Seeds & Fruits

A SeedSeed is a mature ovule, which contains the embryo plant for the next generation.

A FruitFruit is a mature or ripened ovary. A fruit is the packaging structure around the seed.

Seeds

Seed- A matured ovule without accessory parts. Remember, the ovule surrounds the female gametophyte.

Onion Seeds

Remember from the flower chapter that double fertilization occurs.

A) The zygote (2n) becomes the embryo (2n).

B) The primary endosperm becomes the endosperm.

Remember that the endosperm is the nutrient rich storage tissue that will feed the seed when it germinates. Endosperm is common in monocots and remains in the seed as a food reserve. This food is absorbed through epidermal cells for food for the embryo. Endosperm is not common in dicots. In dicots, the endosperm’s energy is transferred to cotyledons (seed leaves) as the seed forms (matures).

CotyledonsCotyledon – seed leaf; there are two in the embryo/seedling of dicotyledonous plants (dicots) they generally store food and can expand and become photosynthetic; in monocotyledonous plants (monocots), only one cotyledon is present, generally a digestive organ.

Dicots have 2 seed leaves emerge from the embryo. Dicots include the eudicots, magnoliids, and a few basal angiosperms.

Monocots have 1 leaf emerge from the embryo. This is the coleoptile. Monocots, later, often have parallel venation on leaves, sheaths instead of petioles and usually no secondary growth. Examples: Lily, Corn, Onion, Grasses

Embryo & Seed Development in CottonThe zygote goes

through mitosis to form the

proembryo and suspensor

(supports embryo in endosperm).

The Cotyledons Form. These are seed leaves that store food. After

germination, cotyledons expand

and become photosynthetic.

In the seed, the radicle

(embryonic root) is at one end and the shoot

tip is at the other end.

The cotton seed has a

seed coat and contains the mature ovule

(mature embryo within).

The Embryo

Develops.

Onion Seed (A Monocot)

Seed Structures

Raphe – ridge on seeds, formed by the stalk of the ovule. Hilum – Large scar left when the seed breaks away from its placental connections (the seed’s belly button).Micropyle – This is where the pollen tube entered the ovule.

Notice that this dicot does not have endosperm, which is typical for the

majority of dicots.

Radicle – embryonic root (1st Root to emerge & first structure to emerge from seed as well).Epicotyl – after seed germination, this is the portion of the embryonic shoot that will be above cotyledons.Hypocotyl – after seed germination, portion of the embryonic shoot below the cotyledons but above the radicle.

Common Bean

Note: Seed structures do vary.

Castor Bean (A Dicot with Endosperm)

Caruncle – A spongy outgrowth of the seed coat that absorbs water, which is necessary for germination.

Yellow Foxtail Grass (A Monocot)

The seedseed of grasses is joined to its fruitfruit. This is called the grain or caryopsis.

Coleoptile – the first leaf in germination of monocots that sheaths the succeeding leaves; ensheathes the shoot apex.

Food is transferred from the endosperm to the scutellum to the growing embryo.

Germination

Germination is the beginning or resumption of growth of a seed, spore, bud, or other resting structure. Germination starts the metabolism of stored food (endosperm or cotyledons) in the seed.

1) The first step of germination is imbibition (the uptake of water). Seeds by

themselves are very dry (only 5-10% water).

2) After imbibition, the swelling of the radicle (embryonic root) starts first and

bursts the seed coat.

The Germination Process Differs Among Plants

Note: The Hypocotyl (below cotyledons)

elongates first.

Stages in the Epigeal

Germination of a Bean

Seed (Dicot).

Epigeal Germination – The hypocotyl raises the cotyledons and shoot apex towards the light. The cotyledons emerge from the

soil and become leaf-like photosynthetic structures.

Note: The Epicotyl (above cotyledons) elongates & hypocotyl (below cotyledons) remains below ground.

Stages in the Hypogeal Germination of a Pea Seed (Dicot).

Hypogeal Germination – The epicotyl straightens, the cotyledons remain below ground and only the apex and first leaf are raised upward.

Note: The first

leaves will start at the base of the cotyledon.

Stages in the Germination of an Onion Seed (Monocot).From a previous slide during this chapter, we saw an onion seed diagram. Onion

seeds have a single coiled cotyledon.

Stages in the Germination of a Corn Seed (Monocot).

After the primary root emerges, it branches to form the root system. Adventitious roots emerge from the lower stem and prop roots form to hold the stem upright. The

emerging young leaves are protected by a sheathing-like coleoptile.

Note: The Coleoptile is the emerging sheath that protects the shoots. The Coleorhiza is an emerging

sheath, which

protects the roots. The

primary root rapidly pushes

through the coleorhiza.

Germination May Be Delayed By DormancyDormancy - Growth and Development are temporarily suspended. During dormancy, seeds have reduced physiological activity.

Some seeds remain dormant and can later be viable for long periods of time (Some up to 100 years). The Oriental Lotus seeds have been known to last 1000 years in a dormant state and then be viable.

Different Items Can Break Dormancy

Some simply need just a little water. Lettuce needs light. Some legumes, locust for example, need heavy scaring of their seed coats. Some need moisture and freezing temperatures at correct levels and times. Certain pines will not germinate unless they have been subjected to the rather high heat of a fire.

BIO 141 Botany with Laboratory

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