I Q2 BIOLOGY OF SPECIALIZED GROUPS 



cally no physiological importance. The pigment granules 

 are usually excreted as rapidly as they are formed. In only 

 those cases where the pigment is combined with the proto- 

 plasm of the cell (chromatophorous) is it of any importance. 

 In this case it undoubtedly serves the functions of the chloro- 

 phyll in the higher plants. It can, therefore, absorb carbon 

 dioxide (CO 2 ) and give off oxygen (O) in the presence of sun- 

 light. This fact has been demonstrated in the case of the 

 bacteriopurpurin of the sulphur bacteria. In all chromo- 

 parous bacteria, the pigment granules can only be regarded 

 as excretory products of the cell. No connection with the 

 nutritive processes of the cell can be demonstrated. Chromo- 

 genesis seems to be an unessential process. 



In regard to the conditions under which pigment formation 

 takes place it may be said in the beginning that pigment for- 

 mation rarely takes place in the absence of oxygen. In cul- 

 tures it is noted that only the superficial layers of bacteria 

 produce any pigment, those beneath being colorless. Oxygen 

 seems to be a prerequisite in most cases. This point will be 

 again referred to. Pigment formation in most species of 

 bacteria takes place equally well in the light and in the dark. 

 In a few species pigment formation is inhibited by darkness 

 (chromatophorous). Bacillus mycoides var. roseus produces 

 pigment only in the dark. High temperatures usually inhibit 

 pigment formation (37 C. and above). Colored lights have 

 no effect on chromogenesis. 



Some pigments are closely related chemically to the lipo- 



