Page:An introduction to physiological and systematical botany (1st edition).djvu/58

 28 more or less, pervade the wood and other parts of these plants, but usually in a less concentrated form.

When a portion of the bark of a tree is removed, the remainder has a power of extending itself laterally, though very gradually, till the wound is closed. This is accomplished by each new layer, added to the bark internally, spreading a little beyond the edge of the preceding layer. The operation of closing the wound goes on the more slowly, as the wood underneath, from exposure to the air, has become dead, and frequently rotten, proving an incumbrance, which though the living principle cannot in this instance free itself from, it has no power of turning to any good account. If, however, this dead wood be carefully removed, and the wound protected from the injuries of the atmosphere, the new bark is found to spread much more rapidly; and as every new layer of bark forms, as will be proved in the next chapter, a new layer of wood, the whole cavity, whatever it may be, is in process of time filled up.

This operation of Nature was turned to great advantage by the late Mr. Forsyth of