246 BOTANY 



PART I 



derived from other organisms, they cannot take the dominant place 

 in nature which the green 2)lants do. 



Tlie transformation of ammonia to nitric acid is of use to the higher phmts for 

 which nitric acid is the best source of nitrogen (p. 226). 



With the nitro-bacteria tlie so-called sulphur-bacteria may be associated ; these 

 oxidise sulphuretted hydrogen to sulphuric acid, sulphur being an intermediate 

 product, and being stored in the body of the plant. Probably some of them 

 {Bcggiatoa) are able to assimilate carbon dioxide by the help of chemical energy and 

 without chlorophyll or light. It is doubtful whether the oxidation of ferrous to 

 ferric oxide by the iron-bacteria is a source of energy. On the other hand, the 

 oxidation of methane and hydrogen, which has been shown to be eflected by certain 

 bacteria, may have this significance. 



C. Fermentation. 



With the removal of oxygen intramolecular respiration begins, but 

 this cannot supply the necessary energy to maintain life in the higher 

 plants, although it may do so in lower organisms. Many Bacteria, 

 Fungi, and certain Algae (Characeae) are notably independent of a 

 supply of oxygen ; they succeed with slight traces of this gas, or they 

 avoid it altogether and live in situations where oxygen is absent. 

 Such organisms are called anaerobes or anaerobionts in contrast to 

 the typical aerobes or aerobionts. All intermediate stages connect the 

 two extremes. The true anaerobionts decompose large amounts of 

 organic substances, and this decomposition, which is in principle the same 

 as the process of intramolecular respiration, is termed fermentation. 

 As in intramolecular respiration these processes are concerned with 

 obtaining combined oxygen. The prototype of fermentation is the 

 alcoholic fermentation brought about by the yeast fungus. In this 

 sugar is split up into alcohol and carbon dioxide, and the process has 

 great technical importance in the production of beer, wine, and brandy. 

 The chemical process is the same as that of intramolecular resj^iratio)! 

 in a green plant ; in contrast to this the yeast plant obtains in the 

 fermentation a complete substitute for respiratory activity. It is, 

 however, only independent of oxygen when it is supplied with a 

 suitable fermentable material (sugar). In the absence of sugar, oxygen 

 is indispensable, and normal resjiiration takes place. When both sugar 

 and oxygen are supplied, respiration and fermentation go on simul- 

 taneously ; part of the sugar is transformed into C.^HyO and CO., and 

 another part into H.,0 and CO.,. Obviousl}^, the transformation of 

 sugar into alcohol and carbon dioxide will provide much less energy 

 than the complete combustion to carbon dioxide and water. It is 

 thus easy to understand that yeast utilises enormous quantities of 

 sugar. Only about 2 per cent of the sugar in the nutrient solution is 

 used in the construction of the substance of the plant, i.e. is assimilated; 

 the rest is fermented. For effecting this extensive decomposition of 

 the sugar, yeast employs a specific enzyme (zymase), the existence of 

 which was demonstrated by E. BuciiNER {^^). 



