THE PLANT AND THE SEASONS 21 



be forgotten that when the tree is in its fullest 

 state of activity, there are two very distinct 

 currents of sap the ascending and the descending 

 flow. The starting into growth of the tree has 

 been compared to the lighting of a lamp. To an 

 extent the developing leaves exercise just that 

 drawing power upon the liquid imbibed by the 

 roots from the soil which the burning wick does 

 upon the oil in the reservoir. This is not a com- 

 plete explanation of what is really a great mystery, 

 for it must be borne in mind that a very vigorous 

 sap flow starts before the foliage is expanded. 



The course which the sap pursues in a typical 

 tree may be briefly outlined. Starting at the root, 

 the fluid is carried upwards through the cells of 

 the newer wood to the upper surface of the leaves. 

 At this stage the sap is really little more than 

 plain water, combined with certain inorganic 

 matter taken from the soil. The force with which 

 the sap rushes upwards is very great, and by way 

 of experiment a vine stem was severed a few feet 

 from the ground in the springtime, and a bladder 

 tied over the cut end. In a short while the bladder 

 was greatly extended, and finally burst, unable to 

 sustain the tremendous pressure of the sap flow. 

 It is not until the passage of the sap along the 

 upper surface of the leaves that the great change 



