256 Development of a Standardised Agar Medium 



It ■will be seen that with 2 per cent, agar the spreading ceases within 

 five days, while with 1 per cent, agar it continues for a longer period, 

 although the normal retardation of spreading is well shown. 



With 0-5 per cent, agar an interesting result appeared. As will be 

 seen from the curve, the increase in colony area took place more slowly, 

 but there was no falling off of the growth, the increase in area taking 

 place quite steadily till the end of the experiment. This steady increase 

 was accompanied by an entire change in the form of growth. The colony 

 was quite circular, nearly transparent and had an indistinct edge, which 



50 r 



l-%Agar __^ 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 



Days growth. 



Fig. 5. B. dendroides. Spreading growth with varying percentages of agar. 



was not produced into processes. The alteration in the mode and form 

 of colony growth was due to the stiffness of the agar having been reduced 

 to a point at which the motile organisms were able to penetrate the 

 substance of the gel and progress slowly through it. When the growth 

 was examined under a |-inch objective it was found that instead of 

 forming a layer of surface growth, the rods were distributed throughout 

 the agar, each rod lying separately and moving through the gel with a 

 restricted, jerky motion. The colony grows as an ever- widening disc, 

 since the forces which normally lead to the formation of branches do not 

 operate. Also there is no faUing off of the rate of growth, since the 



