Life Cycle of Angiosperms
Life cycle of angiosperms (Angiospermae) is the regular sequence of events and alternation of generations in which a seed germinates and grows into a mature plant (the sporophyte), which produces specialized reproductive structures—flowers. Within the flowers, haploid spores are formed, giving rise to extremely reduced male (pollen grain) and female (embryo sac) gametophytes. After pollination and double fertilization (Roland & Roland 1980), they give rise to a new diploid sporophyte enclosed in a seed inside a fruit (Raven et al. 2013; Mauseth 2017).
The key feature of angiosperms is the maximal reduction of gametophytes among all vascular plants (Hafidh et al. 2016). The male gametophyte (pollen grain) consists of two to three cells; the female gametophyte (embryo sac) typically consists of seven cells and eight nuclei (Raven et al. 2013). Double fertilization, discovered by S.G. Navashin, ensures the formation of a diploid zygote (embryo) and a triploid endosperm (nutritive tissue), which is a unique trait of flowering plants (Roland & Roland 1980; Yakovlev et al. 2003).
Unlike gymnosperms, in angiosperms the ovules are completely enclosed within the ovary, and after fertilization the ovary develops into a fruit, which enhances protection and seed dispersal efficiency (Lersten 2004). Thus, the life cycle of angiosperms represents the highest level of adaptation to terrestrial life, where the alternation of nuclear phases (haploid and diploid) is tightly coupled with the morphological specialization of the flower, pollination, double fertilization, and seed and fruit formation (Yakovlev et al. 2003; Mauseth 2017).
