FIGURE 1.

Summary of the different forms of embryogenesis in plants, showing the embryo and endosperm originated from a mature embryo sac (A,B) or the embryo ectopically (C,D) after sexual (A) or asexual (B–D) reproduction, with orange indicating the sexual process, blue the asexual or apomictic process, pink apomictic reproduction with fertilization of the central cell, and N = chromosome set after reduction division: (A) zygotic embryogenesis, involving chromosome reduction (N) and gamete fusion (N+N for the embryo, 2N+N for the endosperm), (B) apomictic embryogenesis, occurring in the ovule, either gametophytic apomixis in which an embryo sac arises from an unreduced megaspore (diplospory) or sporophytic cell of the ovule, usually adjacent to a sexually derived spore or developing embryo sac (apospory), and parthenogenetic (spontaneous) embryo development and autonomous (spontaneous) or pseudogamous (after fertilization of the central cell) endosperm formation, or sporophytic apomixis in which the embryo arises directly from a sporophytic cell of the ovule, often as polyembryony and alongside a sexually derived embryo and endosperm (C) somatic/sporophytic embryogenesis, involving ectopic embryo development from sporophytic cells, and (D) gametophytic embryogenesis, idem from a gametophytic cell. The latter two (C,D) omit the formation of an embryo sac, endosperm, and a seed coat, and occur naturally, for example, from leaf margins or ovular cells (C), gametophytic tissue in lower plants or, e.g., a synergid (D), but are particularly known from in vitro embryogenesis in which embryos are formed in culture, after external induction, particularly from protoplasts, leaf, the hypocotyl or other plant tissues (C), or microspores (D).