BACTERIA. 85 



according to Fliigge, species occur amongst the peptonising 

 bacteria of milk, the spores of which will withstand boiling 

 for four hours. Spores of hay bacillus will also withstand 

 boiling for hours. Spores can usually stand dry heat better 

 than boiling in steam or water. On the other hand, many 

 spores show special resistance to heating in milk, and the 

 same is true of neutral or feebly alkaline liquids, whereas 

 acid liquids are unfavourable to their existence. Spores are 

 generally difficult to stain. On the other hand, colouring 

 matters once taken up by spores are retained better than by 

 vegetative cells ; and after bleaching, therefore, coloured 

 spores become visible in a colourless cell. 



As soon as favourable conditions of nutriment and tem- 

 perature recur, spores germinate. They first swell up by 

 absorption of water, and the contents lose their strong re- 

 fractive power. A bacterium then grows out from the spore ; 

 the wall of the latter is sometimes seen to burst or to unfold 

 into two valves (Figs. 16, 24). The full-grown rod then multi- 

 plies in the usual manner. Spores may maintain their germina- 

 tive power through a long period, sometimes for many years. 



In addition to the endosporic bacteria just mentioned, 

 *' arthrosporic " bacteria were formerly described which do 

 not form spores in the interior of the cell, but in which it was 

 believed that members split off from vegetative cells form 

 the starting point of fresh vegetative generations. A micro- 

 scopically discernible difference between the " arthrospores " 

 and other cells, however, occurs only in a few cases, in that 

 the walls of the latter thicken (Chlamydospores). Perhaps 

 by continued investigation endogenous spores will be found 

 in all such species. 



The gonidia which occur in Crenothrix constitute a special 

 kind of cell ; in the case of some bacteria, as, for instance, Clado- 

 thrix, they are motile. These cells are organs of propagation. 



Temperature plays an important part in the life processes 

 of bacteria. We distinguish the minimum, optimum, and 

 maximum temperature at which life can exist. These three 

 cardinal points differ, not only for each species, but also for 

 the individual functions of each, such as its rate of growth 

 and its fermentative activity. 



