Seed structure

Monocot and dicot seeds

A seed is a mature, fertilised ovule. It has a seed coat, an embryo and stored food (in cotyledons or endosperm).

Dicot seed (gram, bean, pea):

• Seed coat with outer testa and inner tegmen.
• Hilum: scar where the seed was attached to the fruit; micropyle: small pore near it through which water enters.
• Embryo: an embryonal axis with plumule (future shoot) and radicle (future root), and two cotyledons, which store food.
• Most are non-endospermic (exalbuminous): the endosperm is used up and food is stored in the cotyledons. Castor is a dicot with endosperm.

Monocot seed (maize, wheat, rice):

• The maize or wheat "seed" is a fruit called a caryopsis, where the fruit wall is fused with the seed coat.
• A large endosperm stores starch; its outer protein-rich layer is the aleurone layer.
• One shield-shaped cotyledon, the scutellum, absorbs food from the endosperm.
• The plumule is covered by a sheath, the coleoptile; the radicle by the coleorhiza.
• Most are endospermic (albuminous).

Germination types:

• Epigeal: cotyledons are pushed above the soil (bean, cotton, castor, groundnut).
• Hypogeal: cotyledons stay below the soil (gram, pea, maize, wheat, rice).

Key points

• Seed = ripened ovule; fruit = ripened ovary.
• Dicot: two cotyledons, usually no endosperm.
• Monocot: one cotyledon (scutellum), endosperm, coleoptile, coleorhiza.
• Aleurone layer: protein-rich outer endosperm.
• Epigeal: bean; hypogeal: gram, pea, maize.

Exam tips

Questions usually name a part and ask its function or crop type. Remember coleoptile ↔ plumule, coleorhiza ↔ radicle.

Common mistakes

• Calling the scutellum an endosperm layer; it is the cotyledon.
• Thinking all dicots lack endosperm (castor has it).