Explain the grafting metaphor in Shakespeare's Sonnet 18.

The subtle reference to grafting occurs in line 12 of
Shakespeare's Sonnet 18:

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When in eternal lines to time thou
growest:

The specific words
relevant to grafting are "in ... lines to time thou
growest."

Grafting is an ancient horticultural process of
combining branches from one plant with the body of another plant. For instance, walnut
varieties may be grafted together by embedding the branch of one into the trunk of
another so that one tree produces more than one variety of walnut,

The
idea the sonneteer is expressing is that his beloved will become part of immortal Time
through being grafted in and thus live forever as Time itself "lives" forever. This
metaphor is also dependent upon a personification of time, such that time may receive a
graft and such that it may "live."