polyptoton

from WordNet (r) 3.0 (2006)

polyptoton
    n 1: repetition of a word in a different case or inflection in
         the same sentence; "My own heart's heart"

from The Collaborative International Dictionary of English v.0.48

Polyptoton \Pol`yp*to"ton\, n. [L., fr. Gr. ? having, or being
   in, many cases; poly`s many + ? case.] (Rhet.)
   A figure by which a word is repeated in different forms,
   cases, numbers, genders, etc., as in Tennyson's line, -- "My
   own heart's heart, and ownest own, farewell."
   [1913 Webster]