The Collaborative International Dictionary of English v.0.48:

Zygosperm \Zyg"o*sperm\, n. [Gr. zygo`n a yoke + E. sperm.]
   (Bot.)
   A spore formed by the union of the contents of two similar
   cells, either of the same or of distinct individual plants.
   Zygosperms are found in certain orders of algae and fungi.
   [1913 Webster]