Source : Webster's Revised Unabridged Dictionary (1913)

Microspore \Mi"cro*spore\, 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}.

Source : WordNet®

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