OXYGEN FROM COLOURED BACTERIA. 155 
varies somewhat according to the nutrient medium on which the 
bacterium is grown. 
2(b). The formation of pigment often takes place only under 
very specialized conditions, and colourless varieties are com- 
monly very readily produced, which may have permanently lost 
the power of forming pigment even when brought under the 
most favourable possible conditions again (Bacillus prodigiosus). 
In many cases the pigment is excreted as a colourless chromogen, 
which later is oxidized and becomes coloured. Frequently the 
pigment may pass through a series of colours before it reaches 
its most highly oxidized condition. This class is simply a hetero- 
geneous assemblage of all the other pigment-forming Bacteria ; 
but a further classification is impossible until more is known 
about them. It is quite possible that in certain of them the 
pigment may have functions as important as in those coloured 
Bacteria which can evolve oxygen. 
Conclusions. 
A number of coloured Bacteria possess the power under 
Ppropriate conditions of evolving oxygen in greater of less 
amount. In certain of these the oxygen evolved appears to be 
occluded oxygen absorbed from the air by the pigment-substance 
excreted by the Bacteria. The process is not a vital one. The 
substances contained in an alcoholic extract appear to have, 
though to a less extent, the same power of occluding oxygen, but 
this property is soon lost. 
ue S Purple and green Bacteria, in which the pigment forms an 
adini part of the bacterial plasma, show, when exposed to 
an indefinite à very weak evolution of oxygen, continuing for 
former " : ength of time if conditions are favourable. In the 
in the | ese the assimilatory pigment is “ bacterio-purpurin, 
e latter chlorophyll. The process is a vital one, and the 
or i . . 
catbon die ìs apparently derived from the assimilation of 
Botanisches Institut, 
Leipzig, 
