II. (r. Thornton 255 



against the time of incubation. It will be seen that the growth increases 

 in area at a perfectly even rate there being no retardation in the rate of 

 spread even after eleven days. 



The result of this experiment also disposes of the view that the 

 retardation of spreading is the result of an increasing accumulation of 

 metabolic products of growth. 



It therefore seems clear that the progressive retardation of spreading 

 observed on normal platings is the effect of drying. This drying could 

 operate either by reducing the film of surface moisture in which spreading 

 takes place or by producing an unfavourable increase in the concentra- 

 tion of salts in the medium. This question can be examined by measuring 

 the rapidity of spreading on media in which the concentration of agar 



4 5 6 7 8 10 11 



Days gi-owth. 



Fig. 4. Z). deudroidcs. Spreading over agar plates in a saturated atmosphere. 



is varied. An increase in percentage of the agar will not appreciably 

 increase the concentration of salts, but will reduce the relative amount 

 of free water in the medium and in consequence shorten the time taken 

 for the surface moisture film to evaporate. So that if the mere drying 

 of the surface moisture is the cause, the period of spreading should be 

 shortened as the percentage of agar is increased. 



A "synthetic" agar medium was therefore made up with three per- 

 centages of agar — 0-5, 1 and 2. Five platings of each medium were poured 

 and after being kept at 20° C. for 24 hours in order to start the drying, 

 were inoculated with B. dendroides. The plates were incubated at 20° C, 

 and the area of growth measured at intervals. The mean areas of growth 

 on the sets of five parallel plates are shown in Fig. 5 in which the area 

 of growth is plotted against the time. 



