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. 2019 Feb 18;10:109. doi: 10.3389/fpls.2019.00109

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Copyright © 2019 Egertsdotter, Ahmad and Clapham.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Zygotic embryo development in gymnosperms and angiosperms differs in some major aspects. (A) Early stages of conifer embryo development. The conifer zygote results from a single fertilization event. At the beginning of embryogenesis, there are free nuclear stages followed by a pro-embryo stage. Main characteristics of conifer embryo development are the polyembryogenic features of some conifer species where the zygotic embryos to different degrees proceed through a process of embryo-cleavage that results in multiple embryos that are eventually eliminated by programmed cell death. (B) In angiosperm embryo development the sporophytic generation is initiated by a double fertilization event resulting in one embryo. (A) pU, primary upper tier; pE, primary embryonal tier; U, upper tier; S, suspensor tier; EM, embryo mass; dS, R: dysfunctional suspensor tier; Es, embryonal suspensor tier; dEM, degenerating embryo mass; ROC, root organization center; sEs, secondary embryonal suspensor cells. (B) A, apical cell; B, basal cell; EP, embryo proper; S, suspensor. Figure adapted from Egertsdotter (1996).