CULTURE METHODS. 283 



and often characteristic colour, than by microscopical means. 

 For developmental purposes it is of the greatest importance to em- 

 ploy such cultures when information as to the relations of definite 

 forms is required. Lastly upon such macroscopic cultures often 

 depends the problem of determining the presence in certain media 

 of spores capable of germination, and perhaps even to settle their 

 number. The most important condition for all these investigations 

 is that the culture shall start, and shall remain, free from any kind 

 of accidental contamination. The problem how to carry on " pure 

 cultures " has therefore assumed the highest importance. 



To foster and perfect the methods of bacterial culture has now 

 become the special duty of institutes for hygienic purposes. We 

 will ourselves be satisfied with obtaining general information as 

 to these methods ; and only turn our attention more closely to 

 them in so far as they may also have a bearing upon the botanical 

 problems of bacteriology, and the further culture of the lower 

 plants. 



The rearing of bacteria was formerly carried on almost ex- 

 clusively in fluid media ; a transparent solid nutrient substratum 

 was first brought into general use by Robert Koch. This solid 

 transparent nutrient substratum is prepared from a gelatinous 

 stiffening basis, to which one adds a suitable nutrient solution. 

 Thus, according to the* needs of the organism to be cultivated, the 

 nutrient gelatine is prepared with hay-infusion, flesh-extract, 

 peptone-solution, or blood-serum. In order to cultivate our hay- 

 bacillus upon a solid nutrient material this latter would be prepared 

 from hay-infusion and gelatine ; for pathogenic bacteria a nutrient 

 medium, to which is added peptone-sodium-chloride-bouillon, plays 

 the chief part. The nutrient solution in this latter case is prepared 

 from infusion of meat (beef-tea) with peptone and table salt, and 

 made feebly alkaline with basic sodium phosphate or sodium car- 

 bonate. Without gelatine this nutrient fluid may be spoken of 

 simply as a nutrient broth ; if combined with ordinary gelatine it 

 is called Koch's nutrient gelatine ; with agar-agar, nutrient agar. 

 Nutrient gelatines to suit his special purposes the botanist will 

 have to prepare for himself ; those used for pathogenic organ- 

 isms, in the preparation of which special precautions are required, 

 are better procured ready made. A considerable number of firms 

 concern themselves with the preparation of such gelatines for 

 micro-chemical use, amongst others that of Dr. C. Grubler of 

 Leipzig (whose products can be obtained from Southall Bros. & 



