THE PHYSICAL ENVIRONMENT OF ORGANISMS 545 



One Calorie is the quantity of heat necessary to raise the temperature 

 of 1 kilogram of water l^C. 1 



One pound of bread will give approximately 1200 Calories. 



Man needs 1500 to 5000 Calories per day. 



One acre produces from 1 to 4 tons of carbohydrates per year. 



One square meter of green-leaf surface synthesizes 1 to 2 grams of 

 sugar per hour. 



Bread contains 53.3 per cent carbohydrate, 9.1 per cent protein, 1.6 

 per cent fat, 35.0 per cent water (from which no energy is obtainable). 



THE PHYSICAL ENVIRONMENTS IN WHICH ORGANISMS LIVE 



In considering the various interrelationships that bind all the organic 

 world into a huge economic and social complex, it is necessary to see 

 something of the problems and needs imposed upon organisms by the 

 conditions of the physical world in which they live. For purposes of 

 analysis, we may subdivide the influences of physical environment into 

 those of the medium in which the organisms live, and those of the physical 

 (and chemical) factors that condition the medium and determine the 

 reactions and metabolic processes of the organisms within it. 



The Media in Which Organisms Live 



All life of which we have any knowledge is confined to a shallow zone 

 at the surface of our earth, where soil, water, and air meet and inter- 

 mingle to a slight depth. Thus any organism must live in water or air 

 or soil or in some combination of the three. These three media differ 

 markedly in many physical properties, and each presents its own special 

 problems to the organisms that live within it. The requirements for 

 mechanical support and the problems of locomotion, adjustment to 

 temperature changes, respiration, food gathering, and reproduction in 

 aquatic, aerial, and terrestrial media are so unlike that they impose many 

 structural adaptions upon organisms. Ordinarily we can determine from 

 morphological inspection whether a given organism is fitted for life in 

 water, on the land, in the soil, or for borderline or alternate existence in 

 two (or more) of these media. 



The medium and mechanical support. Because of the great buoying 

 power of water, aquatic organisms can attain much greater size and 

 weight and a much greater expanse of surface with a given amount of 

 skeletal or supporting tissue than can land animals. This buoyancy of 

 water is utilized in two quite different ways. On the one hand it has per- 

 mitted the skeletonless development of such large marine organisms as 

 the jellyfish and the octopus among animals and the giant kelps and 



1 More exactly, the amount of heat required to raise the temperature of 1 kilogram 

 of water at 15°C. to 16°C. 



