The Collaborative International Dictionary of English v.0.48:

Microspore \Mi"cro*spore\ (m[imac]"kr[-o]*sp[=o]r), n. [Micro- +
   spore.] (Bot.)
   One of the exceedingly minute spores found in certain
   flowerless plants, as Selaginella and Isoetes, which bear
   two kinds of spores, one very much smaller than the other.
   Cf. Macrospore.
   [1913 Webster]

WordNet (r) 3.0 (2006):

microspore
    n 1: smaller of the two types of spore produced in heterosporous
         plants; develops in the pollen sac into a male gametophyte