The Collaborative International Dictionary of English v.0.48:

Membranaceous \Mem`bra*na"ceous\, a. [L. membranaceus.]
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   1. Same as Membranous. --Arbuthnot.
      [1913 Webster]

   2. (Bot.) Thin and rather soft or pliable, as the leaves of
      the rose, peach tree, and aspen poplar.
      [1913 Webster]