106 OUTLINES OF BACTERIOLOGY 



Aspergillus niger, one of the aerobic higher fungi, can complete its 

 life-cycle when oxygen is present only to the extent of 5 mm. of air- 

 pressure. This last fact is another demonstration of the smallness of 

 the gap that separates the two classes of organisms. Finally, we may 

 add that it is possible both to accustom anaerobic bacteria to a greater 

 and aerobic bacteria to a smaller pressure of oxygen. The breaking 

 down of the sharp line of demarcation between the two classes lends 

 support to the suggestion of Frankland, that anaerobic bacteria have 

 arisen by a process of selection from aerobic organisms ; for if we 

 imagine representatives of the latter, in past ages, growing in places 

 where the supply of oxygen was small or where it was rapidly used 

 up, these organisms would either have to give up the struggle or learn 

 to do with less oxygen. In course of time, therefore, the breaking down 

 of organic matter would be accomplished without the aid of oxygen. 



As to the mode of decomposition employed by anaerobic bacteria, 

 very little is known, but inasmuch as the products are quite different 

 from those formed by aerobic bacteria, we know that oxygen does not 

 play a part in it. Hence it is improbable that they obtain oxygen from 

 within themselves as a substitute for that of the atmosphere. How- 

 ever, the trend of scientific opinion is in favour of the supposition that 

 anaerobes have found means of breaking down organic matter, and 

 thus securing energy without calling oxygen to their aid, as is done by 

 the whole of the animal world and the vast majority of the members 

 of the vegetable world. 



THE SAPROPHYTIC BACTEKIA. 



The term saprophyte is applied to any organism that feeds on dead 

 organic matter, whether this be of animal or of vegetable origin. These 

 organisms perform the work of scavengers, for they initiate processes 

 whereby the constituents of the carcases of animals and plants are 

 changed into substances which can serve as food-material to later 

 generations. In this way they prevent cessation of life, for were their 

 activities to cease, all animate nature would soon succumb for want 

 of food. The changes that are accomplished by the saprophytes we 

 term putrefaction, and they are usually accompanied by the evolution 

 of foul-smelling odours. If very little odour is manifest, as when 

 decomposition takes place in the open air, it is usual to apply the 

 term decay instead of putrefaction, although there is no hard and fast 

 distinction between the two terms. 



