The Collaborative International Dictionary of English v.0.48:

Instructive \In*struct"ive\, a. [Cf. F. instructif.]
   Conveying knowledge; serving to instruct or inform; as,
   experience furnishes very instructive lessons. --Addison.
   [1913 Webster]

         In various talk the instructive hours they past.
                                                  --Pope.
   -- In*struct"ive*ly, adv. -- In*struct"ive*ness, n.
   [1913 Webster]

         The pregnant instructiveness of the Scripture. --Boyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

instructively
    adv 1: in an informative manner [syn: informatively,
           instructively] [ant: uninformatively,
           uninstructively]