ABSORPTION OP FLUID BY THE ROOTS. 87 



ally torn off, has been known to pour forth from the wound many 

 gallons in a few hours ; and the loss could only be restrained, by 

 nailing a leaden plate very firmly over the part. If the stem of 

 a Vine be cut through, when the sap is ascending, and a piece of 

 bladder be tied over the surface of the lower part, this is soon 

 distended with fluid, and in a few hours will burst. Or if to 

 this portion be fixed a bent tube, in such a manner that the fluid 

 which rises shall have to lift a column of mercury, the absolute 

 force may be measured ; and this is found to be very great. 



117. The question, then, arises, by what power do the roots 

 thus absorb and force upwards through the stem the fluid of the 

 surrounding soil ? Is this power peculiar to the living system, or 

 can we in any way account for it on the laws of Mechanics ? 

 Something analogous may be effected by the experimenter, with 

 materials which he obtains from dead structures, or even from 

 inorganic matter. If a glass vessel, of the shape delineated in the 



figure, have its wide open mouth covered with a 

 piece of bladder, and its cavity filled with rather 

 thick syrup or gum- water, whilst the under side 

 of the bladder is immersed in simple water, the 

 volume of fluid within the glass will be much in- 

 creased, by the passage of water from without, 

 through the bladder, to mix with the fluid in the 

 interior ; and this entrance will continue, as long 

 as there is much difference in density, between 

 the fluids on the two sides of the bladder. But there is also a 

 contrary current to a less amount, the interior fluid passing out 

 to mix with the surrounding water. The increase in the volume 

 of the fluid within the vessel is, therefore, equivalent to the dif- 

 ference in the amount of the opposite currents. If, on the other 

 hand, the glass were filled with water, and immersed in syrup, it 

 would be partly emptied by this action. 



118. The principal current is termed Endosmose (flow in- 

 wards) ; and the lesser one is called Exosmose (flow outwards). 

 Bladder or animal membrane is by no means the only porous 

 substance, which may be used for this purpose ; though it is the 

 most fitted to exhibit the experiment, from the rapidity of the 



