Dicotyledonous Embryo

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

A typical 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:

(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:

• The embryo is the young developing plant formed from the zygote after fertilisation, developing at the micropylar end of the embryo sac.
• A typical dicot embryo has an embryonal axis with two laterally attached cotyledons that store food and act as first seed-leaves.
• On the axis, the epicotyl (above cotyledons) ends in the plumule (shoot tip), and the hypocotyl (below) ends in the radicle (root tip), covered by a protective root cap.
• In Capsella bursa-pastoris, the zygote divides transversely into a larger basal cell (forms the suspensor) and a smaller terminal cell (forms the embryo proper).
• The basal cell forms a 6–10 celled suspensor; its first cell becomes a haustorium absorbing nutrients, and its lowermost cell (hypophysis) forms part of the radicle and root cap.
• The terminal cell forms the octant stage (8 cells: upper tier → plumule and cotyledons, lower tier → hypocotyl and radicle), then the dermatogen (future epidermis) at the globular stage.
• In the heart-shaped stage, two lobes grow into cotyledons; further growth curves the embryo through the torpedo stage into the mature horseshoe-shaped embryo, and the suspensor degenerates.