The Collaborative International Dictionary of English v.0.48:

Lenticel \Len"ti*cel\ (l[e^]n"t[i^]*s[e^]l), n. [F. lenticelle,
   dim. fr. L. lens, lentis, a lentil. Cf. Lentil.] (Bot.)
   (a) One of the small, oval, rounded spots upon the stem or
       branch of a plant, from which the underlying tissues may
       protrude or roots may issue, either in the air, or more
       commonly when the stem or branch is covered with water or
       earth.
   (b) A small, lens-shaped gland on the under side of some
       leaves.
       [1913 Webster]

WordNet (r) 3.0 (2006):

lenticel
    n 1: one of many raised pores on the stems of woody plants that
         allow the interchange of gas between the atmosphere and the
         interior tissue