520 TEXTBOOK OF BOTANY 



noted above, certain species may grow as aerobes at one temperature and 

 as anaerobes at another. Many of the thermophihc bacteria, for ex- 

 ample, will not grow at temperatures below 110° F. when exposed to 

 air, but will grow under anaerobic conditions at 95° F. 



At temperatures near the freezing point bacteria grow very slowly, 

 but may survive for weeks or even months. When freezing occurs and 

 the medium becomes solid, diffusion is extremely slow and life processes 

 are reduced to a minimum. It is interesting to note, however, that 

 typhoid bacteria for die most part are destroyed immediately upon the 

 freezing of water and few individuals live longer than three or four 

 weeks. Certain pathogenic bacteria and some of the common micro- 

 organisms of the soil are known to survive for a few days when exposed 

 to the temperature of liquid air (about —310° F.). Cultures of certain 

 bacteria have been exposed to the temperature of liquid hydrogen 

 (about —425° F. or about 35° above absolute zero) for short intervals 

 with no apparent harm to the organisms. 



What are bacteria? Bacteria are at once the simplest in structure, the 

 smallest in size, the most abundant, and the most generally distributed 

 of all plants, both green and non-green. Only the blue-green algae are 

 comparable in these respects. Bacteria are one-celled organisms, but 

 the individuals often cohere and fomi aggregates of visible size. The 

 cells have gelatinous sheaths; and when many of the sheaths coalesce, 

 the bacterial scums often seen on water and on damp objects are foraied. 



The cells of bacteria are so small that their exact structure is difficult 

 to ascertain. Definite cell walls exist in most bacteria as is shown by 

 their rigidity, by their various shapes, and by the effects of plasmolysis. 

 The composition of the cell walls is not definitely known, but chitin and 

 waxes have been reported in some species. The protoplasm is apparently 

 undifferentiated as cytoplasm and nucleus, but both nucleic acids and 

 nucleo-proteins have been obtained from some bacteria by chemical 

 analysis. The cell evidently carries on all the processes usually associated 

 with more differentiated cells, and it is unnecessary to assume any unique 

 hereditary mechanism in this great group of plants. The most important 

 constituents of the cell are proteins; carbohydrates, including starch, 

 glycogen, gums, and simple sugars; fats, waxes, and phospholipoids; 

 enzymes; and various organic salts. It has not been established experi- 

 mentally which salts are essential in the growth of bacteria. Some bac- 

 teria apparently can synthesize vitamin B, but neither A nor C. 



A few bacteria move about by a gliding movement not understood 



