Light and electron microscopy of embryo development in an annual × perennial Medicago species cross

Using light and transmission electron microscopy, we studied embryo development from 3 to 14 days after pollination in an interspecific Medicago cross (perennial M. sativa L., 2n = 4x = 32 × annual M. scutellata (L.) Mill, 2n = 4x = 32) to determine factors contributing to abortion in wide crosses. An intraspecific M. sativa cross and an M. sativa self-mating were used for control embryos. Nucellus and integumentary tapetum at the base of the embryo sac appear to be important sources of nutrition during the first 2 weeks of embryo development. Micrographic evidence supports the conclusions that maternal and embryonic tissue of interspecific ovules fail to carry out a timely sequence of metabolism involving lipid, starch, and nucellar crystals. Delayed breakdown of starch and lipid in the integumentary tapetum and nucellus is a probable factor in reduced development of coenocytic endosperm. At the late heart stage of embryo development, relative inactivity of dictyosomes and endoplasmic reticulum in hybrid...