Source : Webster's Revised Unabridged Dictionary (1913)

Mesophloeum \Mes`o*phl[oe]"um\, n. [NL., fr. Gr. me`sos middle +
   floio`s bark.] (Bot.)
   The middle bark of a tree; the green layer of bark, usually
   soon covered by the outer or corky layer, and obliterated.