INOKGANIC CELL-CONSTITUENTS. 127 



Plants get rid of water through evaporation from their entire external 

 'surface, while water is absorbed by their roots. 



Water leaves the animal body through the kidneys, skin, lungs, and 

 intestines, that passing daily through the kidneys being about half of the 

 total amount of water eliminated. The relative proportion between the 

 amounts eliminated by these organs is subject to very great variation, and 

 depends upon numerous external and internal conditions, which will sub- 

 sequently be alluded to. It may, however, be here mentioned that of the 

 water taken as food but a small amount leaves the body in the faeces ; in 

 man the amount so eliminated is only 4 per cent,, while the remaining 96 

 per cent, leaves the body through the kidneys, lungs, and skin. Water, 

 therefore, does not simply pass through the alimentary canal, but is ab- 

 sorbed by its mucous membranes, enters the blood, and thence becomes 

 a constituent of the different tissues. 



Water is a necessary solvent for various organic and inorganic con- 

 stituents of the body, and it alone, by entering into the condition of im- 

 bibition in the tissues, enables the various physical and chemical proc- 

 esses which constantly occur in cells to take place, and occasions their 

 semi-solid state, their elasticity, flexibility, and transparency. Through 

 its evaporation from the external surface and through the lungs it serves 

 to abstract heat, and therefore is, to a certain extent, a temperature regu- 

 lator. As water is an essential constituent of organic bodies, its loss, 

 which is constantly taking place, must- be continually replaced ; in the 

 higher animals a demand for an increased supply of water is indicated 

 b}' what is known as thirst. This will subsequently demand consideration. 



The removal of water from lower forms of cell life entirely suspends 

 all evidences of vitality ; through desiccation life in such forms is said 

 to be rendered latent. A renewed supply of water will again restore all 

 the phenomena of cell life. None of the higher plants or animals can 

 support loss of water beyond a very moderate amount without causing 

 permanent loss of vitality ; seeds and infusoria may be completely dried 

 and recover their vital properties when supplied with heat and moisture. 

 Although water is an essential constituent of all cells, it may, neverthe- 

 less, act as a poison if absorbed in too great amount. 



Protoplasm of all kinds is killed by immersion in distilled water ; 

 this fact may be partly due to the diffusion currents which are thus in- 

 augurated, the essential salts being removed from the protoplasm, and 

 their place being taken by water. 



Freezing of various parts of plants and then subjecting them to 

 rapid thawing by exposure to the rays of the sun causes their death by 

 first abstracting water from the solids and causing its aggregation in a 

 crystalline form, and then by sudden melting causes drowning out of 

 neighboring parts while more remote portions still suffer from want of 



