THE WONDERS OF LIFE 



In the same general way as light the heat-stimulus 

 acts on organisms, and causes the sensations, sometimes 

 pleasant and sometimes unpleasant, which we call the 

 subjective feeling of heat, warmth, coolness, or cold. 

 The sense-organ that receives these impressions of 

 temperature is the surface of the unicellular plasmic 

 body in the protists, and the skin (epidermis) that 

 protects the surface from the outer world in the histona. 

 In all living things the temperature of the surrounding 

 medium (water or air) has a great influence in regulating 

 the life-processes; in the stationary animals and plants 

 it is the temperature of the ground to which they are 

 attached. This temperature must always be between 

 the freezing-point and boiling-point of water, as fluid 

 water is indispensable for the imbibition of the living 

 matter and the molecular movements within the plasm. 

 At the same time, some of the lower protists (chromacea, 

 bacteria) can endure very high and very low temperatures, 

 but only for a short time. Some protists (monera and 

 diatomes) can stand a temperature of 200° C. for several 

 days, and others can be heated above boiling-point 

 without being killed. Arctic and High-Alpine plants 

 and animals may be in a frozen condition for several 

 months, yet live again when they are thawed. How- 

 ever, the resistance to these extremes of cold lasts for 

 only a limited time, and in the frozen state all vital 

 functions are at a standstill. 



In the great majority of living things the vital activity 

 is confined within narrow limits of temperature. Many 

 plants and animals in the tropics which have been 

 accustomed for thousands of years to the constancy of the 

 hot equatorial climate can endure only very restricted 

 variations of temperature. On the other hand, many of 

 the inhabitants of Central Siberia, where the climate is 

 very hot in the short summer and very cold in the long 

 winter, can stand great variations, Thus the living 



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