NUTRITION. THE MACHINERY. 41 



composition throughout. Near the root the juice of the 

 plant has one composition, near the leaf another. The 

 word " sap," then, though convenient, must not he used or 

 conceived of as indicating the existence of a current abso- 

 lutely fixed in its direction or uniform in its composition. 

 In other words, it has but a remote analogy to the blood, 

 with which it is so often compared. There is an upward 

 current of watery fluid, well marked in spring, there are 

 downward and cross currents varying in direction and 

 intensity according to the requirements of the growing 

 tissues and their conformation. These have only indirect 

 connection with the main upward flow just referred to. 



