80 THE GARDEN OF EARTH 



its upward journey the sap has to pass into and out of 

 countless multitudes of them. This is a very different 

 matter from a clear run through one long channel, 

 and it is impossible fully to explain. 



The subject as a whole is far too difficult to be gone 

 into here. All I can tell you is that the sap does so 

 journey from cell to cell, from tube to tube, filtering 

 gently through the enclosing skins. Not only in spring- 

 time upwards to the leaves, but later on, when it has 

 been fashioned into the completed sap, it travels again 

 in like manner, away from the leaves to all parts of the 

 tree, both above and below, wherever it may be needed. 



One other fact, which bears on this question of the 

 rising sap, is easy to grasp. You have heard how 

 the roots suck in liquid and send it upward. Such 

 constant pressure from below of more and more sap 

 ever rising must, it is thought, help the forward move- 

 ment of that which is in front, very much as people 

 in a dense crowd are pushed on in front by the pressure 

 from behind. 



Also the constant giving off by the leaves of quantities 

 of water is believed to lend some little assistance. 

 Water vapour is poured into the air from tiny leaf- 

 passages, and as it goes, more and more water is needed 

 from below to take its place. 



So we may say that probably both the roots and the 

 leaves do, to some extent, lend their aid in the upward 

 journeying of the raw sap. But with all these proposed 

 explanations it cannot be said that the rise of the sap 

 is fully understood. 



Perhaps hardly anything is more surprising than the 



