300 VEGETABLE PHYSIOLOGY 



of ammonia and the consequent formation of a nitrite; 

 others can convert the nitrite into a nitrate, but neither 

 can do the work of the other. Others have not such 

 limited powers ; a certain bacterium can cause the oxidation 

 of alcohol to acetic acid, and after the exhaustion of what 

 alcohol may be present, can further oxidise the acetic acid 

 to carbon dioxide and water. The exact way in which the 

 protoplasm acts as a carrier of the oxygen without 

 apparently undergoing decomposition is very obscure. It 

 may perhaps combine with the oxygen and pass it on to 

 these oxidisable substances, acting as a carrier only. 



It has recently been found that besides exerting a direct 

 oxidative power, protoplasm can secrete an enzyme, or 

 perhaps a variety of enzymes, each with a special peculiarity, 

 through whose instrumentality the oxidation is effected. 

 These enzymes have been termed oxidases, and they are 

 probably wide-spread in the vegetable kingdom. A dis- 

 cussion of their peculiarities would be beyond the scope of 

 this volume, but we may call attention to their general 

 features. 



The first one discovered is known as laccase ; it has a 

 very wide distribution, occurring in the roots, stems, and 

 leaves of various plants, and in a very large number of 

 fungi. It appears to oxidise various constituents of plants, 

 but particularly the colouring matters. Another, known 

 as tyrosinase, occurs in other fungi, and oxidises chiefly 

 ty rosin. Others oxidise various colouring matters, together 

 with tannin. 



Many very complex disturbances set in when a normally 

 respiring plant is cut off from a supply of oxygen. Death 

 does not immediately supervene as might almost be expected. 

 Instead, the partial asphyxiation or suffocation stimulates 

 the protoplasm to set up a new, and perhaps supplementary, 

 series of decompositions, resulting in the liberation of 

 energy as do those of the respiratory process. We have 

 already noticed that under such circumstances the exhaling 

 stream of carbon dioxide can still be observed. This led 



