Origin and early development of secondary embryos in Quercus suber L.

Somatic embryos of the cork oak (Quercus suber L.) obtained after many generations of recurrent embryogenesis do not show signs of major structural abnormalities except for the appearance of more than two cotyledons or embryos fused at their embryonic axes. Secondary embryogenesis is mainly of multicellular origin and takes place after a slight proliferation of cells in the external layers of the primary embryo root cap. The first sign of secondary embryo formation is the appearance of meristematic primordia on the periphery of the proliferating tissue. They soon develop into bipolar structures and form cotyledonary primordia, and then the new embryos emerge from the proliferation mass. However, the unicellular pathway for secondary embryogenesis cannot be disregarded. In some cases the proliferation mass becomes brownish and takes on a more friable appearance. In this case, single cells, groups of cells, and globular proembryos appear isolated from the surrounding necrotic tissue. However, no further dev...