THALLOPHYTES 



11 



reproduction only by vegetative multiplication, the cell-divisions being 

 simple but in remarkably rapid succession. However, in most forms 

 there is no chlorophyll, so that bacteria in the main are parasites and 

 saprophytes. 



The immense economic importance of bacteria has stimulated their 

 investigation to such an extent that bacteriology has become a distinct 

 held of research, with its special technique. An outline of plant mor- 

 phology can only indicate the existence of this great region of research, 

 for to enter it would demand a course in bacteriology; but bacteria are 

 plants, and their general place among other plants must be considered. 



Bacteria include the smallest known organisms, cells having been 

 measured that are only 0.0005 mm - i n diameter. The cells are either 

 solitary or they may form 

 filaments, as among the 

 Cyanophyceae. For general 

 purposes, individual cells are 

 often referred to three form 

 groups : coccus forms, in 

 which the cells are spherical; 

 bacterium or bacillus forms, 

 in which the cells are oblong 

 or have the form of short 

 rods; and spirillum forms, 

 in which the cells are curved 

 (figs. 13-20). When these 

 various forms of cells enter 

 into the structure of fila- 

 ments, corresponding varia- 

 tions in the form of the 

 filaments follow. 



The occurrence of bacteria may be described as almost literally every- 

 where: in waters of every depth, in air, in soil, in all organic bodies, 

 living or dead, etc. Their resistance to conditions impossible for other 

 plants to endure is remarkable — a feature suggested by their asso- 

 ciates, the Cyanophyceae. Extreme cold, high temperatures, and desic- 

 cation that would destroy ordinary plants are successfully withstood 

 by bacteria. 



Structure. — The structure of the bacterial cell appears to be extremely 

 simple ; in fact it may be said to be almost structureless. The proto 



Figs. 13-20. — Bacteria: 13, coccus form, from 

 pus; 14-18, bacillus forms (14-17, hay bacillus); 

 14, motile cell; 15, filament of motile cells; 16, non- 

 motile cells; 17, cells with " spores " ; 18, typhoid- 

 fever form {Bacillus typhi) ; 19, 20, spirillum forms; 

 19, cholera form {Vibrio cholerae); 20, Spirillum 

 undula. — After A. Fischer. 



