58 Practical Plant Biology. 



but little chance of accurately investigating the metabolism of 

 any one form unless we are able to isolate the different types and 

 investigate them separately. Otherwise it will be impossible to 

 decide whether any product observed is the result of the combined 

 action of a number of different kinds of organisms, or is due to 

 one alone. Hence we find that the first great advances in our 

 knowledge of the metabolism of bacteria were only possible after 

 the pioneers such as Pasteur, Tyndal, and Koch had elaborated 

 methods of isolated culture. 



The observation that bacteria grow and multiply most abun- 

 dantly in organic substances furnished by living or dead plants 

 and animals suggested the use of meat extract as a suitable 

 medium for the cultivation of bacteria. For various reasons 

 which will appear later on it is convenient that the culture 

 medium should be transparent and solid. These two conditions 

 are fulfilled by filtering the meat extract and adding to it sufficient 

 gelatine to make it set at a suitable temperature. 



Having thus obtained a suitable medium we are at once con- 

 fronted with the difficulty that, being suitable to the growth of 

 bacteria, very soon such quantities develop in it that it becomes 

 rapidly decomposed. This difficulty also raises a problem which 

 caused much discussion during the last century, and in its solu- 

 tion evoked a mass of experiment which added greatly to our 

 knowledge and also largely improved bacteriological technique. 

 In this discussion one side maintained that some or all of the 

 bacteria or germs which caused decay and putrefaction originated 

 in the medium by spontaneous generation. The other side held 

 that they were the descendants of spores or cells introduced into 

 the medium from contact with surfaces on which these cells lay, 

 or that they entered into it as dust borne in the air. 



It was found that as a general rule when the vessel containing 

 the medium was raised to a high temperature and sealed her- 

 metically the bacteria did not develop and decay was arrested ; 

 but, on the other hand, undoubted cases were recorded in which 

 development of bacteria and progressive decay took place in 

 sealed vessels, which had been raised to a temperature of 100 C. 

 Inasmuch as it was known that protoplasm is killed at a tempera- 

 ture a little over 70 C. it seemed as if spontaneous generation 

 had occurred, subsequently to the heating, in those vessels in 

 which organisms and decay appeared. The later discovery which 

 showed that spores might withstand a temperature of 100 C. 

 put another complexion on the matter, and it soon transpired 

 that no development of micro-organisms occurred in those vessels 



