CHAPTER III. 



SOLID CULTIVATION MEDIA. 



Nutrient Soils. 



38. To cultivate any organism artificially, apart from the tissue of its 

 host, it must be supplied with a nutrient soil, which will support its 

 vital necessities, and which is perfectly free from the presence of any 

 other organism. If the soil be lacking in the former respect, the 

 organism rapidly dies from inanition ; if in the latter, a struggle for 

 existence occurs between the organism already present and that 

 newly implanted, and one or other is shortly exterminated, or each is 

 modified in many respects from the close contact of so near a 

 neighbour. Perfect sterility of the soil is therefore a sine qua non for 

 the successful artificial culture of organisms. 



The nutrient soil used may be either a liquid or a solid ; and 

 certain facilities of observation are offered by one or the other in the 

 case of different organisms. The employment of solid media is now 

 generally adopted, for in them an organism frequently exhibits speci- 

 fically characteristic methods of growth in a way which is not attain- 

 able where liquids are used. On the other hand, speaking generally, 

 organisms grow with greater rapidity in liquids than in solids, and 

 thus sooner reach their state of maturity, and the transparency of 

 most liquids so used affords facility in making observations, which is 

 not the case with many of the solid materials. A few solid media, 

 however, have been devised, in which the great advantage of trans- 

 parency has been attained, and these offer such a series of advantages 

 over other methods that they are now very largely employed in 

 making observations. Their advantages are as follows : — 



i. Convenience in manipulation : can be inverted (see footnote, 

 p. 90, on gravitation of germs). 



2. Slower growth of organisms, allowing more complete observation 

 of the various stages of development. 



