The Seed

The seed develops from a fertilised ovule once fertilisation is complete, and it represents the next generation of the plant in a protected, often dormancy-capable form, ready to germinate under suitable conditions. Although seeds vary greatly in size, shape and colour across different flowering plants, their internal organisation follows one of two consistent basic plans, corresponding to whether the parent plant is a dicotyledon or a monocotyledon.

The seed coat. Every seed is covered by a protective seed coat, which develops from the integument(s) of the ovule and typically has two distinguishable layers: a thicker, tougher outer layer called the testa, and a thinner inner layer called the tegmen.

Structure of a dicotyledonous seed. Within the seed coat of a dicot seed lies the embryo, made up of an embryonal axis and two large cotyledons attached to it -- in many dicot seeds, such as pea and gram, these cotyledons are fleshy and swollen because they themselves store the seed's reserve food. The short embryonal axis has two distinguishable portions relative to where the cotyledons attach: the part above this point is the epicotyl, which terminates in the plumule, the embryonic shoot that will grow upward after germination; the part below this point is the hypocotyl, which terminates in the radicle, the embryonic root that will grow downward, usually protected by a small root cap. Dicot seeds are further divided by whether they retain the endosperm (the nutritive tissue that surrounds the embryo after fertilisation) at maturity: albuminous seeds, such as castor, retain a substantial endosperm layer that supplies food to the germinating embryo, while exalbuminous seeds, such as pea and gram, use up the entire endosperm during the embryo's own development, so that reserve food is instead stored directly within the swollen cotyledons themselves.

Structure of a monocotyledonous seed. A monocot seed, such as maize, generally retains a large, bulky endosperm at maturity, occupying most of the seed's volume and clearly visible in cross-section; such seeds are described as endospermic. The embryo itself is comparatively small and lies embedded to one side of this endosperm, usually in a small pocket near one end of the seed. The monocot embryo has only a single cotyledon, called the scutellum, positioned against the endosperm. On either side of the scutellum, the embryonal axis carries the plumule (enclosed within a protective sheath called the coleoptile) at one end and the radicle (enclosed within a protective sheath called the coleorhiza) at the other -- these two protective sheaths are themselves a distinctive monocot feature, absent from the dicot seed plan described above. …