146 PLANT RESPIRATION , 



is the usual respiration of the seed-plants and the strongly 

 aerobic microorganisms in which there is a complete combustion 

 of sugar even with low oxygen tension.^ The following 

 observation, already described, can serve as an illustration of 

 the unequal behavior of strongly aerobic fungi, such as Aspergil- 

 lus niger, on one hand, and the aerobic, fermenting fungi which 

 are rich in zymase, such as yeasts and mucors, on the other 

 hand, in the case of a somewhat restricted access to oxygen.^ 

 If Aspergillus niger is immersed in a sugar solution it does not 

 produce a trace of alcohol if the surface of the solution is left 

 in equilibrium with the air. The oxidising capacity of this 

 fungus is so strong that even the small amount of oxygen dis- 

 solved in the solution effects a complete oxidation of the respira- 

 tory material. The fermenting yeasts and mucors behave 

 quite differently. Under the same experimental conditions 

 they give rise to a vigorous alcoholic fermentation and the 

 combustion of sugar is discontinued. 



Third Case. — The plant cells contain an insuihcient amount 

 of enzymes of fermentation. Hence the oxidising enzymes are 

 not only able to oxidise completely all the intermediate products 

 of fermentation but they also attack the unchanged sugar," 

 since their working capacity is not spent on the insignificant 

 quantities of the fermentation products. An incompletely 

 proven example of this case is that of the formation of gluconic 

 acid by Aspergillus niger. M. Molliard^ has found that with 

 a deficiency of mineral nutrients, Aspergillus niger oxidises 

 grape sugar to gluconic acid: 



2CHoOH^(CHOH)4— CHO + O2 = 2CH0OH— (CH0H)4— 



COOH. 



It is characteristic of oxidising enzymes of plants that they 

 are able neither to break carbon chains nor to unite carbon 



' Stich. Flora. 74: i. 1891; Amm. Jahrb. f. wiss. Bot. 25: r. 1893; Johannsen. 

 Untersuch. aus d. Bot. Inst. Tubingen 1 : 716. 1885. 



2 Kostytschew, S. und M. Afanassjewa. Jour. d. russ. bot. Ges. 2: 77. 191 7. 



5 MoUiard, M. Compt. rend. 174: 881. 1922; 178: 41. 1924. 



"There seems to be no proof that the sugar is "unchanged" {unzersetzlen) 

 under these conditions. A number of points previously mentioned by the author 

 favor the view that the sugar must be changed to some isomer or derivative 

 before it can be attacked by the oxidising enzymes. — Ed. 



