition of carbon dioxide. Recent 

 observations, however, have raised the suggestion that chloro- 

 phyll may be the result, not the cause, of the decomposition. 

 The question needs further investigation, but the presence of 

 chlorophyll and the decomposition of carbonic acid always go 

 together.) 



A very important point to remember is, that the plant does 

 not absorb free nitrogen from the atmosphere. All the nitrogen 

 required by the plant is taken in either as ammonia or nitrates. 



3. As a breathing organ. — Plants, like animals, carry on 

 a process of respiration, i.e. they take in atmospheric oxygen, 

 which combines with the carbon and hydrogen of the tissues 

 forming carbon dioxide and water, which are restored to the 

 air. The respiration, however, of plants is much less than 

 that of animals ; it differs in its amount in various plants, and 

 in the different parts of the same plant. The more energetic 

 the growth, the greater the amount of respiration : hence it is 

 especially well seen in quickly germinating seeds, and unfolding 

 leaf and flower buds. 



The oxygen passes into the interior of the plant by means 

 of the stomata, and by the same openings the carbonic acid 

 gas is passed out into the atmosphere. This change goes on 

 continually, irrespective of the presence or absence of light. 

 During the daytime the feeding by the decomposition of carbon 

 dioxide and assimilation of the carbon is so greatly in excess of 

 the respiratory act, that it completely overshadows it, and ap- 

 parently nothing is going on but the decomposition of the car- 

 bonic acid ; but in the night time^ when, owing to the absence 

 of the light, no assimilation is taking place, the breathing can 

 be perceived. 



4. As an organ of transpiration. — A large portion of the 

 water which is taken in by the roots of the plant escapes by 

 the leaves. By means of this evaporation the sap becomes 

 thickened as it ascends the stem. The transpiration takes 

 place by means of the epidermis, and especially through the 



