30 



METABOLISM 



kinds of soil, with different amounts of water, even when the leaves were exposed in 

 the dark to a damp atmosphere. The following table shows this more in detail: — 



100 g. of soil contained in the first case I2'3 g. 



Fig. 6. Potometer. (From Detmer'S Smaller Practical Physiology, 1903.) 



in the second 8 g., and in 

 the third 1-5 g. of water, 

 which were not available 

 for the plant's use. These 

 quantities correspond ap- 

 proximately to those left 

 in the soil when it is air- 

 dried. 



By what force then 

 does a roothair overcome 

 the adhesion of the water 

 to the soil particles ? 

 After what we have 

 learned as to the osmotic 

 characters of a single cell, 

 we can have no hesita- 

 tion in ascribing this 

 power to osmotic acti- 

 vity. In fact, the plas- 

 molytic method enables 

 us to demonstrate with 

 ease an osmotic pressure 

 in the roothairs. Owing 

 to this osmotic pressure, 

 as we have seen, the cell- 

 wall will be stretched 

 until its elastic expan- 

 sion equals the turgor, 

 and the water will be 

 sucked into the cell 

 cavity so enlarged by the 

 stretching of the wall, as 

 by a suction pump. The 

 cell-sap will first of all 

 withdraw the water from 

 the protoplasm, which, 

 in its turn, owing to its 



power of imbibition, will endeavour to draw fresh supplies of water from the 

 membrane. The wall will then contain less water than it can hold in virtue of 

 its power of swelling, and in consequence will suck up the water which we have 

 spoken of as adhering to the soil particles. 



If now the water osmotically absorbed by the roothair be retained, move- 

 ment of water, after a time, on the restoration of equilibrium, must come to 

 an end. In reality the movement of the water is not completely stopped, 

 a condition of equilibrium only is reached in which the osmotic entry of water 

 is balanced by the outflow effected by the pressure of the cell-wall. However, 



