150 PLANT PARASITES. 



ficial layers of the soil, where they may be present in enormous numbers. 

 They are especially abundant among the habitations of man, or wherever 

 under favorable conditions of moisture and temperature animal or vege- 

 table substances are undergoing decay. They cling tenaciously to moist 

 surfaces, but when dried, and especially when dried upon comminuted 

 material, they may float in the air as dust. In quiet air they gradually 

 settle with other forms of dust on to horizontal surfaces, and thus in 

 closed, still rooms the bacteria-laden air may in a few hours almost wholly 

 free itself of its living contaminations by a process analogous to sedimen- 

 tation in water. 



This widespread transportation of bacteria as dust by moving air, and 

 the spontaneous cleansing of the latter by the settlement of the germs, are 

 important factors in the sanitary problems which the complex conditions 

 of modern life present. 



While bacteria may live for long periods in the dried state in dust 

 they do not in this condition multiply. But the upper three or four feet 

 of the soil forms the great abiding, and when moist the breeding, place 

 of the myriads of germs which are concerned in the salutary work of 

 food preparation for higher plants. Large numbers of mould spores are 

 frequently mingled with the bacteria in dust and soil. 



Surface waters almost always contain bacteria, which may have en- 

 tered by aerial dust or from the wash of adjacent soil or from direct 

 human or animal contamination. Many bacteria find in water favorable 

 conditions of life and flourish on what to other forms would be but scanty 

 nutriment. Many pathogenic bacteria may remain alive for considerable 

 periods in water, but they do not usually thrive there. 



The water which in many places lies in hollows of the rocks, bathing 

 the deeper layers of the soil or gathered in caverns and recesses beneath, 

 is called ground water. This under favorable conditions is almost wholly 

 free from micro-organisms, these, through the complex process of filtra- 

 tion, germ metabolism, etc. , which go on in the upper soil layers, having, 

 together with inorganic contaminations, been largely retained or trans- 

 formed as the surface water has slowly sought the lower levels. 



The Relationship of Bacteria to Other Living Beings. So far as we 

 know, with few exceptions the bacteria whose natural habitat is the soil 

 or air or water are not under usual conditions harmful to man. On the 

 other hand, it is germs from the bodies of men or animals who are the 

 victims of infectious disease, gaining access in one way or another to 

 these great reservoirs and sources of distribution, which occasionally ren- 

 der the bacterial flora of soil, and air, and water of direct personal sig- 

 nificance to man. 



It will be seen from what has been said about bacteria and their vari- 

 ous modes of life that some live in or upon and at the expense of other 

 living beings the hosts these are parasites. Others which live and 

 grow apart from a living host are called saprophytes. In either class there 

 are forms which, through the capacity of adapting themselves to their 

 environment, can maintain at one time a parasitic, at another a sapro- 



