SECT. II PHYSIOLOGY 245 



It may be assumed that oxygen -respiration and intramolecular 

 respiration are expressions of one and the same property of the plant, 

 in other words that on withdrawal of oxygen normal respiration passes 

 over into intramolecular respiration. If this is true, it follows that the 

 essence of res])iration does not consist in an oxidation process but in a 

 breaking down of organic substance in which products arise that readily 

 take up oxygen. The materials which are respired in the plant, such as 

 carbohydrates and proteid, ai'e not easily oxidisable at ordinary tempera- 

 tures. Fats, it is true, which may also serve as material for respiration, 

 are oxidisable, but in this case we know that they are transformed 

 into carbohydrates before they are used for resj)iration by the plant. 

 The plant must thus have at its disposal special means in order to 

 carry on the oxidation and the preceding decompositions that are 

 involved in respiration. It is scarcely to be doubted that enz3mes are 

 concerned in this, but we have at present no insight into their jjrecise 

 action (^''). 



At first sight respiration appears a contradictory jDrocess, since in 

 it organic material which has been built up in assimilation is again 

 broken down. If a supply of energy is required for the construction 

 of organic material, energy will be set free on its decomposition. On 

 this liberated energy the plant is dependent for the driving force in 

 many of its vital phenomena. Movement of protoplasm, growth, and 

 movements due to stimuli cease on the withdrawal of oxygen from 

 the plant. All these vital phenomena begin again on the restoration 

 of a supply of oxygen, if this is not too long delayed. It might have 

 been expected that the organism would possess arrangements by the 

 help of which the external energy of light or heat could be employed 

 as driving power. Practically, however, it is found that the plant 

 proceeds to store up the energy of the sun's rays in the form of 

 potential chemical energy, and then utilises this at need. 



B. Oxidation of Inorganic Material (^^) 



While most plants use organic compounds, especially carbohydrates 

 in respiration, certain Bacteria utilise other sources of energy which 

 are more economical. Thus, the nitrite bacteria which commonly 

 occur ill tlie soil oxidise ammonia to nitrous acid, and the associated 

 nitrate bacteria further oxidise the nitrous acid to nitric acid. By 

 the help of the energy thus obtained they can then — as has already 

 been pointed out on p. 220 — assimilate carbon dioxide ; the chemical 

 energy takes the place for them of the sun's energy in the typical auto- 

 trophic plant. It is uncertain whether a true respiration exists in 

 addition to this process. Probably, however, this is not the case ; 

 the whole of the assimilated nutritrive substance is thus retained, so 

 that the woi'king of these organisms is very economical. Since, how- 

 ever, only a limited amount of ammonia is available, and this is 



