154 DR. A. J. EWART ON THE EVOLUTION OF 
of a power of assimilation in purple Bacteria is at present 
justifiable, but until further research has been made in this 
direction it cannot be regarded as a conclusively established 
fact. Allattempts to isolate Chromatium Okenii &c., on agar &., 
with and without SH,, in darkness or in light failed. 
It has been shown above that in certain coloured Bacteria an 
evolution of oxygen takes place because the pigment possesses 
the power of holding oxygen in a state of loose combination as 
hemoglobin does, whilst in others the evolution of oxygen is 
the result of a process of assimilation, and takes place only when 
an absorption of radiant energy is able to take place. Beyer- 
inck’s classification of pigment Bacteria into parachromophores, 
chromophores, and chromopares does not seem to be a correct 
one. So far as at present appears, the only Bacteria in which 
the pigment forms an essential and integral part of the 
bacterial plasma is in those green chlorophyllous and red 
bacterio-purpurin containing Bacteria, in which it has apparently 
an assimilatory function. Even here, as has been shown, under 
abnormal conditions the formation of pigment may be sup- 
pressed. This in the case of Streptococcus varians is perhaps 
comparable with the degenerative changes which may be induced 
in the chlorophyll grains of Elodea and Funaria under some 
what similar abnormal conditions *. In all the other coloured 
Bacteria examined the pigment appears to be an excrete product. 
They may be divided into two classes :—(a) Those which form 2 
lipochrome pigment insoluble in water and having the property 
of hoiding oxygen in a state of loose combination, and hence 
having an important biological significance, especially as regards 
the respiration of the Bacteria. (5) All other pigment-forming 
Bacteria, in which the pigment is often soluble in water and is 
an excrete product which is either functionless or the function 
of which has not as yet been discovered. 
In (1) under normal conditions the pigment is always formed, 
and always has precisely the same colour. -In 2 (a) under 
normal conditions the pigment is always formed, but its formation 
is more readily affected by external agencies, and varieties in which 
but little or no pigment is formed are rather readily produced. 
The colour of the pigment, as well as its amount, in many Case 
* "On Assimilatory Inhibition,” in Journ. Linn, Soc. (Bot.) xxxi. (1899) 
p. 422, etc. 
