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Embryo

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Estimated time: 27 minutes
CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Definition: Embryo

The embryo is the young developing plant formed from the zygote after fertilisation. It develops at the micropylar end of the embryo sac.

CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Structure of the Dicot Embryo

A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons attached laterally to it. The embryonal axis has distinct upper and lower regions:

Part Location on Embryonal Axis Terminates In / Function
Epicotyl Above the cotyledons Terminates in the plumule (shoot tip / stem tip)
Hypocotyl Below the cotyledons (cylindrical portion) Terminates in the radicle (root tip)
Radicle Lowermost end Develops into the primary root
Root Cap (Calyptra) Covers the radicle tip Protects the root tip during soil penetration
Plumule Apical shoot tip Develops into the shoot system; first leaves emerge from here
Cotyledons (×2) Attached laterally to embryonal axis Store food; function as first seed-leaves

(a) A typical dicot embryo; (b) L.S. of an embryo of grass

CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Step-by-Step Development - Dicot Embryo (Capsella)

(a) Fertilised embryo sac showing zygote and Primary Endosperm Nucleus (PEN); (b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)]

The following stages describe embryogenesis in Capsella bursa-pastoris:

1. First Transverse Division of the Zygote

The zygote divides transversely into two unequal cells:

  • Basal cell (larger, towards micropyle) → will form the suspensor
  • Terminal / Apical cell (smaller, towards chalaza) → will form the embryo proper
2. Formation of the Suspensor

The basal cell divides transversely to form a filamentous suspensor of 6–10 cells.

  • The first cell of the suspensor (micropylar end) becomes swollen and functions as a haustorium — it absorbs nutrients from the surrounding endosperm and passes them to the embryo.
  • The lowermost cell of the suspensor is the hypophysis — this later gives rise to part of the radicle and root cap.
3. Octant Stage

The terminal (embryonal initial) cell undergoes three successive mitotic divisions. Planes of division are at right angles to each other, yielding 8 cells arranged in two tiers:

  • Upper tier (4 cells) → forms the plumule and cotyledons
  • Lower tier (4 cells) → forms the hypocotyl and radicle
4. Dermatogen / Globular Stage

Periclinal divisions (parallel to the surface) of the outer cells of the octant produce a distinct outermost layer called the dermatogen, which will become the future epidermis. The embryo now takes on a spherical (globular) shape. The inner regions differentiate into procambium and ground meristem.

5. Heart-shaped (Cordate) Stage

Cells of the upper tier begin to divide unevenly. Two lateral lobes (future cotyledons) grow rapidly, giving the embryo a characteristic heart shape. The terminal plumule and cotyledon initials become clearly visible.

6. Torpedo → Mature (Horseshoe-shaped) Embryo

Enlargement of the hypocotyl and further growth of the two cotyledons causes the embryo to curve. The final shape is horseshoe-shaped (U-shaped), characteristic of a mature dicot embryo. The suspensor degenerates at this stage.

CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Structure of the Monocot Embryo

In monocotyledonous plants (e.g., the grass family), the embryo possesses only one cotyledon situated towards one side (laterally) of the embryonal axis. This single cotyledon is called the scutellum.

Two specialised protective sheaths are present — one for the shoot and one for the root — which are unique to monocots:

Part Location Function
Scutellum Lateral side of embryonal axis (one cotyledon) Absorbs nutrients from the endosperm during germination; shield-shaped
Epicotyl Above scutellum attachment point Bears the shoot apex
Coleoptile Hollow, foliar sheath enclosing the epicotyl Protects the shoot (plumule + leaf primordia) during germination and soil emergence
Epiblast Opposite side of the scutellum Rudimentary / vestigial structure (considered vestigial second cotyledon)
Radicle Lower end of embryonal axis Gives rise to the primary root
Root Cap Covering the radicle tip Protects the root tip
Coleorhiza Undifferentiated sheath surrounding the radicle + root cap Protects the root (radicle) during germination

Development of Monocot (grass) Embryo

CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Key Points: Embryo

  • Embryogenesis is the process of development of the zygote into an embryo, which begins after fertilisation and some endosperm formation.
  • The embryo develops at the micropylar end of the embryo sac.
  • The zygote divides to form a two-celled proembryo: a terminal (embryonal) cell and a basal (suspensor) cell.
  • The suspensor pushes the developing embryo into the endosperm and helps in nutrient supply, while the embryonal cell forms the main embryo.
  • The embryo passes through stages like the octant and heart-shaped stage, forming structures such as cotyledons, plumule, radicle, and hypocotyl.
  • In monocots, a single cotyledon (scutellum) is present with protective sheaths, the coleoptile (plumule) and coleorhiza (radicle), while in dicots, two cotyledons are formed.
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