The Collaborative International Dictionary of English v.0.48:

Protophyte \Pro"to*phyte\, n. [Proto- + Gr. ? a plant.] (Bot.)
   Any unicellular plant, or plant forming only a plasmodium,
   having reproduction only by fission, gemmation, or cell
   division.
   [1913 Webster]

   Note: The protophytes (Protophyta) are by some botanists
         considered an independent branch or class of the
         vegetable kingdom, and made to include the lowest forms
         of both fungi and alg[ae], as slime molds, Bacteria,
         the nostocs, etc. Cf. Carpophyte, and Oophyte.
         [1913 Webster]