THE RESPIEATION OF PLANTS 469 



formation of certain organic acids which are foiuid in abundance 

 in the tissues of the plant. 



In the case of certain micro-organisms there is no absorption 

 of free oxygen ; indeed these so-called ana'erohic plants are 

 killed by exposure to the gas. We must not, however, conclude 

 that their metabolism is of a. totally different kind from that of 

 others, but rather that they obtain what oxygen they require 

 from internal decompositions, or fermentative changes which 

 they set up. 



The meaning of these complex metabolic processes must be 

 looked for in relation to the question of the energy which 

 the plant requires for its vital functions. There are but two 

 sources of energy available for its use ; one of these, the 

 radiant energy of the sun's rays, can only be utilised through 

 the chloroplasts, and is not directly at the disposal of the proto- 

 plasm. But the construction by them of various complex 

 materials, food and plant substance, involves the expenditure 

 upon the latter of a considerable amount of this energy, which 

 can be again set free by the decomposition of the complex bodies 

 and the production of simpler ones from them. It is, of course, 

 an every-day experience that the combustion of coal or wood, 

 which is really the oxidation of its carbon and hydrogen to 

 the condition of COo and water, is attended with the liberation 

 of a great deal of energy which takes the form of heat. So 

 with the decompositions of the protoplasm which are set up, 

 or at any rate facilitated, by the access of oxygen. The splitting 

 up of the complex molecule, and the formation of simpler ones, 

 at once set free certain potential energy, the amount being 

 proportionate to the extent of the changes. Part of this energy 

 is required at once by the protoplasm for the reconstruction 

 or building up of its substance from some of the residues, but 

 as some comparatively- simple bodies, such as COo and water, 

 are always left to be given off, there is always liberated a 

 certain amoimt of energy available for other work. In many 

 cases this takes the form of heat. In germinating seeds and in 

 the opening of flower-buds, where respiration is particularly 

 active, there is always an appreciable rise of temperature, 

 suflacient indeed to be measured bj' a thermometer. In other 

 cases the released energy is iitilised in causing movement ; in a 

 few instances occurring among certain fungi, it takes the form 

 of light or phosphorescence. 



The importance of respiration thus becomes evident ; it marks 

 the initiation, and is the accomraniment, of the catabolic pro- 



