LOGARITHMIC CURVE OF GROWTH 273 



Comparatively little was known of the life history of any 

 Schizomycete. Ward therefore made a detailed and exhaustive 

 study of that of Bacillus raniosus, the Wurzel bacillus of German 

 authors, which is common in Thames water, and bears a super- 

 ficial resemblance to the anthrax bacillus, but is innocuous. It 

 proved convenient for study, as it ran through its entire life 

 history in from thirty to sixty hours at ordinary temperature. 

 It forms long filaments, the growth of which Ward was able to 

 measure under the microscope with great precision. On plot- 

 ting out his measurements he obtained a regular curve, from 

 which he found that, under constant conditions, the filament 

 doubled itself in equal times. This he called " the law of 

 doubling." It is the same as the so-called "law of compound 

 interest," and leads to the expression of the growing quantity 

 as an exponential function of the time, so that the time is 

 proportional to the logarithm of that quantity. This relation 

 has, of course, long been familiar in chemical reactions, but, as 

 far as I know, Ward was the first to detect it in any vital process 

 in a plant. This, which was in 1895, has, I think, been over- 

 looked. Stefanowska has since, in 1904, obtained a logarithmic 

 curve for the early period of the growth of maize, which doubles 

 its weight every ten days, and the subject has since been pursued 

 by Chodat and others. 



In speculating on the cause of the destructive action of 

 light on bacteria, Ward adopted the view of his friend Elfving, 

 that it inhibited metabolic processes necessary to nutrition. He 

 suggests that the "constructed metabolites" at the moment of 

 assimilation are in a highly unstable condition, and liable to 

 destruction by oxidation promoted by light. He points to the 

 fact that plant structures are frequently provided with colour 

 screens, which would cut off the blue-violet rays and check their 

 action in promoting the rapid oxidation of reserve materials,, 

 and he quotes the suggestion of Elfving that chlorophyll itself 

 may serve as such a screen against "destructive metabolic action 

 in synthesis." Ward seems to have attributed little importance 

 to the fact that substantially the same view had long before been 

 put forward by Pringsheim, though received with little favour. 

 His own view that when red and orange predominate in the 



o. B. 18 



