Plant Ontogeny
The individual development of a plant, like that of any living organism, covers the entire path from the moment of origin to natural death. This path includes not only an increase in size but also sequential qualitative changes – the emergence of new organs, changes in tissue types, the transition from the vegetative phase to the reproductive phase, and finally, aging.
Plant ontogeny (from Greek ontos – being and genesis – origin) is the set of regular, irreversible changes in an individual from its inception (e.g., from a zygote or a vegetative rudiment) to death (Strasburger, 1971; Gatsuk et al., 1980). This concept should be distinguished from the “grand life cycle” (or cycle of reproduction), which involves the alternation of generations – sporophyte and gametophyte (Evert, 2006; Raven et al., 2005). Ontogeny describes the development of a specific, usually diploid individual (the sporophyte in higher plants), whereas the life cycle encompasses the succession of nuclear phases and generations over time.
To understand why modern plants have their particular organization, one must turn to evolution. The form and internal structure of plants are the result of a long history of adaptation to terrestrial life. Evolutionary development (phylogeny) leaves a deep imprint on individual development: many features of ontogeny (characteristics of organ initiation, growth patterns, age-related changes) can only be explained in light of the origin of the group in question (Strasburger, 1971).
The first land plants – rhyniophytes – had a simple body consisting of dichotomously branching axes (telomes) without true leaves and roots. They rooted in the soil using rhizomoids and conducted water and nutrients through primitive tracheids. The transition to land required the development of protective coverings, stomata, mechanical tissues, and an efficient conducting system (Evert, 2006; Raven et al., 2005).
A key evolutionary acquisition that radically changed ontogeny was the appearance of the seed. In seed plants (gymnosperms and angiosperms), the female gametophyte and the embryo developing from the zygote are protected and supplied with nutrients inside the ovule, which after fertilization turns into a seed. This allowed the embryo (the young sporophyte) to survive unfavorable periods in a dormant state and to disperse via seeds. In the ontogeny of a seed plant, a latent period (seed dormancy), a pre-reproductive, a reproductive, and a senile period are distinguished (Gatsuk et al., 1980).
The evolutionary trend toward sporophyte dominance over the gametophyte in vascular plants reached its maximum in angiosperms, where the female gametophyte – the embryo sac – is reduced to a few cells and depends entirely on the sporophyte (Raven et al., 2005). Simultaneously, heterospory arose – the emergence of two types of spores: microspores (from which male gametophytes develop) and megaspores (giving rise to female gametophytes). This, in turn, led to the appearance of the flower as a specialized reproductive shoot that ensures efficient pollination and fertilization.
Thus, modern ontogeny of higher plants is the result of a complex evolutionary restructuring, combining conservative early stages (embryogenesis, germination) and plastic, ecologically dependent later stages (growth, flowering, fruiting, aging). Understanding ontogeny is impossible without knowledge of the evolutionary history of groups and comparative morphology, but within the framework of this article, the main focus will be on the structural and functional organization of individual development – from zygote to the death of the individual.
