METABOLISM OF COLORLESS PLANTS 



45 



could not long continue without their services. It is this 

 aspect of the Bacteria which concerns us at present. 



Among the Bacteria are the smallest organisms known. 

 Some species are less than one fifty-thousandth of an inch 

 in length and much less in breadth. None of the typical 

 forms comes within the range of unaided vision, indeed 

 there is room and to spare for thousands of millions of Bac- 

 teria to live in a thimble-full of sour milk. The small 

 size and similarity of structure of many of the Bacteria 

 render their study particularly difficult, and accordingly 







CD 



FIG. 13. Chief types of Bacteria. A, cocci; B, bacilli; C, spirilla; D, branched 

 filamentous form. (From Buchanan.) 



they are grouped and classified largely on the basis of chem- 

 ical changes which they produce, rather than on structural 

 characteristics. However, there are three chief morphologi- 

 cal types: the rod -like forms or BACILLI; the spherical forms 

 or cocci; and the spiral forms or SPIRILLA. Bacilli or cocci 

 may be associated in linear, branching, or plate-like series, 

 or grouped together in colonies. (Fig. 13.) 



The individual bacterium is generally regarded as a single 

 cell though in most species there is no definite nuclear body; 

 the chromatin material being distributed in the form of 

 granules throughout the cytoplasm. A cell wall chemically 

 similar to protein is usually present. Some forms show 

 active movements by means of prolongations of the cytoplasm, 



