PURPOSES OF RESPIRATION. 40;^ 



the most obvious indication of the processes of oxidation connected with normal 

 respiration ; and that these oxy-acids, which evidently arise at the expense of the car- 

 bohydrates and fats, in their turn constitute important points in the complex of vital 

 processes, may be concluded from the universality of their occurrence. Moreover, 

 Hugo de Vries' idea that the vegetable acids play an important part in the turges- 

 cence of the cells, and therefore in growth also, is scarcely to be put aside. On the 

 one hand it is a direct or indirect consequence of the process of respiration that in 

 germination and in the growth of buds also, compounds containing very little or no 

 oxygen arise from the splitting of carbo-hydrates and fats, since it is only during 

 normal respiration and the growth depending upon it that resins and ethereal oils 

 are formed. There is nothing extraordinary in the view that by means of a process 

 of oxidation (and respiration is a very intense process of oxidation) compounds poor 

 in oxygen as w'ell as those abounding in it arise ; but of course further investigations 

 will first have to render clear the details in these processes. 



It may appear absurd that plants which decompose carbon dioxide by means of 

 their organs containing chlorophyll, in order to produce carbonaceous vegetable 

 substance, on the other hand destroy such carbonaceous substance again during their 

 whole life by means of respiration, and thus effect a loss of the capital gathered 

 together by themselves. It may have been this reflection which drove so keen 

 a mind as that of Justus von Liebig to the quite unwarranted decision that no 

 respiration at all takes place in plants. But what was said at the beginning of this 

 lecture leads us to the right conclusion, in opposition to Liebig's, which depended 

 upon mere perplexity. It is in fact not simply a matter of accumulating a quantity 

 of organic vegetable substance by assimilation, but rather, this gain in substance is 

 only to serve the purpose of promoting the vital processes. Starch, fats, and proteid 

 substances are of course products of assimilation, but by and for themselves they are 

 inert material, just as bricks and mortar constitute merely the material for the con- 

 struction of a house. For these to be set in motion, and for the structure actually 

 to come into existence, moving forces are necessary, and it is respiration which 

 provides these in the organism. The loss of substance which results in addition 

 from respiration, serves to develope mechanical forces by means of which the atoms 

 and molecules of the remaining substance are set in those movements from which 

 growth and the other functions of the living plant result. In a word, respiration is 

 the source of the energy from which all the phenomena of life derive their vital 

 forces; while assimilation in the organs which contain chlorophyll supplies the 

 materials which are subsequently to be set in motion for the purposes of life. 

 This, expressed quite generally, is the physiological significance and object of 

 respiration, and it is certainly not too dearly bought by a relatively small loss of 

 substance. 



In respiration, as we have seen, carbon dioxide and water are produced at the 

 expense of organic substance. This process, according to a general law of nature, 

 is connected with the development of heat. As in every other process of combustion 

 of carbon and hydrogen to carbon dioxide and water, so also in respiration a 

 definite quantity of heat must be produced, though not exactly the same quantity 

 as when carbon and hydrogen in the elementary state burn with oxygen, since 

 a portion of the heat-producing force is destroyed in the former case, because 



D d 2 



