THE ROLE OF WATER 419 



A plant is said to require 200 to 400 lb. of water to produce a 

 pound of dry matter; and the circulation of water in animals is 

 enormous. So great are the incoming and outgoing of water in 

 organisms that the amount in tissues would appear not to be 

 precisely maintained. But this is not the case. On the con- 

 trary, the balance between water and the other constituents is 

 very accurately controlled. Water is not free to enter a cell 

 to an unlimited extent. We ordinarily think of it as passing 

 freely into and out of the cell — because we are in the habit of 

 regarding the plasma membrane as wholly permeable to water — 

 but this is not true. There is, as Hofler points out, a very 

 accurate control. This regulatory power of protoplasm is 

 assumed to be parallel to that of jellies, e.g., gelatin, which swells 

 a definite amount in water and no more (see hysteresis and mis- 

 cibiUty, pages 145, 207). Such a viewpoint ascribes the entrance 

 of water into protoplasm to imbibition forces. Water enters 

 plant cells principally by osmosis, in the pure and strict sense; 

 osmosis is thought by some to be also primarily responsible in 

 the case of animal cells (see page 196). 



Just as the water supply of the cell is very accurately controlled 

 by the regulatory power of the membrane or the protoplasm as 

 such, so also is the quantity of water taken into the body con- 

 trolled, an excess or a deficit being harmful. A fasting animal 

 may lose all its fat and half its protein but not more than one- 

 tenth of its water. 



Where a disturbed metabolic condition results from too little 

 or too much water, the changed water content may be only the 

 first cause leading to a second disturbance which is the final 

 cause. This is true in the case of miner's cramp (page 450), 

 where excessive loss of water (sweating) is the primary cause, 

 leading to decreased salt content, the ultimate cause of the cramp. 



Of the many evident uses of water in the animal body — as the 

 medium for metabolic processes, the distribution of foods, the 

 regulation of temperature (through circulation and evaporation 

 from the skin), keeping mucous surfaces moist, eliminating 

 poisons from the kidneys, etc. only a few instances can be 

 selected for consideration. 



McQuarrie, Trumper, Fischer, Beutner, and others call atten- 

 tion to the importance of water metabolism in connection with 

 epilepsy, diabetes, edema, and kidney function. 



