ABSORPTION OF LIQUID NUTRIMENT. 



15 



erally such is the case. The amount of any one substance 

 necessary to estabUsh equiHbrium is very small, and as soon as 

 this amount is acquired, absorption of that substance ceases. 

 When the plant withdraws any of the substances from the cell- 

 sap solution to build up tissue, another quantity of that sub- 

 stance is absorbed from the soil. Thus different quantities of 

 the various substances are absorbed. It is to be noted that 

 all substances in the soil are not invariably extracted even in 

 the minutest quantity by any one plant. The " rotation of 

 crops " has its value for the farmer because different plants do 

 not require the same soil-salts. 



EXPERIMENT 19. 



INCREASED DIFFUSION. 



Close the lower end of two lamp-chimneys with bladder or 



Fig. 10. 



Apparatus to show selective diffusion. (After Oels.) K K, corks, loosely- 

 fitted ; C C, copper-sulphate solution. 



parchment and fill with distilled water. In the upper end of one 

 cylinder place a stopper into which several iron nails have been 

 driven. Make a saturated solution of copper sulphate and place 

 exactly the same amounts in both cylinders. Now fasten the two 

 chimneys in the cylinder as shown in Fig. 10. In 48 hours take out 



