CHAP. IV. FUNCTION OF NUTRITION. 229 



(225.) Effects of Pruning. The objects to be ob- 

 tained by pruning are various. The gardener employs 

 this resource as the means of improving the general 

 form which he wishes his ornamental shrubs to assume ; 

 and he prunes his fruit trees in order that they may 

 bear fruit of larger size and improved flavour. With 

 these questions we have nothing to do in this place. 

 The results of pruning which we propose to notice 

 are such as are produced internally at places where 

 the knife has been employed, particularly for the pur- 

 pose of improving the quality of timber. This is at- 

 tempted by removing superfluous branches, which com- 

 pels the main trunk to become a straight clean shaft. The 

 effect of every wound of this kind is to expose a portion 

 of the older or innermost parts of the woody layers, 

 which are incapable of generating fresh tissue. The 

 consequence is that such parts cannot be healed over, 

 excepting by the growth of the newest tissue round the 

 edge of the wound. This tissue gradually extends 

 itself from the edges over the whole surface of the 

 wound until the opposite sides meet, and then grafting 

 together unite into one continuous mass : but the new 

 wood contracts no union with the surface of the old 

 wood exposed by the operation of pruning. As the 

 growing tissue which coats over a wound depends 

 upon the returning sap for its supply of nutriment, no 

 wound produced by cutting off a branch at some dis- 

 tance from the main trunk can ever heal. In this case 

 there are no leaves beyond the exposed surface to supply 

 it with proper juice, and whatever descends from the 

 main stem is carried into the branch, and consumed 

 in developing the buds and tissue on the lower part 

 of it before it can arrive at its extremity. But where 

 the branch is lopped near the trunk and a "snag" 

 (as it is technically termed) has been left, the descending 

 sap flows into this stump in sufficient abundance to 

 enable the tissue to close over the exposed extremity. 

 As the trunk increases these snags are completely em- 

 bedded and greatly injure the timber ; especially as they 

 Q 3 



