DIV. ii PHYSIOLOGY 273 



of the sugar which serves as the material for respiration. The molecule 

 of sugar breaks down and forms, in addition to carbon dioxide, other 

 reduced compounds. Sometimes, for example, alcohol according to 

 the formula 



If this empirical formula is replaced by the structural formula 



COH . CHOH . CHOH . CHOH . CHOH . CH 2 OH 

 - C0 2 + CH 3 . CH 2 OH + CH 3 CH 2 OH + CO,, 



it will be seen that the molecule of sugar has broken down into four 

 portions, two of which are poorer and two richer in oxygen than the 

 molecular groups from which they are derived. In this type of 

 respiration certain molecular groups withdraw the combined oxygen 

 from others. 



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 respiration 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, are 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 respiration 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 enzymes are 

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

 action ( 46 ). 



At first sight respiration appears a contradictory process, since in 

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

 broken down. Its meaning only becomes evident when, turning 

 from the changes of substance, those of energy are considered. It 

 is not the production of CO., and H 2 that is important, but only 

 the liberation of energy. This is effected on the breaking down of 

 such substances as carbohydrates, for the construction of which, 

 as has been seen, a supply of energy is requisite. On this liberated 

 energy the plant is dependent for the driving force in many of 

 its vital phenomena. Movement of protoplasm, growth, and move- 

 ments 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 



