120 NUTRITION AND METABOLISM. 



without results. It is not known that the pigment is of any material 

 advantage to bacteria; for it is possible to cultivate colorless strains of 

 pigment bacteria which grow apparently as well as the orginal pigmented 

 culture. Again, pigments cannot take the place of the chlorophyl in 

 plants except perhaps the bacteriopurpurin of the purple bacteria. It 

 does not even protect the cells against intense light, because the pigmented 

 organisms are not more resistant than the corresponding colorless 

 "sports." The only exception are the colored spores of the molds, 

 especially Penicillium and Aspergillus, which are very resistant to light, 

 while the spores of Oidium are killed just as easily as the mycelium. 

 Pigments cannot be considered as reserve substances, since many pig- 

 ments are excreted and remain outside the colorless cells. Pigment 

 production may be incidental. It is possible that the waste products 

 of certain organisms happen to be colored. 



After Beyerinck, the chromogenic bacteria may be divided into three 

 classes: 



1. Chromophorous bacteria, in which the pigment is placed in the cell 

 and has a certain biological significance analogous to the chlorophyl 

 of higher plants. In this division belong the green bacteria discovered by 

 Van Tieghem and Engelmann and the red sulphur bacteria or purple 

 bacteria. 



2. Chromoparous or true pigment-forming bacteria, which set free the 

 pigment as a useless excretion, either as a color-body or as a leuco-body 

 which becomes colored through the action of atmospheric oxygen. The 

 individuals themselves are colorless and may under certain conditions 

 cease to form pigments. To this class belong B. prodigiosus, Ps. Syn- 

 cyanea, Ps. pyocyanea, and others. 



3. Parachrome bacteria, which form the pigment as an excretory pro- 

 duct but retain it within their bodies, as B. janthinus and B. violaceus 



When the pigment is soluble in water, as those produced by Ps. pyo- 

 cyanea and the fluorescent bacteria, it diffuses through the medium. 

 When the pigment is not soluble, it either lies within the cell wall or be- 

 tween the individuals. 



This classification furnishes some details concerning the methods of 

 pigment production, which depends upon the presence of certain media; 

 according to Sullivan, sometimes certain mineral salts, sometimes sugar 

 will stimulate chromogenesis. The same is true with molds. Very 

 brilliant colors appear with certain species of molds if grown on cellu- 



