The plant embryo is a small, rudimentary plant contained within a seed. This microscopic structure represents the beginning of the sporophyte generation, which is the diploid phase that constitutes the familiar green plant body. The embryo is an immature sporophyte, developed from a fertilized egg, holding the complete genetic blueprint for the mature organism. Encapsulated within the seed, the embryo pauses its development until conditions are right for germination and growth.
How the Plant Embryo Forms
The formation of the plant embryo, known as embryogenesis, begins immediately after fertilization occurs deep within the flower’s ovule. In flowering plants, a unique process called double fertilization takes place, where one sperm cell fertilizes the egg to form the diploid zygote (2n), while a second sperm cell fuses with two polar nuclei to form the triploid endosperm (3n). The zygote is the single-celled origin of the embryo, and the endosperm becomes the nutrient-rich tissue that will sustain the developing embryo.
The zygote then divides asymmetrically into an apical cell and a basal cell, establishing the future polarity of the plant body. The basal cell divides to form a stalk-like structure called the suspensor, which anchors the embryo to the maternal tissue and serves as a conduit for transferring nutrients from the endosperm. The apical cell gives rise to the embryo proper, progressing through distinct morphological stages that establish the basic body plan.
The first stage of the embryo proper is the globular stage, a spherical mass of cells where the initial tissue layers are differentiated. This is followed by the heart stage, where the appearance of two lobes marks the emergence of the cotyledons, or embryonic leaves. The structure then elongates into the torpedo stage, where the shoot apical meristem (SAM) and root apical meristem (RAM)—the regions of future growth—become clearly defined at opposite ends of the structure. By the time the embryo reaches maturity, it has established all the primary structures of the adult plant, though in a highly miniaturized form.
Anatomy of the Mature Plant Embryo
The mature plant embryo is organized around an embryonic axis. The cotyledons, or “seed leaves,” are often the most prominent part of the embryo and primarily function in nutrient storage or absorption. For example, in non-endospermic seeds, the cotyledons store food reserves, while in endospermic seeds, the cotyledon (scutellum) acts as an absorptive organ, transferring nutrients from the surrounding endosperm.
At the lower end of the embryonic axis is the radicle, which is the embryonic root that will be the first structure to emerge during germination. Just above the radicle is the hypocotyl, the transitional stem portion that lies below the attachment point of the cotyledons. The hypocotyl is responsible for pushing the cotyledons and the developing shoot above the soil surface in many species.
The upper part of the axis, above the cotyledons, is the epicotyl, which contains the plumule, or the embryonic shoot. The plumule is composed of the shoot apical meristem and the first true leaf primordia, representing the future stem and leaves of the plant. The plant kingdom is broadly categorized by the number of cotyledons: monocots possess a single cotyledon, while dicots possess two.
From Embryo to Seedling: The Role of Germination
The mature embryo often enters a state of dormancy, an adaptive trait that prevents it from germinating until conditions are favorable for seedling survival. Dormancy is maintained by the low moisture content of the seed and the presence of plant hormones like abscisic acid. The breaking of dormancy and the transition to a free-living plant is initiated by germination, which is the process of metabolic reactivation and growth.
Germination begins when the seed imbibes, or takes up, water, which hydrates the embryo and activates the stored enzymes. This water uptake makes the stored food reserves accessible to the growing embryo, supplying the energy needed for cell division and expansion. The radicle is typically the first part of the embryo to break through the seed coat, anchoring the young plant and beginning water absorption.
The successful continuation of germination is highly dependent on external environmental factors. Sufficient moisture is necessary for imbibition and metabolic activity, while appropriate temperature determines the rate of biochemical reactions within the seed. Some seeds also require specific exposure to light or darkness to signal whether they are at the soil surface or buried at a safe depth.