66 PLANT PHYSIOLOGY 



and 1863) the proof that plants make very varied demands on oxygen. At 

 the one extreme there are those which can go through their normal life only 

 in its presence, and at the other those to which oxygen is a poison ; the former 

 are termed aerobes or aerophiles, the others anaerobes or anaerophiles, but the 

 difference is not so great as one at first sight might suppose. 



213, 11. 29-36, for Since nothing . . . with a loss read Ordinary aerophiles 

 can, as we have seen, support life for a long time by intra-molecular respira- 

 tion. Alcoholic fermentation forms a substitute, though certainly a very 

 inferior one, for respiration ; but in other organisms it forms a complete sub- 

 stitute. Thus Mucor stolonifer, when supplied with air, decomposes sugar 

 completely into C0 2 and H 2 O ; when air is deficient, however, alcoholic fermenta- 

 tion begins which, in contrast with typical aerobes (e.g. Aspergillus niger), 

 goes on for days (KOSTYTSCHEW, 1904 a). Other species of Mucor, how- 

 ever, e.g. M. racemosus, M. javanicus (KOSTYTSCHEW, 1904 a; WEHMER, 

 1906), never use up the sugar in presence of air uniformly, but always form 

 alcohol. When air is deficient normal respiration is inhibited, but not the 

 formation of alcohol. This behaviour certainly appears to support the 

 view that there are two different enzymes, one for respiration and one for 

 fermentation. 



The yeasts are closely related to Mucor racemosus, for they possess, in 

 addition to the power of inducing fermentation, the capacity for normal respira- 

 tion, and if oxygen be present in the fermentable medium, part of the sugar 

 is respired and the rest fermented. A definite amount of yeast produces all the 

 more alcohol the less it is able to respire normally, and if oxygen be entirely 

 absent all the sugar disappearing in metabolism is fermented. Since the energy 

 in the sugar is available to a far greater extent in respiration than in fermenta- 

 tion, one would conceive that more sugar would be used up in the latter case. 

 In one of GILTAY and ABERSON'S experiments (1894) i g. of yeast (dry weight) 

 used up in the same time 7g. of sugar when air was supplied, but 14 g. when it 

 was excluded. The maximum formation of alcohol, however, is by no means 

 coincident with the best vital condition of yeast. Growth and multiplication 

 are in the higher degree dependent on the supply of oxygen. When the oxygen 

 is greatly reduced vegetative growth ceases entirely, though fermentation con- 

 tinues for a longer time. Unfortunately there is no unanimity among authors 

 as to the extent to which yeast increases in absence of oxygen ; some deny it 

 altogether, while others (BEIJERINCK, 1894) state that the yeast may increase 

 up to 20 or 30 times its original amount. Whichever view be correct, growth 

 is certainly very limited when alcoholic fermentation alone goes on, but is 

 unlimited when respiration ensues. Although, however, the amount of alcohol 

 formed in a unit of time obviously depends on the amount of yeast present, 

 still one cannot say a priori whether a minimal amount of yeast will produce 

 in the long run more alcohol with or without oxygen. Experiments and 

 empirical results teach us that if only a little oxygen be added the yeast increases 

 greatly, and fermentation is so little retarded that a maximal production of 

 alcohol may be observed. If a suitable nutritive solution be inoculated with 

 a small amount of yeast, air being carefully excluded, the organism at first 

 absorbs greedily the oxygen dissolved in the fluid ; it is able to appropriate 

 even loosely-combined oxygen, tearing away oxygen from oxyhaemoglobin, 

 although it is unable to decolourize indigo-carmine. Thus the yeast at first 

 increases to a certain extent and alcoholic fermentation begins to be apparent, 

 bubbles of C0 2 appearing in the fluid ; soon, however, the bubbles become 

 smaller and smaller and finally cannot be seen at all. It is very remarkable 

 how the addition of a minute air- bubble at once raises the intensity of fermenta- 

 tion and how visible bubbles of carbon-dioxide once more begin to come off 



