200 METABOLISM 



as a matter of fact Gerber (1900) found it to be as much as 471 in Ricinus, i. e. 

 almost five times as much carbon-dioxide was formed as oxygen absorbed. 



The fact that sugar may be formed by oxidation in destructive metabohsm 

 is worthy of special attention, because it shows better than any other illus- 

 tration that a physiological classification of materials cannot be made to 

 agree with a chemical classification, since one and the same substance, in the 

 present instance sugar, can constitute at once a product of assimilation in 

 constructive metabolism and a respiratory product in destructive metabolism. 

 Further, we do not always meet with sugar in the germination of oily seeds, as 

 for example, in the onion ; it is wanting in Cannabis, where it is true it is formed 

 but very quickly changed into starch. Starch and sugar serve later on equally 

 well as supporters of respiration and as constructive materials for the manu- 

 facture of cell-walls. 



Carbohydrates and fats can to a certain extent replace each other in the 

 plant as respiratory material. This is known to be the case also in the animal 

 world, but there the two substances are not sufficient to maintain life, which is 

 dependent on a constant destruction and oxidation of proteid, followed by the 

 excretion of such nitrogenous waste as hippuric acid, urea and uric acid. The 

 question thus arises whether proteid is also destroyed in the plant during 

 respiration and whether that takes place of necessity. It may be easily shown 

 that peptone can act as a respiratory material, in Fungi especially, and it will 

 be sufficient to quote here an experiment of Wehmer's (1892) on Aspergillus. 

 This fungus thrives remarkably well on peptone as a source of nitrogen and sugar 

 as a source of carbon, but there are no obvious indications that the peptone is 

 used as respiratory material also. The fungus is, however, able to supply all 

 its wants both as to carbon and nitrogen when peptone alone is supphed to 

 it. Under these circumstances functions carried out by sugar must be undertaken 

 by the peptone, and it appears that a part of the nitrogen of the peptone is 

 transformed into ammonia and excreted. The insight we thus obtain into the 

 process of respiration is onlysomewhat limited, since the nitrogen of the ammonia 

 produced in this reduction cannot be compared with the carbon of the carbon- 

 dioxide produced in the oxidation of carbohydrates. Ammonia is a secondary 

 product of peptone respiration, which comes off free when the carbon of the 

 peptone is turned into carbon-dioxide in the process of combustion. The 

 appearance of products of reduction in respiration will be dealt with later, at 

 present we need only note how extremely varied a mould fungus can be in its 

 metabolism : ammonia, the same material which in combination with sugar 

 serves as a source of nitrogen, is also given off as a worthless waste product 

 when peptone is supplied as the single organic nutrient. 



The other question, as to whether some proteid must always be respired, is 

 not so easily answered. This was more often referred to the decomposition of 

 proteid especiaUy noticeable in seedlings but which has been clearly demonstrated 

 elsewhere and which leads to the formation of amido-compounds. We have con- 

 sidered these bodies as products of the hydrolytic decomposition of proteid, and 

 have assumed that the formation of these crystallizable and easily diffusible 

 substances was necessary, since proteid as such is not well adapted to trans- 

 location through the plant body. It is quite possible, however, that the amido- 

 compounds arise in respiratory metabolism and are produced from proteids by 

 oxidation. This view, at the present moment, can neither be proved nor disproved, 

 still when the condition of things in animals is taken into account we must regard 

 it in a certain sense as probable. It is easily understood, according to the earlier 

 statements made, that the products of the decomposition of proteid in plants are 

 not excreted. The amido-compounds which are formed under suitable conditions 

 are capable of being again made use of for the regeneration of proteid ; and this 

 fact increases the difficulty of completely solving this problem. If the amide- 



