BACTERIAL PI CM EX TS AND CELL-CONTENTS 13 



Finally, there are bacteria, Z>. I'iolaccus for example, in which the pigment 

 is lodged mainly in the cell-wall ; such forms may be termed ' parachromatO' 

 phorotts. 1 



There is conclusive evidence that the red colouring matter of the 

 sulphur bacteria, ' bacteriopurpurin,' performs the same functions as the 

 chlorophyll of higher plants, assimilating carbonic dioxide under the in- 

 fluence of light and giving off free oxygen (see Chap. VII). Only the outer 

 layer of the cell contents is charged with pigment, the central mass being 

 colourless. 



In all probability the colouring matter is of importance physiologically 

 only where it is bound up with the protoplasm. In all the ' chromoparous ' 

 bacteria the pigment is an excretion merely, and, as might have been 

 expected, chemical and spectroscopic analysis fail to show that it has any 

 connexion with assimilation. 



Some of the bacterial pigments are of the nature of lipochromes and 

 are related to the fats ; others have a basic character and resemble the 

 ptomaines. Many seem to belong to the proteins, and the colouring matter 

 of B. cyaneo-fnscus has a composition similar to that of indigo. 



Differentiated cell-contents, such as the starch or aleurone grains of 

 higher plants, are wanting in most bacteria. The protoplasm stains a 

 uniform golden yellow when treated with iodine. Some of the butyric acid 

 bacteria, however, as well as various species inhabiting the human mouth, 

 form an exception to this rule, and take on a blue or deep violet tint. 

 Very little is known about the substance to which this reaction (granulose 

 reaction) is due ; it has been called granulose merely on the strength of 

 its behaviour towards iodine. Whether its chemical composition is similar 

 to that of the granulose of starch cannot be decided by this reaction alone. 

 It seems to be a carbohydrate, however, and carbohydrates as food are 

 necessary for its growth. The mouth bacteria obtain these of course from 

 our food, and the butyric acid organisms from the fermenting substances in 

 which they grow. The ' granulose ' makes its appearance in the bacterial 

 cell in the form of very minute grains, so that after treatment with iodine 

 the yellow protoplasm seems full of black points. These granules become 

 considerably larger, and then seem to diffuse through the cell contents, so 

 that finally the whole cell stains blue or violet. 



In the butyric acid bacteria granulose is at first absent, but is formed 

 when the time of sporulation draws near. In that part of the bacterium, 

 however, where the spore appears, none is deposited, the protoplasm staining 

 from first to last yellow with iodine. It would seem that we have here an 

 instance of division of labour, the swollen end of the bacterial cell being 

 devoted to sporulation, and the cylindrical portion serving as a manufactory 

 and storehouse for granulose, from which the spore may be nourished 

 (Fig. u, cf). 



