30 BOTANICAL GAZETTE [ JANUARY 
pigment production is materially influenced by such slight fluc- 
tuations. It has invariably happened in these experiments that 
the cultures kept in diffuse daylight show a distinctly fainter 
color than the control tubes kept in the dark. In several instances 
fluorescence failed to appear in cultures grown in the light. This 
is true, for example, of a culture of B. #2. albus in a solution of 
0.2 per cent. asparagin, 0.1 per cent. sodium phosphate, 0.0001 
per cent. magnesium sulfate. In other solutions too, in which the 
amount of pigment produced is at best not great, a similar com- 
plete inhibition of the fluorescigenic power can be noticed. Thumm 
(2. c.) does not state whether the cultures in his experiments 
were kept in the dark or in the light, and it may be suggested 
that if the latter was the case some of his negative results (as, 
for example, that with B. 7. zenuis in ammonium tartrate solution) 
might be accounted for. 
The question whether in such cases the light acts upon the | 
pigment or upon the metabolic activity of the bacteria is a diffi- 
cult one to come at. The turbidity of cultures kept in the dark 
and in the light. seems to be about the same. If cultures in 
which abundant pigment production has taken place be trans- 
ferred to the light (this was done with a set of cultures in 
ammonium succinate solution), a very slight fading out can be 
noticed at the end of two weeks. The fading is accelerated by 
placing the tubes on the window-sill (north light), but even 
after the expiration of three weeks no very extensive divergence 
can be noted when the tubes are compared with the control tubes 
which have remained all the while in the dark. The cultures of 
some species, however, show a greater tendency to fade than 
others; the pigment produced by B. ff. albus fades out quite 
readily, while that formed by B. fl. tenuis retains its brightness 
with comparative persistence. The fading out consists in the 
gradual yellowing of the pigment, which continues until all trace 
of green is eliminated ; cultures kept in the dark show the same 
change, but it takes alah as I have indicated, more slowly- 
The change is probably one of oxidation, and the acceleration 
of the process in the light is precisely what might be anticipated. 
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