196 BIOLOGY OF SPECIALIZED GROUPS 



Micrococcus citreus var. flavus as a typical example. The 

 chemical structure of these insoluble pigments is unknown. 



The pigments of chromogenic bacteria have also been 

 classified as diffusible (Ex. Ps. pyocyanea) and nondiffusible 

 (Ex. B. prodigiosus). This classification is of no practical 

 value. 



Some species of chromogenic bacteria which attack plants 

 produce a brown stain on the plant at the point of inoculation 

 (Bact. campestre (yellow) on stems and leaves of cucumbers 

 and tomatoes), while other non-chromogenic forms produce 

 the same effect on the plant (Bacterium solancearum on stems 

 and leaves of cucumbers and tomatoes). Plants, under vari- 

 ous conditions, show stains which cannot be attributed to any 

 chromogenic bacterium or other microorganism. These stains 

 are oxidation products. For example, when Bacterium vas- 

 cularum attacks sugar cane very often the plant shows a 

 red stain. It has been noted, however, that fungus growths 

 and insect stings will produce the same color. This coloring 

 of plants has been noted in many species and under various 

 conditions. It is frequently noted in green fruits and flowers. 



The subject of the chromogenic bacterial pigments from the 

 various species of bacteria has not been exhaustively studied 

 as yet. Most of the investigations so far have been on the 

 blue-green and fluorescent pigments pyocyanin and fluorescin, 

 which were referred to above. 



Historical Allusions. The red chromogenic bacterium 

 known as Bacillus prodigiosus has a very interesting history. 



