midrib

from WordNet (r) 3.0 (2006)
midrib
    n 1: the vein in the center of a leaf [syn: {midrib}, {midvein}]
from The Collaborative International Dictionary of English v.0.48
Midrib \Mid"rib`\, n. (Bot.)
   A continuation of the petiole, extending from the base to the
   apex of the lamina of a leaf.
   [1913 Webster]
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