Definitions [1]
Define the following:
Embryo
An embryo is the early stage of development of an organism after fertilization, when the zygote starts dividing and differentiating into tissues and organs.
Key Points
Key Points: Classification of Seeds
- Based on cotyledons, Seeds are classified into dicot (two cotyledons) and monocot (one cotyledon).
- Based on endosperm: Seeds are albuminous (endospermic) or exalbuminous (non-endospermic).
- Albuminous seeds: Endosperm persists and provides nutrition; cotyledons are thin (common in monocots).
- Exalbuminous seeds: Endosperm is used during development; cotyledons become thick and store food (common in dicots).
- Perisperm: In some seeds, remnants of the nucellus persist, forming perisperm (e.g., black pepper, beet).
Key Points: Structure of a Dicotyledonous Seed
- Seed coat: Consists of two layers—testa (outer) and tegmen (inner).
- Hilum: A scar on the seed coat showing the point of attachment to the fruit.
- Micropyle: A small pore present above the hilum helps in water absorption.
- Cotyledons: Two fleshy cotyledons present, storing reserve food material.
- Embryo: Contains an embryonal axis with radicle and plumule at opposite ends.
- Endosperm: May be present (e.g., castor) or absent (e.g., bean, gram, pea) in mature seeds.
Key Points: Structure of Monocotyledonous Seed
- Monocot seed: Usually endospermic, with endosperm storing food (except orchids).
- Endosperm: Large and bulky, separated from the embryo by a protein-rich aleurone layer.
- Embryo: Consists of a single cotyledon called the scutellum and a short embryonal axis.
- Cotyledon: Only one large, shield-shaped cotyledon present.
- Radicle and plumule: Radicle is enclosed in the coleorhiza, and plumule in the coleoptile.
- Seed structure: The seed coat is often fused with the fruit wall, and the embryo lies at one side of the endosperm.
