Cell wall differentiation during early somatic embryogenesis in plants. I.Scanning and transmission electron microscopy study on embryos originatingfrom direct, indirect, and adventitious pathways

In three modes of somatic embryogenesis, direct (Cichorium), indirect (Citr us), and adventitious (Asparagus), a layer enveloped the proembryo and sepa rated it from parenchyma or callus cells. This layer was either torn or par tly dissolved during embryo expansion and led to a reticulated fibrillar st ructure linking the peripheral cells of the embryo prior to protoderm diffe rentiation. Scanning electron microscopic and transmission electron microsc opic analyses revealed it as a net-like structure composed of thick fibres restricted to the embryo surface during a specific developmental stage. Ste reo pairs of micrographs revealed its two-dimensional arrangement. The spat io-temporal localization of this fibrillar network defined it as an interme diate stage of somatic embryogenesis in monocotyledonous and dicotyledonous species irrespective of their modes of development. Hypotheses concerning the origin of the fibrillar network are provided, and its possible function s during somatic embryogenesis are discussed.