WATER. 



103 



of tissue conduct more rapidly than others. In ordinary- 

 plants the elongated wood-cells convey the water mainly. 

 The rapidity of the ascent is dependent on the rate of 

 evaporation ; but in general it may be considered to vary 

 between the limits of five and fifty inches per hour. The 

 so-called root pressure may be shown by cutting ofi" a 

 vigorously growing plant at the ground, and attaching a 

 glass tube in which the water will rise to a 

 considerable height (36 inches in Viiis, 84 

 inches in Betula). This is supposed to be due 

 to a purely physical (endosmotic) force. 



131. The water absorbed from the soil by 

 the rootlets holds, in solution, inorganic food- 

 materials for the plant. Even pure water is 

 a solvent for many kin.ds of rocks and min- 

 erals, which, in a more or less finely divided 

 state, constitute soil. In consequence of this 

 fine state of division, the surface exposed to 

 the action of water, and consequently the 

 amount of material dissolved is enormously 

 increased. Water charged with carbonic diox- 

 ide (called carbonated water), as the soil water 223 

 invariably is, has its solvent power greatly increased. When 

 such water is heated, or so exposed as to loose its carbonic 

 dioxide, it is forced to precipitate the minerals contained, 

 as illustrated by the lime-coating in boilers and in tea- 

 kettles. The solvent power is still greater when the water 

 contains alkalis — ammonia, potash, and soda : in fact, up 

 to a certain point, " its solvent power increases with the 

 amount and number of matters dissolved." Besides, the 

 root-hairs themselves (Fig. 223) attack and dissolve the 



Fig. 223. A root-hair liighly magnified, showing its attachment to the soil-particles (x). 



