IMPORTANCE OF THE WATER-SUPPLY 415 



spoken of it as if it were pure water only ; this is. however, not 

 the case, for the water, when absorbed, contains small quanti- 

 ties of various substances in solution. Though the protoplasm 

 opposes the entry of anything like a strong solution of inorganic 

 salts, say 5 to 10 per cent., it allows very dilute ones to enter 

 much as it does pure water. In this way the slowly diffusing 

 stream brings to the protoplasm of each cell the inorganic 

 materials which are absorbed from the earth, and enables the 

 matters elaborated or formed from them by the protoplasm to 

 pass fi'om cell to cell. The feeding or nutrition of tlie various 

 cells IS thus dependent on the transit of fluid about the plant 

 in the way described. The access of various gases is similarly 

 made possible, for these are dissolved in the liquid stream. 

 Thus the oxygen, upon the presence of which life depends, is 

 transported to each cell, and the carbon dioxide of respiration 

 is removed from the seats of its liberation. The condition 

 of turgescence is necessary also for growth and for various 

 movements of different parts, enabling them to adapt them- 

 selves to varying conditions of their environment. Some 

 plants, particularly those which are aquatic in habit, and such 

 parts of terrestrial plants as have but little woody tissue, are 

 dependent upon the turgidity of their cells for the rigidity w^hich 

 enables them to retain their positions in the medium in which 

 they live. 



The importance of the water supply, and, indeed, its necessity 

 to the plant, explains the presence of the aqueous tissue which 

 we have seen to occur in various parts. The cells of this tissue 

 contain little else than water, and may be regarded as an addi- 

 tional reservoir, supplementing the vacuoles of the ordinary 

 cells. In plants that inhabit dry arid soils, such as sandy deserts, 

 there are often other adaptations relating to water storage. 

 Such plants are often covered with large bladder-like hairs, 

 which hold a considerable quantity of liquid. In many plants 

 the epidermal cells form a further reservoir. Plants exposed to 

 conditions threatening too copious evaporation are generall3=' 

 furnished with a very prominent cuticle, tending to check undue 

 escape. 



