The Collaborative International Dictionary of English v.0.48:

Gemma \Gem"ma\, n.; pl. Gemm[ae]. [L., a bud.]
   1. (Bot.) A leaf bud, as distinguished from a flower bud.
      [1913 Webster]

   2. (Biol.) A bud spore; one of the small spores or buds in
      the reproduction of certain Protozoa, which separate one
      at a time from the parent cell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gemma
    n 1: small asexual reproductive structure in e.g. liverworts and
         mosses that detaches from the parent and develops into a
         new individual