554 INTERRELATIONSHIPS OF ORGANISMS 



The limits of toleration and the optima of different species vary widely. 

 Some species have wide limits; others have very narrow ones; for some 

 the limits of toleration lie within the range of temperatures characteristic 

 of the tropics, while for others the limits are within the temperature 

 ranges of the temperate or boreal climates. Here we find a part of the 

 explanation of the geographic distribution of numerous organisms, both 

 plants and animals. 



Temperatures also play an important part in governing the daily 

 activities and habitat selections of organisms. This is shown if we watch 

 a wandering animal when it encounters a difference in the temperature 

 of its medium. If the new temperature is farther from the optimum than 

 the old, the animal will ordinarily show an avoiding reaction and turn 

 back or away from it ; if it is nearer the optimum than the old, the animal 

 ordinarily moves into the new situation. 



Such reactions or turnings in response to temperature are typical of a 

 large number of more or less fixed and automatic responses that organisms 

 make to stimuli from their environment. All these automatic responses 

 are termed taxes, 1 or tropisms (Greek, trop, "a turning"), and the par- 

 ticular turnings in response to temperature are termed thermotropisms- 

 negative thermotropism when the organism turns away from a given 

 temperature, positive thermotropism when it turns toward a given tem- 

 perature. Thus Epiphragma solatrix, a crane fly found in Florida, if in a 

 temperature of 15°C, is ordinarily negatively thermotropic to a tem- 

 perature of 10°C. but positively thermotropic to a temperature of 20°C. 



Humidity. Another environmental factor of much importance to 

 many terrestrial organisms is the amount of moisture in the air. All the 

 space occupied by air has a capacity for water vapor. Since the molecules 

 both of the air and of the water vapor are widely dispersed, a given cubic 

 space, at a given temperature and pressure, has a water capacity that is 

 independent of the air present. When an air space contains less than its 

 capacity of water vapor, it tends to take up water by evaporation from 

 alljavailable sources. Among such sources is the water contained in living 

 organisms. Organisms have very different degrees of protection against 

 water loss, and we find that they have very different limits of toleration 

 for the water capacity of the air space about them. 



One of the most useful measurements of the amount of water vapor 

 in the air is the relative humidity — the percentage of saturation with 

 water vapor that exists for a given situation at a given time. The lower 

 the relative humidity the more rapidly and the more powerfully will 

 evaporation go on; the greater the relative humidity the less difficulty 

 will an organism have in retaining its water content. At midday during 

 May, various types of natural situations in northern Florida vary from 



1 Pronounced tax'ees; singular, taxis. 



