208 Elementary Biology. 



the complex compounds of which we have found protoplasm 

 to be composed becomes available. 



KatabDlism. The chief agent in the decomposition of 

 protoplasm is oxygen. The oxidation of carbon compounds 

 and the liberation of carbonic acid we have already termed 

 respiration, so that we may conveniently consider that 

 subject first. 



The inhalation of oxygen and exhalation of carbonic 

 acid must be most carefully distinguished from the absorp- 

 tion of carbonic acid and liberation of oxygen. In the 

 case of the animal, the determination of the ratio between 

 the gas inhaled and the gas exhaled is a comparatively easy 

 process ; in the plant it is complicated by the reabsorption 

 of ths carbonic acid so produced in the anabolism which 

 goes on during sunlight. In the dark, however, in the great 

 majority of green plants, and also in colourless plants both 

 at night and in daylight, it is easy to prove the exhalation 

 of carbonic acid in respiration. In the animal the oxygen 

 which is inhaled is almost immediately employed in the 

 oxidation of organic compounds ; in the plant, on the other 

 hand, the oxygen may be absorbed and stored up as intra- 

 molecular oxygen, and used as an oxidising agent long 

 afterwards. The chief products of respiration are the same 

 in both plant and animal, viz. water-vapour and carbonic 

 acid. It has been found, however, that in some plants car- 

 bonic acid is replaced as a product of respiration by other 

 so-called organic acids, such as oxalic and malic acid. No 

 doubt also much of the carbonic acid and water is trans- 

 formed into cellulose, as suggested at page 60, or excreted 

 directly as such. 



The other products of katabolism are some of them 

 useful (secretions), some of them useless (excretions). It 

 has already been seen that what are considered as waste 

 products in the animal world, e.g. carbonic acid, water, and 

 such like, are far from being so in the plant world. In 

 addition to these substances, however, there is a large 



