64 Practical Plant Biology. 



becomes available for these organisms. But nitrous acid is 

 capable of further oxidation and consequently further liberation 

 of energy. This fact is utilised by a second class of nitrifiers, 

 namely, the nitro-bacteria which perform this oxidation and are 

 able to eke out a subsistence by this means. After their respir- 

 ation the nitrogen is fully oxidised and appears as nitrates. The 

 energy set free in the stages of oxidation of the nitrogen is not 

 only used for the work of the cells such as is usually carried out 

 by respiratory energy, but, as was mentioned before, it enables 

 these organisms to build up organic substances from carbon 

 dioxide and water, effecting thus by means of chemical energy 

 (Chemosynthesis) what green plants are able to achieve by solar 

 energy (Photosynthesis). 



Another very interesting example of respiration is afforded by 

 the sulphur bacteria. These organisms are found in any other- 

 wise suitable medium where sulphuretted hydrogen is to be had, 

 e.g. in sulphur springs, or in water where that gas is set free by 

 putrefaction. Their respiration is divisible into two stages. In 

 the first the sulphuretted hydrogen is oxidised, and water and 

 elemental sulphur are produced. Microscopic observation of 

 the sulphur bacteria, which are among the largest known, often 

 shows granules of this sulphur embedded in the protoplasm 

 of the cells. The second stage consists in the oxidation of the 

 sulphur to sulphuric acid. The two processes may be represented 

 in a simplified form by the equations 



2H 2 S + O 2 = 2H 2 O + 28 

 28 + 2H 2 O + 3<3 2 = 2H 2 SO 4 



In both stages energy is liberated. 



The examples just given illustrate a few points in the nutrition 

 and metabolism of bacteria ; however, they must be regarded as 

 representing only in a very limited degree the vast number of 

 different modes of existence which are found in this great group 

 of organisms, which is as yet but very imperfectly explored. 



PRACTICAL WORK. 



Apply the method described in the foregoing chapter of making a pure 

 culture of one of the gelatine-liquefying bacteria from the mixed growth in 

 seaweed. 



Similarly make a pure culture of one of those which do not liquefy 

 gelatine. 



Make stained smear-preparations of several isolated colonies in the petri 

 dish, and examine microscopically. 



