40 PLANT LIFE ON THE FARM. 



augmented by the swaying of the branches or the move- 

 ments 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 neces- 

 sarily those where the conditions for osmosis are most pro- 

 pitious. In proportion, 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 forma- 

 tion 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 out that the direction is 

 not exclusively upward, but that it is manifested in what- 

 ever direction the resistance is least and where growth may 

 be going on most actively at the time. Again, it is neces- 

 sary 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 animals 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 con- 

 vey the fluid, and that fluid is uniformly arterial or venous. 

 It is quite otherwise with plants ; there is no continuous tube 

 or set of tubes, and there is no fluid of uniformly the same 



