246 GENERAL BIOLOGY 



on the conditions of life for all organisms. The 

 oxygen, which constitutes 20 per cent of ordinary air, is 

 absolutely essentially for practically all living things. 1 

 The nitrogen, as we have seen, is the raw material 

 which the soil-bacteria transform into salts available 

 for plants. The CO 2 is the source of the carbohy- 

 drates. Almost equally important in its effect on 

 organisms, particularly plants, is the amount of 

 moisture in the atmosphere. The difference in 

 appearance between the vegetation of a dry arid 

 region and one supplied with abundant moisture is 

 familiar to every one. Of course, there is a very 

 intimate relation between the moisture of the soil and 

 that of the adjacent air, but the general appearance 

 of a region is largely determined by the moisture 

 brought to it by the wind in the form of rain-clouds. 



The Nature of Organic Response. From one 

 point of view an organism may be considered to be 

 always in a condition of stimulation, which we call 

 tone. There must always be a certain amount of 

 heat, for example, to make life-conditions possible. 

 The increase or decrease of the external temperature, 

 however, causes a readjustment on the part of the 

 organism, which is the evident or perceptible reac- 

 tion. This response may be of two sorts. On the 

 one hand, the vital phenomena may be changed in 

 character, or qualitatively, especially if the reaction 

 is a very violent one, but more often they are merely 



1 With the exception of the anaerobic bacteria, which get their oxygen 

 in other ways. 



