Jan. 1S91.] BOTANICAL SOCIETY OF EDINBURGH, 



43 



the needle track as a centre, forming in this way a most 

 beautiful arborescence, and will not show any definite growth 

 in the needle-track itself. If, however, one uses a gelatine 



Fig. ]. 



Fig. 2. 



Fig. 3 



Diagrams illustrating tlie growth of Bacillus arhorescens in gelatine of 

 varying density. Fig. 1 in 5 per cent ; Fig. 2 in 7 '5 per cent ; Fig. 3 in 10 

 per cent. 



of 7*5 per cent., this arborescence will be somewhat coarser 

 and less copious, and in the track of the needle there will be 

 an evident growth of a denser kind. 



If a still stronger gelatine be used, 10 per cent., then no 

 arborescence at all will be found, but instead a firm, dense, 

 coherent growth in the track of the needle, and slightly 

 heaped above its site. Between these two culture forms, 

 the 5 and the 10 per cent., we thus have nothing at all in 

 common, and one ignorant of these facts would be certain of 

 their being indeed altogether different organisms. In much 

 the same way the familiar characteristic growth of the 

 Pneumococcus of Friedlander, which consists in a nailhead- 

 like growth upon the surface of the gelatine, can only be got 

 in a strong gelatine. If a weaker gelatine is made use of, 

 the surface growth tends to spread more over the surface of 

 the medium, and hence to be not at all characteristic. 



