36 



PROTOPLASM AND THE CELL 



fluences as well as internal influences produced by the bacteria 

 themselves. The environment is soon changed because of their 

 own physiological activities and multiplication is soon checked. 

 Reproduction is quickly stopped by natural factors like dessi- 



cation, unsuitable temperature, acid- 

 ity or alkalinity of the medium, but 

 many bacteria have the power to 

 resist such adverse conditions by 

 forming internal spores or Dauers- 

 G poren (enduring spores). These are 

 FIG. 17 Spore formation usually spherical, ellipsoidal or oval 



and germination of spores in ... . _ . 



bacteria. A, A pair of rods in form and possess a dense envelope 



hour later; C, one hour later of the chromatin granules and some 



still; D, a five-celled rod with , /^. -.-^^ m, 



three ripe spores which were Cytoplasm (Fig. 17, D). These spores 



placed in a nutrient medium possess a much higher resistance to 



after drying for several days; r 



E, F, the same spores from one external influences than do the cells 

 o? ^S&3$* "and ^om which they are formed (many 

 movement. (From de Bary f or example, can withstand a tern- 

 after Sedgwick and Wilson.) N 



perature of from 70 to 100 C.). 



One spore per cell is the rule, but in rare instances, two similar 

 spores may be formed. Spore formation in bacteria, therefore, 

 is not always a method of reproduction but may be an adaptation 

 for the preservation of the organism corresponding to what 

 is known as the "encysted state" of many unicellular animals. 

 Physiology of Bacteria. The food of bacteria is most diverse. 

 The majority are known as saprophytes, that is, they obtain 

 their nourishment from dead organic matter. Many are para- 

 sites, getting their food from other living organisms in the form 

 of complex chemical compounds of protein substance or protein 

 derivatives. Some live in the soil, and get their food supply and 

 their energy from purely inorganic materials. Among these are 

 the so-called nitrifying bacteria, one of which, Nitrosomonas, 

 converts ammonia salts into nitrites while another, Nitrobacter, 

 changes the nitrites into nitrates. Other bacteria utilize free 

 ammonia (NH 3 ) and still others, free nitrogen (N) in the manu- 

 facture of nitrates (see p. 129). These organisms thus per- 



