100 PHYSIOLOGICAL ACTIVITIES 



hence they are frequently found on various sea foods, espe- 

 cially when these are allowed to decompose, such as fish, 

 oysters, clams, etc. The light is due to the conversion of the 

 energy of unknown easily oxidizable compounds directly into 

 visible radiant energy through oxidation without appreciable 

 quantities of heat. The light produced may be sufficient 

 to tell the time on a watch in absolute darkness, and also 

 to photograph the growths with their own light, but only 

 after several hours' exposure (Fig. 71). None of the phos- 

 phorescent bacteria so far discovered produce disease in the 

 higher animals or man. 



PRODUCTION OF PIGMENT OR CHROMOGENESIS. 



One of the most striking results of bacterial activity is 

 this phenomenon. The particular color which results may 

 be almost any one throughout the range of the spectrum, 

 though shades of yellow and of red are of more frequent 

 occurrence. 



In the red sulphur bacteria the " bacteriopurpurin" which 

 they contain appears to serve as a true respiratory pigment 

 in a manner similar to the chlorophyl in green plants, except 

 that these bacteria oxidize H2S in the light as a source of 

 energy instead of splitting up CO2. The red pigment pro- 

 duced by certain bacteria has been shown to have a capacity 

 for combining with resembling that of hemoglobin," and 

 some investigators have believed that such bacteria do 

 store O in this way for use when the supply is diminished. 

 With these few exceptions the pigments seem to be merely 

 by-products of cell activity which are colored and have no 

 known function. 



The red sulphur bacteria above mentioned and one or 

 two other kinds retain the pigments formed within the cell. 

 Such bacteria are called chromophoric as distinguished from 

 the chromoparic bacteria whose pigment lies outside the 

 cell. 



The chemical composition of no bacterial pigment has 

 been determined up to the present. Some are soluble in 

 water, as shown by the discoloration of the substances on 



