The Collaborative International Dictionary of English v.0.48:

Procumbent \Pro*cum"bent\, a. [L. procumbens, -entis, p. pr. of
   procumbere to fall, bend, or lean forward; pro forward +
   cumbere (in comp.), akin to cubare to lie down: cf. F.
   procombant. Cf. Incumbent.]
   [1913 Webster]
   1. Lying down, or on the face; prone. " Procumbent each
      obeyed." --Cowper.
      [1913 Webster]

   2. (Bot.) Lying on the ground, but without putting forth
      roots; trailing; prostrate; as, a procumbent stem.
      [1913 Webster]

WordNet (r) 3.0 (2006):

procumbent
    adj 1: having stems that trail along the ground without putting
           down roots