22 GENERAL MORPHOLOGY AND BIOLOGY. 



it is a frequent experience that, when a species is sown on 

 a mass of artificial food medium, it does not in the great 

 majority of cases go on growing till the food supply is 

 exhausted, but soon ceases to grow. Effete products 

 diffuse out into the medium and prevent growth. Such 

 diffusion may be seen when the organism produces pig- 

 ment, which frequently can be observed in a transparent 

 medium far beyond the limit of the growth of the organism, 

 e.g., B. pyocyaneus growing on gelatine. In supplying 

 artificial food for bacterial growth, the general principle 

 ought to be to imitate as nearly as possible the natural 

 surroundings, though it is found that there exists a con- 

 siderable adaptability among organisms. With the patho- 

 genic varieties it is usually found expedient to use media 

 derived from the fluids of the animal body, and in cases 

 where bacteria growing on plants are being studied, in- 

 fusions of the plants on which they grow are frequently 

 used. With some bacteria special substances are necessary 

 to support life. Ttius some species, in the protoplasm of 

 which sulphur granules occur, require sulphuretted hydrogen 

 to be present. In nature the latter is usually provided by 

 the growth of other bacteria. When the food supply of a 

 bacterium fails, it degenerates and dies. The proof of 

 death lies in the fact that when it is transferred to fresh 

 and good food supply it does not multiply. If the bacterium 

 spores, it may then survive the want of food for a very 

 long time. It may here be stated that the reaction of the 

 food medium is a matter of great importance. Most 

 bacteria prefer a slightly alkaline medium, and some, e.g. t 

 the cholera vibrio, will not grow in the presence of the 

 smallest amount of free acid. 



Moisture. The presence of water is necessary for the 

 continued growth of all bacteria. The amount of drying 

 which bacteria in the vegetative stage will resist varies very 

 much in different species. Thus the cholera spirillum is 

 killed by two or three hours' drying, while the staphylo- 

 coccus pyogenes aureus will survive ten days' drying, and 

 the bacillus diphtherise still more. In the case of spores 



