Simple Plants Without Chlorophyll — Fungi 169 



fide to free sulfur and then to sulfuric acid, thereby releasing energy for 

 the synthesis of organic compounds from carbon dioxide and other in- 

 organic substances. The sulfuric acid undergoes chemical changes in 

 the soil to form sulfates which are the principal sources of sulfur for 

 green plants; (2) the iron bacteria which live in iron-containing waters 

 and oxidize the iron compounds, thereby releasing energy for the syn- 

 thesis of organic compounds; (3) the hydrogen bacteria which live in 

 soils and oxidize molecular hydrogen to form water, thereby releasing 

 energy; (4) the nitrifying bacteria which live in soils— one group oxidizes 

 ammonia to nitrites and the other group oxidizes the nitrites to nitrates, 

 thus releasing energy. Other bacteria, the symbiotic nitrogen-fixing bac- 

 teria, live symbiotically in the nodules of the roots of leguminous plants 

 where they fix the free nitrogen to form nitrates. Still other soil bacteria, 

 the nonsymbiotic nitrogen-fixing bacteria, fix the free nitrogen to form 

 nitrates (not in roots). Hence, the essential nitrate supply of the soil is 

 aflfected by the actions of these various bacteria (Fig. 325) . 



Some typical photosynthetic bacteria include: (1) the purple sulfur 

 bacteria which, because of their purple pigment, are able to synthesize 

 organic compounds in a manner similar to that used by chlorophyll-bear- 

 ing plants; (2) the purple nonsulfur bacteria which synthesize organic 

 compounds by the utilization of molecular hydrogen in the presence of 

 light; (3) the green bacteria which synthesize organic compounds by 

 oxidizing hydrogen sulfide and reducing carbon dioxide. 



Bacteria are considered to be plants, rather than animals, because 

 (1) their methods of reproduction resemble those of certain algae and 

 true fungi, (2) their cell walls often contain cellulose, (3) they synthesize 

 vitamins like those of certain plants, (4) some species are able to utilize 

 simple inorganic compounds from which more complex organic com- 

 pounds may be synthesized. 



Bacteria are unicellular and the simplest and smallest of living organ- 

 isms, being visible only under high magnifications (Fig. 34). When 

 growing under certain conditions many individuals may associate them- 

 selves to form a colony whose color and other characteristics are more or 

 less specific for each species. Typically, bacteria are not over 4 to 5 

 microns long (a micron is one-thousandth part of a millimeter). 



The forms of bacteria (Fig. 34) include: (1) the coccus (spherical 

 or ovoid), (2) the rod-shaped (cylindrical), (3) the spiral-shaped, (4) 

 the filamentous (which may be branched). There are various types of 

 each of the four described above. For example, there are Staphylococci 



