From The Collaborative International Dictionary of English v.0.48:
Liber \Li"ber\ (l[imac]"b[~e]r), n. [L. See Libel.] (Bot.)
The inner bark of plants, lying next to the wood. It usually
contains a large proportion of woody, fibrous cells, and is,
therefore, the part from which the fiber of the plant is
obtained, as that of hemp, etc.
[1913 Webster]
Liber cells, elongated woody cells found in the liber.
[1913 Webster]