38 



OUTLINES OF BACTERIOLOGY 



individuals of the same species. This, however, is not pleomorphism, 

 but simply morphological variations which are found more or less in 

 most bacteria of the rod variety, and in two species which change 

 alcohol to acetic acid, viz. Bac. aceti and Bac. Pastorianum, most 

 remarkable structures have been figured by Prof E. Chr. Hansen 

 (Figs. 56 and 57). These varieties of form, however, depend on the 



Fig. 56. — Bac. aceti. (After Hansen.) 



Fig. 57. — Bac. Pastorianum. (Ater Hansen.) 



environment, and do not represent definite phases in the life-history 

 of the species in which they are exhibited. The exact nature of the 

 causes influencing the formation of morphological variations has not 

 been sufficiently studied, but the following are probably among the 

 number : 



1. The constitution of the nutrient medium in which the species is 

 growing. 



2. The nature and amount of excretion products. 



3. The presence of definite chemical substances. 



Finally, among growth-forms mention must be made of what are 

 known as involution structures. These are unhealthy structures, and 

 when they occur it is a sign that the species is growing under 

 unfavourable conditions. Under involution forms are included all 

 irregular structures, such as unnatural swellings, branchings and dis- 

 tortions. The best known are those which occur on the nodules that 

 are found on the roots of plants belonging to the Leguminosae (Pea 



