BIOLOGY OF SOILS 139 



desiccation and other unfavourable conditions which would 

 be fatal to the organism in its more active state. Under 

 favourable conditions, the spores germinate and the 

 organism is reproduced in a state of vital activity. 



Appearance. Bacteria can be propagated by growth in 

 a suitable medium, and can often be recognised by the 

 appearance of the cultures, whiclTsomewhat resemble that 

 of mould growths but are generally denser. Individu- 

 ally they can be distinguished only under high powers of 

 the microscope, and are generally classified according to 

 their shape as micrococci and bacilli. The former includes 

 all the spherical and oval forms, and the latter the more 

 elongated, rod-like and filamentous forms, but the line 

 of division is not a sharp one. The micrococci are often 

 associated in pairs, groups of four, or multiples of four, 

 arranged in chains or scattered indefinitely. Of the bacilli 

 some are straight, others are curved like a comma, doubly 

 curved like a corkscrew, and sometimes intertwined. The 

 names given by bacteriologists to these various types need 

 not now be considered. 



Food. In general, bacteria require the same mineral 

 elements as other plants, but are adapted for growth in a 

 very dilute medium. As they contain no chlorophyll they 

 are unable 1 to decompose carbon dioxide but obtain their 

 carbon chiefly from carbohydrates, e.g., sugar and other 

 similar compounds. Nitrogen, as a rule, is derived from 

 organic compounds, though some bacteria can obtain it 

 from ammonium salts and probably also from nitrates and 

 the free nitrogen of the air. Some are parasitic and 

 others saprophitic, i.e., some grow only on a living host 

 usually a particular organism and not infrequently a 



1 Kecent researches have shown that, under certain conditions, 

 some bacteria can obtain carbon from carbonates and from carbon 

 dioxide, notwithstanding the absence of chlorophyll. 



