RISE AND FALL OF SAP. 



Itfl 



he ascertained it to be in a state of consid- 

 erable activity in mid-winter. Among other 

 things he found that frost had considerable 

 influence upon the direction in which the 

 sap moves. In mild weather, the sap was 

 constantly rising ; but when frost was ex- 

 perienced, the sap flowed back again — a 

 phenomenon which he referred to the con- 

 tracting power of cold on the vessels of the 

 trunk and branches, the effect of which was 

 to force the sap downwards into the roots 

 lying in a warmer medium ; then, again, 

 when the frost reached the roots themselves, 

 and began acting on them, the sap was 

 forced back into the trunk ; but as soon as 

 a thaw came on, and the ground recovered 

 its heat, the roots, out of which a part of 

 the sap had been forced upwards, were 

 again filled by the fluids above them, and the 

 sap was forced to fall. A large Poplar tree, 

 in the latter state, having been cut across 

 at the ground line, the surface of the stump 

 was found to be dry, but the end of the 

 trunk itself dripped with sap. Sap, then, 

 is always in motion ; and if it ever settles 

 to the root in a visible manner, that is owing 

 to temporary causes, the removal of which 

 causes its instant re-ascent. 



As to the idea that the bleeding of a tree 

 begins first at the root, and, in connection 

 with this supposition, that what is called 

 the rise of the sap is the cause of the ex- 

 pansion of buds, and leaves, and branches, 

 nothing can well be more destitute of any 

 real foundation. If in the spring, when the 

 buds are just swelling, a tree is cut across 

 at the ground line, no bleeding will take 

 place, neither will the sap flow for some 

 distance upwards ; but among the branches 

 the bleeding will be found to have com- 



menced. Let A B represent the trunk and 

 branches of a tree ; let inci- 

 sions be made at c, d, e,f; 

 the sap will run at c first, 

 then at d, next at e, and last 

 at /, next the roots. This 

 was observed some years ago 

 by Mr. Thompson, at that 

 time the Duke of Pokt- 

 land's gardener, who tho't 

 he had discovered that the 

 sap of trees descends in the 

 spring, instead of ascending ; 

 a strange speculation enough 

 it must be confessed. The 

 fact is, that the sap is driven 

 into accelerated motion, first 

 at the extremities of a tree, 

 because it is there that light 

 ^ and warmth first tell upon 



^'°-^^- the excitable buds. The 



moment the buds are excited, they begin to 

 suck sap from the parts with which they are 

 in contact ; to supply the waste so produced 

 the adjacent sap pushes upwards ; as the 

 expansion of the leaves proceeds, the de- 

 mands upon the sap near them become great- 

 er • a quicker motion still is necessary on 

 the part of the sap to make good the loss ; 

 and thus from above downward is that per- 

 ceptible flow of the fluids of trees, which 

 we call bleeding, effected. 



The well known fact of trees sprouting 

 in the spring, although felled in the autumn, 

 proves that the sap had not at that time 

 quitted the trunk to take refuge in the roots. 

 Such a common occurrence should put peo- 

 ple on their guard against falling into the 

 vulgar errors on this subject. 



