54 MICRO-ORGANISMS AND FERMENTATION. 



forms ; when they intertwine like a plait of hair, the form 

 called Spirulina is produced. To these must be added the 

 remarkable irregular, swollen, or curved forms which many 

 bacteria can assume involution forms (i) ; these are often 

 brought into existence by unfavourable conditions of life, but 

 in the case of certain species (acetic acid bacteria) form part of 

 the regular course of development. 



We will now select one of these forms and submit it to a 

 thorough examination with a magnifying power of about 1000 

 diameters. Like every other cell, it contains protoplasm, a 

 homogeneous, feebly refractive mass, in which infinitesimal 

 particles can be detected here and there, especially if the cell 

 is not in its most active growth. Sometimes a bright spot is 

 found in the middle of the cell, which, from analogy to the 

 higher plants, is considered to be a sap-cavity or vacuole. In 

 some bacteria certain solid substances have been detected, as, 

 for instance, sulphur grains in bacteria which live in water 

 containing sulphur ; in some species the plasma can, under 

 certain conditions, be coloured blue by iodine, which indicates 

 the presence of substances resembling starch. 



Surrounding this protoplasmic body we find a cell-wall or 

 membrane. An examination of this by means of staining will 

 generally show that its outer layers are swollen up into a 

 gelatinous mass, which becomes especially distinct when masses 

 of bacteria are aggregated together. From a chemical stand- 

 point it must be provisionally assumed that this cell-wall is 

 of a different nature in different species. In some it reminds 

 us of the cellulose of the higher plants, whilst in others it 

 appears rather to resemble the albuminoids in its properties. 



Many bacteria contain blue, red, yellow, or green colouring 

 matter, which sometimes causes very intense coloration. 

 Under the microscope, however, the individual bacteria appear 

 only very faintly coloured. It has not yet been determined 

 with certainty in what part of the organism the colouring mat- 

 ter is situated. Some species of bacteria are phosphorescent 

 under certain nutritive conditions. 



A remarkable property of many bacteria is their free loco- 

 motion. This is either quick or slow, the bacteria rotating 

 about their longitudinal axes, assuming the forms of open or 



