BLEEDING. 53 



their leaves are formed ; because the leaves gradually empty 

 the tubes, put an end to their distension, and prevent its 

 recurrence so long as they remain in an active state. 



The excessive loss of sap in the above-mentioned cases 

 would not have taken place if the roots had been wounded in 

 the summer or autumn. It is probable, moreover, that the 

 bleeding was increased by the unusual coldness of the spring 

 in which these instances occurred. Hales himself was aware 

 that sap falls back at night in consequence of the contraction 

 of the tubes by cold ; Mr. Knight observed the same fact : and 

 it has more recently been proved experimentally by M. Biot. 

 It may therefore be supposed that excessive cases of bleeding 

 occur, because, in addition to the natural contraction of the 

 tubes of the wood, their mechanical contraction by unusual 

 cold has to be taken into account. 



