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duramen

duramen Bot.
  (djʊˈreɪmɛn)
  [rare L. dūrāmen hardness; a hardened or ligneous vine-branch (Columella), f. dūrāre to harden.]
  The central wood or heart-wood of an exogenous tree.

1837 Penny Cycl. IX. 205/1. 1839 Carpenter Princ. Gen. & Comp. Phys. §329 The deposition of the products of secretion which gives strength and firmness to the duramen, destroys or greatly diminishes its power of transmitting fluid. 1882 Vines Sachs' Bot. 133 The inner layers of alburnum are gradually transformed into duramen..the cell-walls assuming a darker colour, from saturation with resin, colouring-substances, etc. The distinction between alburnum and duramen is very clear and well-marked in the oak, walnut, cherry, elm..brazil wood, etc.

Oxford English Dictionary

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