3.6. Seed and Germination

The highly organized body of a seed plant begins its existence with the product of gametic fusion, the zygote , which develops into an embryo. The mature embryo has a limited number of parts - a stem-like axis bearing one or more cotyledons, the embryonic shoot. Below the cotyledon(s), the embryonic root or the radicle exists. Often, as in the case of cereal seeds like that of wheat, a starchy reserve called the endosperm powers this early heterotrophic growth of the embryonic shoot and root. After the seed germinates, the apical meristem of the shoot forms, while the apical meristem of the root located at the tip of the radicle forms the primary root.  The growth outlined above constitutes the vegetative stage in the life of a seed plant. Under favorable conditions, determined in part by an endogenous rhythm of growth and in part by exogenous environmental factors, especially light, humidity, and temperature, the vegetative apical meristem of the shoot is changed into a reproductive apical meristem, which produces a flower. The vegetative stage in the life cycle of the plant is thus succeeded by the reproductive stage.