Embryogenesis in Plants
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Direct answer
The zygote, sitting at the micropylar end of the embryo sac, divides — usually transversely — into two cells: the small apical or terminal cell generates the embryo proper, and the larger basal cell produces the suspensor that anchors and feeds it. In dicots the embryo passes through globular and heart-shaped stages before the mature form emerges with two cotyledons, an epicotyl ending in the plumule above the cotyledonary node and a hypocotyl ending in the radicle below. In monocots like wheat the single cotyledon is called the scutellum, the plumule is enclosed in a hollow foliar sheath, the coleoptile, and the radicle plus root cap in the coleorhiza. All this begins only after sufficient endosperm has formed.
What you must remember
- First division: mostly transverse, cutting the zygote into terminal (embryo) and basal (suspensor) cells — the suspensor pushes the developing embryo into the nutritive endosperm.
- Sequence in dicots: proembryo → globular embryo → heart-shaped embryo (cotyledon primordia appear) → mature embryo with two cotyledons.
- Mature dicot embryo parts: epicotyl (above cotyledons, bearing plumule), hypocotyl (below cotyledons, ending in radicle) — "epi" above, "hypo" below, relative to the cotyledons.
- Monocot cotyledon: one, termed the scutellum, pressed against the endosperm to absorb food.
- Two sheaths: coleoptile sheathes the plumule and shoot apex; coleorhiza encloses the radicle and root cap — coleoptile for shoot, coleorhiza for root.
- Site: embryogenesis occurs inside the embryo sac at the micropylar end, where the zygote sits after fertilisation.
- Dependence on endosperm: NCERT's timing — the embryo develops only after some endosperm has formed.
From one cell to a seedling blueprint
Follow the terminal cell. It divides in several planes to build first a ball of cells — the globular embryo — and then, as two lobes bulge on one flank, the heart-shaped stage that every labelled diagram in the chapter shows. Those lobes are the incipient cotyledons; their appearance is what makes the stage "heart-shaped" rather than decorative. Between them a notch marks the shoot apex, the future plumule, while the opposite pole organises the radicle. The basal cell, meanwhile, has not been idle: its divisions create the filamentous suspensor, whose job is partly mechanical (embedding the embryo deep in the endosperm) and partly nutritional, and whose terminal cell nearest the embryo contributes to the root-forming region — a detail advanced questions occasionally probe.
In a wheat or maize grain the same body plan is folded into a monocot configuration. The single cotyledon spreads as the scutellum, a shield pressing against the endosperm and funneling digested starch to the growing seedling. The shoot apex must punch through soil, so it travels armoured inside the coleoptile; the radicle departs inside the coleorhiza. These sheaths are the reason etiolated seedlings can elongate safely before photosynthesis begins. The exam's affection for this region is structural: give it a name — scutellum, coleoptile, coleorhiza, epicotyl, hypocotyl — and it will give you a mark.
How the examiner frames embryogenesis
Diagram-labelling is the main weapon: a heart-shaped embryo with the cotyledon lobes and suspensor marked, or a maize grain with the scutellum blank, tests whether you can attach names to geometry. Statement questions exploit the epicotyl-hypocotyl boundary — students place the radicle "at the tip of the epicotyl" when the axis below the cotyledons is the hypocotyl. The coleoptile-coleorhiza pair is reversed under time pressure, so rehearse the pairing as shoot-sheath and root-sheath. One more repeated stem: "the zygote divides longitudinally" — false; the first division is transverse in most flowering plants, and the suspensor, not the embryo proper, arises from the basal cell.
Frequently asked questions
What is the usual plane of the zygote's first division?
Transverse, producing a terminal cell that forms the embryo proper and a basal cell that forms the suspensor.
What is the function of the suspensor?
It anchors the embryo, helps push it into the endosperm for nutrition, and its cells assist in nutrient transfer during early development.
What is the scutellum?
The single cotyledon of a monocot embryo, as in wheat and maize, absorbing food from the endosperm for the seedling.
Which structures respectively protect the plumule and the radicle in a cereal grain?
The coleoptile sheathes the plumule and shoot apex; the coleorhiza encloses the radicle and root cap.
Why is the dicot embryo called heart-shaped at one stage?
Because the two developing cotyledons project as lobes from the globular body, giving the embryo a heart-like outline.