FUNGI 131 



Bacteria are found almost everywhere in the air, in 

 the water, in the soil, in most foods, and in the bodies of 

 plants and animals, as regular inhabitants. Many of them 

 are entirely harmless, some are useful, and others are very 

 dangerous. A laboratory near Paris, arranged for studying 

 bacteria in the air, has found that the average number of 

 bacteria in every quart of air in that locality is eighty. 

 The highest numbers were found during the autumn, and 

 the lowest during the winter; while a wind from the city 

 increased the numbers very much. The "pure" water of 

 springs and wells contains abundant bacteria, while in 

 stagnant water and sewer water they swarm in immense 

 numbers. The slimy deposits usually observed about 

 "iron" and "sulphur" springs, or in the pipes leading from 

 them, are due to the presence of the peculiar bacteria liv- 

 ing in such waters. The presence of dangerous bacteria 

 in drinking water is probably the most common cause of 

 epidemics of infectious diseases, and warnings as to the 

 dangerous condition of a city water-supply should always 

 be heeded. It is very evident that no sewage should find 

 its way into such water-supply. 



It is important to know something about the structure 

 and the habits of bacteria, before considering some of their 

 important relations to man. They are one-celled and occur 

 in three general forms: (1) spherical cells, usually grouped 

 in various ways, and including the minutest forms 

 (Fig. 121, B}} (2) rod-shaped cells, that is, longer than 

 broad, the cells remaining separate or attached end to end 

 and forming filaments (Fig. 121, F and <7);(3) elongated 

 cells, more or less curved, from short curved forms resem- 

 bling a comma to long spirals (Fig. 121, J-Af). Many 

 bacteria swim more or less actively by means of cilia; and 

 this fact first gave the impression that they are minute 

 animals an impression that is still prevalent outside of 

 laboratories (Fig. 121). 



