SIZE AND STRUCTUBE OF THE BACTERIAL CELL. 33 



produces a brilliant pigment. In this case the pigment, or 

 rather its antecedent, is made within the cell and is then 

 excreted into the surrounding medium. 



A few bacteria show a faint red coloration when exam- 

 ined under the microscope. This is due to the presence of 

 a red pigment known as bacterio-purpurin. This substance, 

 in its microchemical reactions, appears to be identical with 

 a similar coloring matter present in certain protozoa. It is 

 believed to exercise a role similar to that of chlorophyll. 



A green chlorophyll-like pigment has been observed to 

 be present in a small number of bacteria. The absence of 

 chlorophyll from the majority indicates that these organ- 

 isms, unlike the higher plants, cannot assimilate carbonic 

 acid. This, however, does not necessarily follow, since 

 certain nitrifying bacteria are capable of assimilating car- 

 bonic acid even in the absence of light. 



A number of bacteria have been met with in the mouth 

 which show a yellowish or brownish tint. The exact nature 

 of the pigment in either of these cases is unknown. 



On contact with iodine, protoplasm in general takes on 

 a light yellow color. A number of bacteria, however, give 

 with iodine a blue or dark violet color. Inasmuch as this 

 reaction is similar to that with boiled starch or granulose, 

 it is spoken of as the granulose reaction. Such bacteria 

 would seem to contain, therefore, a carbohydrate similar to 

 soluble starch. This substance may be present only in 

 scattered granules, or these may accumulate so that the 

 entire cell is deeply stained. In some bacteria the substance 

 is not present in the cell until just previous to spore forma- 

 tion. Bacteria giving this reaction are especially found in 

 the mouth. 



Motility of bacteria. When bacteria are examined in the 

 living condition they will, as a rule, show motion. The 

 movements observed may be apparent or they may be real. 

 In the latter case the organism is in active motion, travel- 



3 



