Chapter 22: Early Development of the Plant Body

Embryogenesis begins with the first asymmetrical cell division of the zygote, establishing apical-basal polarity where a smaller apical cell generates most of the embryonic structure while a larger basal cell produces the suspensor, which physically anchors and nutritionally supports the developing embryo. As development progresses, three primary meristematic tissues emerge and establish the plant's radial organization: the protoderm generates the epidermis, the ground meristem forms ground tissues, and the procambium gives rise to vascular tissues. Eudicot embryos characteristically progress through recognizable morphological stages including globular, heart, and torpedo configurations as their two cotyledons develop, whereas monocot embryos develop a single specialized cotyledon called the scutellum that often dominates the embryonic architecture. Genetic research using Arabidopsis mutants such as gurke, fackel, monopteros, and gnom has illuminated the regulatory gene networks controlling axial organization and tissue differentiation. The mature embryo consists of an axis bearing a hypocotyl and root system with shoot and root apical meristems positioned at opposite ends, plus cotyledons that may function as either storage organs or nutrient-absorbing structures depending on the species. Seed maturation involves dramatic desiccation as water content decreases approximately ninety percent, triggering entry into dormancy, a protective state maintained through mechanical seed coat resistance, chemical inhibitors, or physiological incompleteness of the embryo itself. Breaking dormancy requires specific environmental signals or treatments such as cold exposure, fire, or leaching that remove chemical inhibitors. During germination, water uptake initiates enzyme reactivation and mobilization of stored nutrients, with the root typically emerging first to establish anchorage and water acquisition. Germination pathways differ among species: epigeous germination brings cotyledons above soil via hypocotyl extension, while hypogeous germination maintains cotyledons underground as the epicotyl or specialized protective structures like the coleoptile emerge. Grasses employ unique morphological innovations including the coleoptile to shield emerging shoots and the coleorhiza to protect emerging roots, facilitating successful seedling establishment in challenging environments.