4SO PROrOPHYTA 



the kind at all. Records of the successive occurrence of different 

 forms in the same situation have been substituted for direct observations 

 of continuity, while errors of even grosser kind abound in the vast 

 literature of the subject. Bacteria are the present refuge of those 

 who believe in ' spontaneous generation,' just as higher forms of organ- 

 ised beings were the subject of their speculations in former times, when 

 the instruments of investigation were less perfect. 



Bacteria are either single minute cells of roundish form, or cylin- 

 drical and rod-like cells or rows of cells. As has been said, their very ' 

 minute size has prevented our attaining an exact knowledge of the cell- 

 structure. The cell-cavity is ordinarily filled with homogeneous proto- 

 plasmic contents. Chlorophyll has been discovered tingeing the proto- 

 plasm in three forms — Bacterium viride and Bacillus virens of Van 

 Tieghem, and more faintly in Bacterium chlorinum of W. Engelmann. 

 A red colouring matter discovered by Lankester, and named by him 

 baderiopurpurin, tinges the protoplasm of Beggiatoa roseo-persicina 

 (Zopf), but though colours are often associated with masses of Bacteria, 

 it is difficult to discern in the magnified view the exact seat of it, 

 whether it occur in cell-contents, cell-wall, or substratum. In some 

 forms (which do not contain chlorophyll) a substance resembling starch 

 is found. No one has yet detected nuclei with absolute certainty. The 

 cell-membranes are very delicate, and in such cases as Spirillum 

 (Ehrenb.) highly elastic, but it happens to most Bacteria at one stage of 

 their development that gelatinous outer layers are formed, which either 

 invest single cells and cell-groups, or unite into masses large numbers 

 of cells. A great number of Bacteria have the power of free movement. 

 During such movement rotation takes place round the longitudinal 

 axis, while movements of oscillation also occur in other forms. Cilia or 

 flagella are found in some, but not all, of these moving Bacteria, and it 

 has not been proved that they are motile organs as one might too readily 

 infer. As a matter of fact it is not known, in many cases, whether 

 these flagella are parts of the membrane or of the protoplasm protruded 

 through it ; and since they are not always present in motile Bacteria, 

 they need not be regarded as essential organs of locomotion at all 

 events. 



Various growth-forms occur which were at first associated with dif- 

 ferent Bacteria and received generic names. Individual Bacteria are 

 either roundish or in the form of straight rods, or of twisted rods. As 

 de Bary has remarked, ' a billiard ball, a lead pencil, and a corkscrew so 

 exactly illustrate these three chief forms,' that there is no need of models 

 to convey instruction in this respect. The round growth-forms are 

 termed Coccus (Micrococais Cohn &c.) ; the rod-like forms include those 



