62 Practical Plant Biology. 



be adopted. Opening the petri dish cautiously a sterilised piece 

 of wire is stabbed into one of the colonies which we select for 

 further examination, and thus infected the wire is stabbed deep 

 into the still solid medium in a sterile test-tube. Of course, the 

 ritual previously described is strictly observed when opening and 

 closing this tube. It is now set aside for incubation, as the pro- 

 cess of growth and reproduction under defined conditions is called. 

 The growth in the tube may now be submitted to various 

 observations. 



Sub-cultures taken from this primary pure culture may also be 

 submitted to various experimental tests, such as culture on 

 various media containing certain substances and omitting others. 

 In this way information as to the nature of the nutrition of the 

 special bacteria under observation may be obtained. 



Such observations have proved that the vast majority of bacteria 

 are holozoic in their nutrition, i.e. they depend on organic sub- 

 stances for their supplies of carbon and nitrogen, in contrast to 

 plants containing chlorophyll which draw their supplies of carbon 

 and nitrogen from inorganic substances, as has already been de- 

 scribed. This latter type of nutrition is called holophytic, as 

 previously stated. In the case of some bacteria it is found that 

 the organic substance must be supplied by living organisms or it 

 is unfit to act as food. These are parasites and among them are 

 found many which cause disease. Many of these natural para- 

 sites can accommodate themselves to live upon dead organic 

 substances and so grade insensibly into the saprophytes which 

 normally subsist on dead organic matter. A very common and 

 almost universally distributed saprophyte is Bacillus vulgaris. It 

 is a long bacillus covered uniformly with cilia and usually de- 

 velops freely in protein solutions. By its activities and those of 

 its confederates the proteids are gradually broken down into 

 various simpler bodies, e.g. peptones, ptomaines and various 

 aromatic products and finally the inorganic end products of putre- 

 faction, viz. ammonia, free nitrogen, free hydrogen, carbon di- 

 oxide, sulphuretted hydrogen, etc. Various bacteria are concerned 

 in these processes and research has not yet delimited the role to be 

 assigned to each. The vast number of saprophytic bacteria, and 

 the complexity of their nutritive substances and their products 

 render the unravelling of the various changes and mutual inter- 

 actions in the process of decay an investigation of great difficulty. 

 For putrefactive bacteria proteins can act as a source not only 

 of nitrogen but also of carbon. Culture experiments, however, 

 show that many of them can also utilise simpler compounds to 



