SECT. II PHYSIOLOGY 243 



Since, however, when no potash is present, the volume of gas is not 

 altered by the respiration of the plants, an equal volume of carbon dioxide 

 must be formed for each volume of oxygen that is absorbed. If we 

 assume that sugar is the substance respired, this must take place accord- 

 ing to the formula — 



CgH^.Oe + 602 = 6CO2 + CHoO. 



This is an exactly opposite process to the assimilation of carbon dioxide. 

 The volume of air does not under all circumstances remain 

 unchanged by the respiration of the plant ; the carbon dioxide 

 produced is not always equal in volume to the oxygen which 

 disappears. Small deviations from this ratio occur in all plants, and 

 considerable ones, in, for instance, the germination of fatty seeds, and 

 of the leaves of certain succulent plants (Crassulaceae). This depends, 

 on the one hand, upon other substances than carbohydrates being 

 used in respiration, and, on the other, upon different end products than 

 COo and H^O being formed. 



Ill tlie germination of fatty seeds far more oxygen is absorbed tlian carbon 

 dioxide is given off ; this may go so far tliat in the first days in the dark, in spite 

 of continual respiration, an increase in the dry weight takes place. Most of 

 the oxygen is used in the transformation of fats, which are poor in oxygen, 

 into carbohydrates, and only a small proportion is used in respiration. In the 

 Crassulaceae a large jiroportion of tlie carbohydrate is changed into organic acids 

 in the process of respiration. The oxidation is thus incomjilete ; it does not 

 lead to the formation of CO2, so that less of this gas is formed than the amount of 

 oxygen absorbed would lead us to expect. 



This peculiar respii'atory process which is connected with an accumulation 

 of acids in the cell-sap, as can be recognised by the taste, is not without oecological 

 significance for succulent plants. Since in them gaseous exchange is difficult, they 

 retain the carbon once it has been obtained and diminish their need of absorbing 

 CO2 from the atmosphere (■'^). 



Probably other organic acids, the wide distribution of which in plants has been 

 referred to, are products of incomplete oxidation. 



It is not so easy to demonstrate the formation of water in typical 

 respiration as it is to show the utilisation of oxygen and the pro- 

 duction of carbon dioxide. Quantitative estimates of the loss of dry 

 weight and of the carbon dioxide formed show that the latter does 

 not account completely for the former ; a part of the dry substance 

 must thus have been transformed into water. 



As has been mentioned, respiration is of general occurrence in the 

 higher plants. It not only occurs in the parts of plants which do not 

 possess chlorophyll and are commonly used in experiments on 

 respiration, but can be demonstrated also in cells which contain 

 chlorophyll. In this case the respiration in the light is masked by 

 the quantitatively greater process of assimilation ; it appears only as a 

 diminution in the products of assimilation. If the light is diminished 

 assimilation ultimately ceases and the respiration becomes evident. 



Though respiration goes on in every cell its intensity varies greatly in 



