INOCULATION OF POTATOES. 



63 



of the cut potato surface, and the seed material is thus implanted in the 

 sterile nutrient soil (see Fig. 22). The wire is then withdrawn, and the 



Fig. 22. — Inoculation of potatoes. 



bell jar closed. Note is taken of the date and the substance inocu- 

 lated, and the potatoes are set aside for subsequent observation. 



As a general rule, those organisms which are adapted for cul- 

 tivation on potato are capable of growing actively at ordinary 

 temperatures. In any special case, where it is desired to incubate 

 organisms growing on this soil, the bell jar, with its contents, must be 

 placed in a warmer room, or in one of the larger sizes of incubator 

 subsequently to be described. Incubation at high temperature is not 

 so desirable in the case of potatoes as in that of some other soils, 

 for they become dry superficially, and thus lose their special adapta- 

 bility to the growth of organisms. 



(B.) Cultivation on Sterile Bread Paste. 



41. Slight differences in the chemical constitution of a nutrient soil 

 often determine its suitability, or otherwise, for the growth of an 

 organism ; certain species, which thrive but slightly on potato, are 

 found to flourish luxuriantly when supplied with other forms of starch, 

 as that from cereals. A convenient soil for such organisms is sup- 

 plied by bread paste. This is made simply of bread crumb and 

 water. A stale loaf is thoroughly dried in an oven ; then all the 

 crust is removed, and the remainder is finely powdered. Small 

 clean one-eighth litre Bohemian flasks are charged each with ten 

 grammes of the bread crumb, and to this is added 2-5 c.c. of dis- 



