Post-Fertilization Events: Embryo and Seed Development

While endosperm formation is already under way inside the embryo sac, the zygote -- formed by

syngamy -- begins its own, separate developmental journey, known as embryogeny, which will

eventually produce the mature embryo. In most flowering plants, embryo development proceeds only

after endosperm formation has begun, so that the growing embryo has ready access to a nutrient

supply from a very early stage.

In a typical dicotyledonous plant, the zygote's first division is markedly asymmetric, producing

two cells of quite different character and fate: a smaller terminal cell and a larger basal cell.

The basal cell does not itself become part of the future embryo proper; instead, it divides further

to build up a filamentous structure called the suspensor, whose function is twofold -- it anchors

the developing embryo in position, and it physically pushes the embryo deeper into the surrounding

endosperm tissue, from which the embryo can then absorb nutrients more effectively. The terminal

cell, meanwhile, is the true precursor of the embryo: it divides repeatedly to form what is called

the proembryo, and continued, carefully oriented cell divisions progressively shape this proembryo

first into a rounded, radially symmetric globular stage, and then, as two swellings that will become

the two cotyledons begin to appear on either side, into a heart-shaped stage. Further growth and

differentiation elongate the developing cotyledons and the embryonal axis between them, ultimately

producing the mature dicot embryo, complete with an embryonal axis and two cotyledons -- the part

of the axis above the level of cotyledon attachment differentiating as the epicotyl, which

terminates in the plumule (the embryonic shoot apex that will eventually give rise to the shoot

system), and the part below that level differentiating as the hypocotyl, which terminates in the

radicle (the embryonic root apex, itself protected by a root cap) that will give rise to the root

system.

As the embryo matures within it, the ovule as a whole is simultaneously being transformed into the

seed. The integuments, which had earlier enclosed the nucellus and embryo sac, harden and dry to

become the protective seed coat: an outer layer, the testa, usually thicker and tougher, and an

inner layer, the tegmen, usually thinner. A small pore in this seed coat, inherited directly from

the ovule's original micropyle, generally persists in the mature seed, where it later serves as the

principal passage through which water and oxygen re-enter the seed to trigger germination.

Depending on the species, the fate of the endosperm formed inside the seed varies considerably.

In albuminous seeds, the endosperm is not fully consumed during embryo development and instead

persists as a prominent, distinct food-storage tissue right through to seed maturity -- wheat,

maize and castor are common examples, and in such seeds the embryo remains comparatively small

relative to the bulk of stored endosperm surrounding it. In non-albuminous (or exalbuminous) seeds,

by contrast, the endosperm is entirely used up by the growing embryo during seed development, so …