CHAP. IV. FUNCTION OF NUTRITION. 231 



of water upon the surface of the wound, after the newly 

 developed wood has formed a swollen border round its 

 edges. If the cut is perfectly smooth it will be the 

 sooner healed ; and its surface may be protected by 

 some compost (such as that which is known by the 

 name of Forsyth's mixture) whenever the wound is un- 

 avoidably large. An opinion has gone abroad that it is 

 possible to diminish the blemish which pruning neces- 

 sarily occasions in timber, by lopping the extremities of a 

 branch and causing them to die and rot off in a natural 

 manner. Supposing it were true that a branch thus 

 treated always did die, which is by no means a neces- 

 sary consequence, all that could be gained by such a 

 mode of proceeding would be the introduction of the 

 rotten stump of the lopped branch into the heart of the 

 tree instead of the clean scar which close pruning pro- 

 duces. It is not true, as some suppose, that any na- 

 tural sloughing off of the decayed part takes place 

 or that the old and new wood can ever completely 

 unite together ; but in all cases it will be found that 

 the new wood has grown over the old wound, and that 

 the surface of the latter is preserved exactly in the 

 state in which it was embedded. The knots in deal 

 and other timbers are defects produced by the process 

 of " natural pruning," as it has been termed, and such 

 defects are inevitably greater than those which result 

 from artificial pruning performed on branches of the 

 same dimensions and cut off close to the stem. 



(227.) Grafts. Every one is acquainted with the 

 fact, that certain portions of some plants may be grafted 

 upon others, and that the tissues of the " graft " and 

 " stock " as the two are named will completely unite 

 and vegetate together as though they were parts of the 

 same individual. The effects thus artificially produced 

 are occasionally observed to take place naturally : two 

 branches of the same tree being sometimes found 

 grafted together, where they have been wounded by 

 mutual attrition. When ivy has grown to a consider- 

 able size its branches often interlace and graft together 

 Q 4 



