48 



COUESE OF THE SAP. 



PT. ir. 



give off water, from all parts wliicli are exposed to 

 drought. 



Liebig tells us that leaves, twigs, and branches, 

 when completely matured, as they do not become 

 larger, do not need food for their support.' Why, 

 then, do they droop when the supply is cut off, and 

 revive when the roots only are watered? Let any 

 physiologist of this school apply this reasoning to his 

 own ' completely matured ' body, and cease to supply 

 it with food because it has ceased to become larger. 

 But if leaves ' do not become larger,' the plants which 

 bear them do. The formation of the bud, the down- 

 ward growth, the growth in girthing, and the growth 

 in elongation of the roots, are in full tide when the 

 leaves have ceased ' to become larger ; ' and the leaves 

 are doubtless essential to this increase. Suppose a 

 gardener were to take Liebig's word, and to refuse to 

 water his pot greenhouse plants when turned out for 

 the summer. Not one would survive. And why do 

 they require watering when plants in the open ground 

 do not ? Because the earth in the pots is exposed all 

 round to drouglit from sun and air, and unless the 

 earth were watered the roots would have no mois- 

 ture to absorb. While in reference to plants plunged 

 in the earth, the upper surface only of the ground is 

 exposed, and the roots underneath the surface have 

 always moisture to absorb. 



To feel the force of the dpicrrov ^ilv vScop we must 

 go to the sallow south; but if, on Liebig's principle 



