VEGETABLE PHYSIOLOGY. 71 



of the tree, in the same manner perhaps as bleeding is necessary 

 to the health of the animal. The trunk of the plum and cherry 

 tree seldom expand freely till a longitudinal incision has been 

 made in the bark ; and hence this operation is often practised 

 by gardeners. If the incision affects the epidermis only, it heals 

 up without leaving any scar ; if it penetrates into the interior of 

 the bark it heals up only by means of leaving a scar ; but if it 

 penetrates into the wood, the wound in the w T ood itself never 

 heals up completely ; but new wood and bark are formed above 

 it as before. 



2. Boring. Boring is an operation by which trees are often 

 wounded for the purpose of making them part with their sap in 

 the season of their bleeding, particularly the birch and sugar 

 maple. A horizontal or rather slanting hole is bored in them 

 with a wimble, so as to penetrate an inch or two into the w T ood, 

 from this the sap flows copiously ; and though a number of holes 

 is often bored in the same trunk, the health of the tree is not 

 materially if at all affected. For trees will continue to thrive 

 though subjected to this operation for many successive years ; 

 and the hole, if not very large, will close up again like the deep 

 incision, not by the union of the broken fibres of the wood, but 

 by the formation of new bark and wood projecting beyond the 

 edge of the orifice, and finally shutting it up altogether. 



3. Gii-dling. Girdling is an operation to which trees in 

 newly-settled countries are often subjected when the farmer 

 wishes to clear his land of timber. It consists in making parallel 

 and horizontal incisions with an axe into the trunk of a tree, and 

 carrying them quite round the stem so as to penetrate through 

 the liber into the alburnum, and then to scoop out the intervening 

 portion. If this operation is performed early in the spring and 

 before the commencement of the bleeding season, the tree rarely 

 survives it ; though some trees that are peculiarly tenacious of 

 life, such as the sugar maple and mountain tupelo, have been 

 known to survive it a considerable length of time. 



4. Fractures. If a tree is bent so as to break only a part of 

 the cortical and woody fibres, and the stem or branch but small, 

 the parts will again unite by being put back into their natural 



