The Collaborative International Dictionary of English v.0.48:

Radicle \Rad"i*cle\ (r[a^]d"[i^]*k'l), n. [L. radicula, dim. of
   radix, -icis, root: cf. F. radicule. See Radix.] (Bot.)
   (a) The rudimentary stem of a plant which supports the
       cotyledons in the seed, and from which the root is
       developed downward; the stem of the embryo; the caulicle.
   (b) A rootlet; a radicel.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Caulicle \Cau"li*cle\, n. (Bot.)
   A short caulis or stem, esp. the rudimentary stem seen in the
   embryo of a seed; -- otherwise called a radicle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

radicle
    n 1: (anatomy) a small structure resembling a rootlet (such as a
         fibril of a nerve)