DESCENDING SAP 115 



passage, as for example in the crevice of a rock, and 

 swollen above the obstacle into an unsightly excres- 

 cence. The stoppage of the sap in its downward 

 course explains this phenomenon. 



"If the tree-trunk is not completely encircled by 

 the stricture, if somewhere there is a strip of bark 

 left free to serve as a passage, the nourishing juice 

 takes this way to get around the obstacle, and so 

 pursues its course to the roots. Then the tree con- 

 tinues to live. But if the barrier is absolutely in- 

 superable, as in the case of an unyielding ligature 

 or when the tree has been girdled, the sap cannot 

 descend to the roots to nourish them ; and with the 

 death of these the end of the tree is not far distant. 



"An important lesson remains to be drawn from 

 these details concerning the circulation of this nu- 

 tritive liquid in plants. Henceforth, when we fasten 

 a plant to its prop or supporting stake, we shall be 

 careful not to tie the string too tight or else to loosen 

 it at the proper time, since otherwise we should run 

 the risk of strangling the plant and so causing its 

 death." 



