The Collaborative International Dictionary of English v.0.48:

Proembryo \Pro*em"bry*o\, n. [Pref. pro- + embryo. ] (Bot.)
   (a) The series of cells formed in the ovule of a flowering
       plant after fertilization, but before the formation of
       the embryo.
   (b) The primary growth from the spore in certain cryptogamous
       plants; as, the proembryo, or protonema, of mosses.
       [1913 Webster]