Seed Structure and Seed Germination

Monocots vs Eudicots (Dicots)

Structure of the Mature Seed A seed consists of an embryo, endosperm, and seed coat. Seed coat Embryo Endosperm The embryo and its food supply are enclosed by a hard, protective seed coat The seed enters a state of dormancy

Definitions Cotyledons: the embryo’s “seed leaves.” Embryonic shoots and roots also form. Endosperm: a tissue produced inside the seeds that surrounds the embryo and provides nutrition for the embryo. Seed coat: is a tough outer layer that protects the embryo from damage, dehydration, and predators. Hypocotyl: the stem of a germinating seedling, found below the cotyledons and above the radicle (root). Epicotyl: the embryonic shoot above the cotyledons. In most plants the epicotyl will eventually develop into the leaves of the plant. Radicle: embryonic root Plumule: the rudimentary shoot or stem of an embryo plant.

(a) Common garden bean, a eudicot with thick cotyledons Seed coat Epicotyl Hypocotyl Radicle Cotyledons Figure 38.8a Seed structure (a) Common garden bean, a eudicot with thick cotyledons

(b) Castor bean, a eudicot with thin cotyledons Seed coat Endosperm Cotyledons Epicotyl Hypocotyl Radicle Figure 38.8b Seed structure (b) Castor bean, a eudicot with thin cotyledons

Pericarp fused Scutellum with seed coat (cotyledon) Endosperm Coleoptile Epicotyl Hypocotyl Coleorhiza Radicle Figure 38.8c Seed structure (c) Maize, a monocot

Seed Dormancy: An Adaptation for Tough Times Seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling The breaking of seed dormancy often requires environmental cues, such as temperature or lighting changes Seed Germination and Seedling Development Germination depends on imbibition, the uptake of water due to low water potential of the dry seed The radicle (embryonic root) emerges first Next, the shoot tip breaks through the soil surface In many eudicots, a hook forms in the hypocotyl, and growth pushes the hook above ground The hook straightens and pulls the cotyledons and shoot tip up

Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Figure 38.9a Two common types of seed germination Radicle Seed coat (a) Common garden bean

In maize and other grasses, which are monocots, the coleoptile pushes up through the soil Foliage leaves Coleoptile Coleoptile Figure 38.9b Two common types of seed germination Radicle (b) Maize