The Collaborative International Dictionary of English v.0.48:

Ostiole \Os"ti*ole\, n. [L. ostiolum a little door, dim. of
   ostium a door: cf. F. ostiole.] (Bot.)
   (a) The exterior opening of a stomate. See Stomate.
   (b) Any small orifice.
       [1913 Webster]

WordNet (r) 3.0 (2006):

ostiole
    n 1: a small pore especially one in the reproductive bodies of
         certain algae and fungi through which spores pass