THE GENERAL CONDITIONS OF LIFE 281 



and decreases with the rise and fall of the water-contents. If a 

 slight shower of rain comes, activity immediately begins, the 

 plants grow and bloom, and the sluggish animals awake from their 

 summer sleep. 



In a manner somewhat different from that of the desert-plants 

 and animals, other organisms which at times are obliged to undergo a 

 lack of water are adapted to life in drought, since at such times 

 they assume a quiescent phase and are protected against drying. 

 Such quiescent phases occur especially among unicellular organ- 

 isms, as m the spores of Bacteria (Fig. 128) or the cysts of 

 Rhizopoda and Infusoria (Fig. 84, p. 205), which enclose the 

 living cell-substance in a thick, completely impervious skin. 



FIG. 128. Bacillus butyricus, forming spores, a, Beginning of the process ; b, ripe spores still 

 within the bacilli ; c, spores after the dissolution of the membrane of the mother-cells ; d, 

 spores beginning to germinate and to allow the bacilli to come forth. (After Mjgula.) 



The seeds of plants likewise belong to these permanent conditions 

 of organisms. But in all these cases life is latent ; no trace of 

 vital phenomena can be demonstrated in them by means of the 

 most delicate methods. It would appear that in all such cases 

 life stands still, like a wound-up clock that has been suddenly 

 stopped. 



From these facts the importance of water for the maintenance 

 of life is evident. Without water life cannot exist. With the 

 increase and decrease of the water- contents of living substance 

 within certain limits the intensity of life rises, falls, and becomes 

 zero. 



3. Oxygen 



It was Priestley, the discoverer of oxygen, who recognised the 

 fundamental importance of this gas for life upon the earth ; by 

 his epoch-making discovery of the gas and its properties he gave 

 a real background to Mayow's ingenious comparison of respiration 

 with combustion. In respiration free oxygen is taken up by the 

 living substance, and in return carbonic acid is given off; hence 

 a combustion, an oxidation of carbon, must take place in the 



