EFFECT OF DESICCATION 77 



Mosses and lichens on naked rocks shrivel up to dry friable masses, falling 

 into a resting condition in which they remain for weeks without losing the 

 power of growing again as soon as they receive moisture. The algae of 

 ponds and those that live on soil that is only periodically wetted can also 

 withstand drought for months. In all these cases it is the whole plant that 

 falls into a resting-stage, in which it can retain its vitality for long periods, 

 although not for an unlimited time. Among animals, too, many instances 

 are known (rotifers, tardigrada, anguillulae) in which the whole organism 

 can remain dried for weeks and months, and then be recalled to life by 

 moisture. 



Organisms which are provided with a morphologically specialized resting- 

 stage (spores, cysts, or seeds) can still better resist the effects of dryness. 

 The spores of the Smut fungus, Ustilago carbo, germinate when placed in 

 water, even after lying for ten years in an herbarium ; and wheat grains 

 sprout after ten or even twenty years if they have been carefully protected 

 from moisture. This suspension of vitality cannot, however, go on for 

 unlimited periods, and the miraculous stories of ' mummy wheat ' germinat- 

 ing after thousands of years are certainly fabulous. 



The spores of bacteria (50) can also remain dormant for a very long time ; 

 those of Anthrax, for example, germinate after ten years' quiescence. Thus 

 the resting-stage among bacteria is of similar duration as among other organ- 

 isms, and is shortened in the same manner by moisture or damp air. Many 

 bacteria can resist drying for long periods even in their vegetative condition. 

 Dry tubercle bacilli retain their virulence for weeks, as do also the bacilli of 

 diphtheria and typhoid fever and pus cocci. True aquatic bacteria, on the 

 other hand, such as the cholera vibrio, are killed by drying in from two 

 to five hours *. 



For the purposes of practical disinfection drying is not applicable, but 

 it seems to play an important part in the destruction of bacterial cells in 

 nature. Still, if dried bacteria, during short periods of moisture such as 

 continually occur, can find suitable nutriment they revive again, and multiply, 

 only to fall again into the resting-stage. For this reason it is not to be 

 expected that disease products, or earth contaminated with organic substances, 

 will be spontaneously disinfected by mere exposure to the air. 



* For the relation of these phenomena to infectious diseases see Chaps. XV. and XVI. 



