PHYSIOLOGY 243 



sources of force in the plant (force of pressure in turgescence, vibrations of light 

 and heat rays, etc.). 



The majority of plants sacrifice in physiological combustion a portion of their 

 organic substance (especially carbohydrates) in order to obtain this driving power. 

 The combustion is as a rule so complete that CO., and H.,0 are the resulting products ; 

 this not only obtains the maximum of energy but obviates the accumulation of 

 injurious products of respiration. In other cases, however, considerable amounts of 

 organic acids are formed. This modification, which is found in succulents living under 

 unfavourable conditions for gaseous exchange and consequently for assimilation, 

 avoids the loss of carbon attendant on the liberation of C0. 2 into the air. (Cf. p. 241). 



The energy liberated by the respiratory combustion of carbon-compounds is 

 traceable back to that stored in the form of potential energy of chemical combination 

 in carbohydrates, which were formed by the help of the sun's rays in assimilation 

 (cf. p. 216). Energy is not only liberated in the more or less complete combustion 

 of carbon-compounds but may be obtained in other chemical processes. While 

 most plants consume organic substances in respiration, some lower plants, especially 

 Bacteria, depend on energy obtained from other chemical changes. Thus sulphur- 

 bacteria oxidise sulphuretted hydrogen to sulphur, and this in turn to sulphuric 

 acid ; others, according to NATHAXSOHN, oxidise thio-sulphates. The nitrite-bacteria 

 form nitrous acid from ammonia and amides, while nitrate-bacteria convert the 

 nitrous into nitric acid. It is doubtful whether the iron-bacteria gain a similar 

 benefit from the oxidation of compounds of iron. The energy obtained in such 

 processes may serve to replace the driving power of respiration, or, as has been 

 shown for the uitro-bacteria (p. 189), and is possibly the case for the sulphur- 

 bacteria also, may effect the synthesis of organic carbon-compounds. The working 

 power is thus devoted to one or other process of the life of the plant ("). 



Fermentation and Respiration (). In the section on special 

 modes of nutrition it was stated that when this is effected at the 

 expense of organic food material, fermentation is frequently set up 

 in the substratum. Since fermentation is a process of decomposition 

 associated with a liberation of energy stored in organic compounds, 

 it may be serviceable in the same way as respiration. Its amount 

 may also be more or less influenced by the respiratory needs of the 

 organism. 



Thus, when the yeast plant is living as an anaerobe, the greater part of the 

 organic substratum (about 98-99 per cent) is fermented. When growing as an 

 aerobe, with a full supply of oxygen, a larger amount of the substratum is available 

 for use in the processes of growth and multiplication. 



Since, even in the latter case, a large part of the grape-sugar (according to 

 BUCHNER and RAPP about 85 per cent) is fermented, it may be concluded that the 

 process of fermentation, though under certain circumstances a source of energy to 

 the plant, is to some extent independent of the respiratory needs of the latter, 

 and may have a further significance. Since the products of fermentation are as 

 a rule far better endured by the organisms which have given rise to them than 

 by others, their production may have an oscological value in the struggle with 

 competing organisms. Oxygen-respiration is also to a certain extent independent 

 of the other vital manifestations. It is most active at a temperature a little below 

 that which causes the death of the organism, when all other activities are being 

 arrested by the heat. 



