CH. v.] THE SAP. 157 



the ground 1,006; but the same black Hamburgh, 

 gro^-ing upon its o^vn roots, had specific gravities 

 at corresponding heights of 1,004 and 1,009. This 

 increase is of great importance to a tree's gro^^th, 

 when the quantity of sap passing annually through 

 its vessels is considered. The exact amount of this, 

 it is perhaps impossible to discover, but its extent 

 may be appreciated by the quantity of moisture their 

 roots ai'e kno^ii to imbibe, and by the facts that 

 a small \me branch has poui'ed out sixteen ounces 

 of sap in twenty-four hours ; a birch tree a quantity 

 equal to its own weight during the bleeding season ; 

 and a moderate sized maple about two himdred pints 

 duiing the same period. 



The habit of the stock also is of much more im- 

 portance than is usually considered. If it grows 

 more rapidly, or has larger sap vessels than the 

 scion or bud, an enlargement occurs below these ; 

 but if they grow more rapidly than the stock, an 

 enlargement takes place just above the point of 

 union. In either case the tree is usually rendered 

 temporarily more prolific, but in the case where the 

 stock grows most slowly, the productiveness is often 

 of very short duration, the supply of sap annually 

 becoming less and less sufiicient to sustain the 

 enlai"ged production of blossom and leaves. This 

 veiy frequently occurs in the freer growing cherries 

 when inserted upon the wild species; and still more 



