temperature and pressure. Consequently such 

 animals cannot usually exchange environ- 

 ments without suffering drastic derange- 

 ments in their metabolism. 



Man represents an exception to the rule 

 that land organisms do not encounter signif- 

 icant changes in the environmental pressure. 

 Deep-sea divers and workers in pressure cais- 

 sons may experience sudden compressions 

 and decompressions amounting to several at- 

 mospheres, and a modern aviator may gain or 

 lose altitude so fast that the pressure change 

 becomes a matter of importance. 



The pressure sustained by a caisson worker 

 is usually not greater than five or six atmos- 

 pheres, a pressure that is not sufficient to 

 produce any direct effect upon metabolism. 

 But decompression may have serious indirect 

 effects, unless the pressure is reduced very 

 slowly in carefully graduated steps. This pre- 

 caution is necessary to prevent the formation 

 of gas bubbles in the blood stream. In a 

 caisson, the pressure is transmitted to the 

 body through the surrounding air — rather 

 than through water, as in aquatic organisms. 

 In the caisson, therefore, the high pressure 

 tends to drive excesses of air into solution 

 in the blood and other fluids of the body. 

 During decompression, this gas begins to 

 come out of solution — slowly and without 

 the formation of bubbles, if the decompres- 

 sion is slow. But if the decompression is rapid, 

 gas bubbles are formed in the blood, and 

 these bubbles may choke off circulation in 

 the arterioles and other smaller vessels of the 

 blood system — which accounts for the serious 

 symptoms of caisson sickness. 



Even at atmospheric pressure fairly large 

 quantities of the atmospheric gases are pres- 

 ent in the blood, but ordinarily these gases 

 remain in solution. An ordinary climb to 

 high altitude, for example, is slow enough 

 to permit a gradual escape of blood gases as 

 the atmospheric pressure falls. A modern 

 pilot, however, may climb at a rate approxi- 

 mating a mile a minute. Under such condi- 

 tions aeroembolisms are sometimes encoun- 

 tered, and a study of the process of aero- 



Ecology and Evolution - 585 



embolism has become a matter of practical 

 importance in aviation medicine. 



Water. No organism can live and grow 

 without water, since water is a main com- 

 ponent of all protoplasm. Also it is plain 

 that aquatic organisms can absorb water di- 

 rectly from their surroundings. But most 

 land-dwelling species are constantly losing 

 water to the atmosphere, and such species 

 have developed many adaptations for con- 

 serving and augmenting their water re- 

 sources. 



Land-dwelling animals display a wide va- 

 riety of integumental coverings that serve 

 to minimize the loss of water vapor from ex- 

 posed body surfaces. Thus the skin of terres- 

 trial vertebrates — whether naked or clothed 

 by scales, feathers, or hair — provides an effec- 

 tive barrier against evaporative losses from 

 the tissues and body fluids; and the same is 

 true of the chitinous integuments of insects, 

 arachnids, and other land-dwelling arthro- 

 pods. In land animals, also, the respiratory 

 organs (for example, the lungs of vertebrates 

 and the tracheae of insects) are deeply re- 

 cessed within the body, so that the loss of 

 water vapor from the respiratory surfaces is 

 reduced as much as possible. And finally, the 

 excretory organs of typical land animals (for 

 example, the kidneys of man) are able to cur- 

 tail the excretion of water whenever a dearth 

 of water begins to threaten the welfare of the 

 animal (p. 375). 



Terrestrial plants likewise display many 

 adaptations that are related to the water 

 supply of the environment. For example, 

 desert plants (xerophytes) are apt to have 

 very deep and extensive roots; and the root 

 sap of such plants tends to be very hyper- 

 tonic, which facilitates the absorption of 

 every possible vestige of water from the soil. 

 Moreover, the stem and leaves of many xero- 

 phytes are characteristically modified. Gen- 

 erally, the exposed surface area of a desert 

 plant is sharply reduced; the epidermis is 

 thick, shiny, and heavily cutinized; and the 

 stomata are sparse and deeply sunk below the 

 epidermal surface. In extreme xerophytes 



