PLANT NUTRITION. 43 



squeezing process, is augmented by the swaying of the 

 branches or the movements of the leaves. Even more 

 powerful must be the effect of the atmospheric pressure 

 urging up the liquid to fill the place of that evaporated 

 from the leaf surface. This upward current is naturally 

 most active at the period of growth, and the channels 

 through which it flows are necessarily those where the 

 conditions for osmosis are most propitious. In propor- 

 tion, therefore, as the cells become filled with woody or 

 earthy material does the current become less. As the 

 straw ripens or the timber hardens by the formation of 

 wood in its cells, so does the flow of liquid diminish, the 

 leaves in their turn and degree become obstructed and 

 fall, and the current, deprived of their stimulus, becomes 

 feeble. 



But while in thus alluding to some of the duties of 

 the stem, we have had to note the existence during the 

 period of growth of a current of liquid whose general 

 direction is upward, it is necessary to point oat that the 

 direction is not exclusively upward, but that it is mani- 

 fested in whatever direction the resistance is least and 

 where growth may be going on most actively at the time. 

 Again, it is necessary to guard against the still prevalent 

 fallacy attaching to the use of the word " sap." That 

 term was first employed when it was imagined that a 

 regular circulation of fluid took place in plants from 

 root to leaf, and from leaf back to root just as in ani- 

 mals the blood courses from the heart through the arteries 

 to the capillaries, and back from the capillaries to the 

 heart by the veins. In the case of the higher animals 

 there is a continuous series of tubes to convey the fluid, 

 and that fluid is uniformily arterial or venous. It is 

 quite otherwise with plants ; there is no continuous tube 

 or set of tubes, and there is no fluid of uniformily the 

 same composition throughout. Near the root the juice 

 of the plant has one composition, near the leaf another. 



