170 Plant Biology 



(masses of cocci forms), Streptococci (chains of cocci), Diplococci (pairs 

 of cocci), Sarcina (box-shaped mass of cocci) , etc. 



Certain types of bacteria, like higher plants, require free, atmospheric 

 oxygen for their normal activities and are known as aerobes (a'erobe) 

 (Gr. aer, air; bios, life). Other species do not require free oxygen but 

 secure oxygen by breaking down certain types of oxygen-bearing foods 

 through the action of enzymes. These are called anaerobes (ana' er obe) 

 (Gr. an, without; aer, air; bios, life) . Certain species are at times aerobic 

 and at other times anaerobic. 



A bacterial cell has a cell wall which in some species contains cellulose. 

 In some species an external, slimy layer or capsule is present. A few 

 species form a slimy, gelatinous mass called a zooglea (zoo-gle'a) (Gr. 

 zoon, animal; gloia, glue) in which great numbers of bacteria are em- 

 bedded in a mucilaginous matrix which is frequently iridescent. The 

 protoplasm of the cell is fairly homogeneous and contains vacuoles as 

 well as granules, including chromatin. An organized nucleus and plastids 

 are absent, although certain investigators maintain that certain species 

 possess a structure which resembles a nucleus. 



Each species of bacteria has a temperature at which it grows best and 

 is known as its optimum temperature. As this temperature is decreased 

 or increased, growth is retarded until it eventually ceases. On the basis 

 of optimum growth temperatures, bacteria are grouped into ( 1 ) psychro- 

 philes (si'krofil) (Gr. psychros, cold; philein, to love), or those growing 

 best at temperatures below 14° C; (2) mesophiles (mes'ofil) (Gr. 

 mesos, middle; philein, to love) , or those having an optimum temperature 

 between 20° and 40° C, and (3) thermophiles (ther'mofil) (Gr. 

 therme, heat; philein, to love), or those that grow best at temperatures 

 above 45° C. Psychrophilic organisms are common in cold, deep waters, 

 where they exist as saprophytes. Psychrophiles may decompose foods in 

 cold storage plants. A majority of bacteria are mesophilic. The sapro- 

 phytic types common in soils, water, etc., grow best at room temperature 

 (20° to 25° C). Those growing in animals grow best at temperatures 

 which approximate their animal host. Bacteria which produce human 

 diseases grow best at body temperature (approximately 37° C). Ther- 

 mophilic bacteria may be found in many places but particularly in hot 

 springs, decaying vegetation, etc. Thermophils are not known to pro- 

 duce diseases, although they can be bothersome in food canning, milk 

 pasteurization, etc. 



Not all bacteria are able to locomote in liquids, but when they do, 

 this is accomplished by the rhythmic, vibratile action of whiplike proto- 



