Lecture 25-26. Topics

• Generalized structure – Where the seed comes from – Parts of the seed • Kinds of seeds – Dicots – Monocots


• The seed represents one of the most • When a pollen grain lands on the stigma of significant plant adaptations. the same species, it germinates, forming a pollen tube. • The Seed develops from the Ovule. • Passes between the stigma and style to reach the micropyle of the ovule. • Double fertilization occurs. – One sperm nucleus unites with the egg nucleus, producing a zygote. – Other unites with the polar nuclei, forming a 3n cell.

Parts of seed Structure of Seeds • Seeds are mature, fertilized • The Ovule's Integuments form the ovules. Seed Coat. • : Young sporophyte, • The ability to spread reproduction over a long time-span also diploid (2n), result of fertilization. contributes to long distance The mature embryo consists of transport of seeds which helps to (seed leaves), extend their range. (stem-like embryonic • In many cases this was aided by animal dispersal. This can be due axis below the cotyledons), to ingestion or by adherence to (embryonic ). the feathers of birds or other beasts. • Endosperm: Food storage • Some seeds float and can tissue, triploid (3n), result of withstand long exposure to salt fertilization. water. The coconut is a good example of this and it can spread • Testa (seed coat): Outer naturally over the ocean. protective layer of the seed, developed from the integuments of the ovule, maternal dipoid tissue.

1 Structure of Seeds • Seed Coat – Hilum: place where seed was attached to plant – Micropyle: small opening in the integuments

-The hilum is a mark where the bean was once attached to a plant. - Below it is the micropyle, the tube where the male nucleus first entered the ovule. Seed structure (pea) - micropyle: A small canal in the integument surrounding the ovule of a flowering plant, through which the pollen tube usually enters the ovule on the way to the embryo sac. Water enters the seed via the micropyle prior to . - The hypocotyl is the ’s root, while the is its stem.

Seed structure (pea) Seed structure (pea) - Inside the pod, the seeds are arranged in an alternate manner

2 Seed structure (pea) Seed structure (pea) When the seed is torn apart, two cotyledons are seen, together with the embryonic shoot (= plumule) and the embryonic root (= radicle).

Seed structure (pea) Seed structure (pea) - Nature is imperfect. Some ovules are not fertilized, - Nature is imperfect. Some ovules are not fertilized, and those that are may not get the same amount of and those that are may not get the same amount of nutrients. nutrients.

The micropyle and hilum This pod has three seeds.

3 What are p and r ?

The structure of a seed consists of: Kinds of seeds • Cotyledons - Food storage organs that • Dicot seeds – function as first seed endosperm is partially leaves. Dicot seed and seedling or completely • Plumule - Embryo shoot. absorbed by the • Epicotyl - Stem above embryo by cotyledons . • Monocot seeds – • Hypocotyl - Stem below endosperm is a attachment point. discrete, major structural seed unit • Radicle - Stem tip developing into a root.

Monocot Seed and Seedling

Kinds of seeds Mature Capsella Seed

• Dicot seeds – endosperm is partially or completely • Capsella is used as a absorbed by the embryo by model for Dicot Embryo cotyledons • Monocot seeds – endosperm development and helps to is a discrete, major structural seed unit show the dynamic relationship between the Embryo and the Endosperm.

4 Mature Capsella Seed Legume seeds

• The Seed consists of: • tremendous agronomic significance – Seed Coat (Testa) • Fabaceae is one of the largest and most – Endosperm –Embryo important families of flowering plants. • The Embryo consists of: • The Mature Seed has no visible – Root Apical Meristem (RAM) Endosperm. This is consumed by the – Hypocotyl Shoot Apical Cotyledons. –Meristem(SAM) – Two Cotyledons (Dicot)

Legume seeds: Honey Locust Legume seeds: Honey Locust

(Gleditisia) (Gleditisia) Complete Embryo with both Cotyledons visible as well as the Hypocotyl. The Hypocotyl is the region of the stem BELOW the Cotyledons.

Immature Seed External View Embryo with one Cotyledon Removed: The Epicotyl with Immature Gleditisia Fruit Immature Gleditisia Fruit macroscopic Leaves is visible. The SAM is located at the base of the leaf primordia. The Cotyledon Scar is visible. This marks Immature Seed Split Open: A Dissected Seed: Note the the Cotyledonary Node! H = Cotyledon and Root Apex are presence of a semitransparent Hypocotyl. readily visible. Endosperm plus the Embryo.

Bean (Phaseolus sp.) Bean (Phaseolus sp.)

Longitudinal Section through a Bean Diagram of a typical Legume Seed: Cross-Section of a Bean Seed (Phaseolus) Seed: Note the Funiculus (F) The Plumule represents the "Bud" through the middle of the which attaches the Ovule (a.k.a. the Seed) to or Shoot Tip of the Embryo. The Cross-Section of a Bean Seed above the Embryonic Axis which lies close to the Placenta. The Cotyledons fill most of the Epicotyl includes the Plumule and level of the Plumule: The Cotyledons are the the Funiculus. The Axis consists of seed's volume. Fragments of the Endosperm any other stem tissue above the only visible Structures. everything from the SAM to the may occur between the Cotyledons and the Hypocotyl. RAM Testa.

5 Bean (Phaseolus sp.) Bean (Phaseolus sp.)

High Magnification View of the Funiculus: The Embryonic Axis is composed Note the presence of the Tracheid Bar (T) of a central Vascular Cylinder near the opening (Hilum) in the Testa. Note (VC) which is Diarch. This In some cases like the Aerenchyma which surrounds the suggests that it is the region of the (Glycine max) the Cotyledons tracheids. These probably assist in the Root. The Vascular Cylinder is Cotyledons may contain large quantities of contain large amounts of Protein in uptake of air and water. There are two surrounded by Ground Tissue (G) Starch which has stained brown with IKI. Protein Vacuoles (Blue). This is the Palisade (P) layers composed of and a unicellular Epidermis (E). One Phaeolus Cotyledons contain a lot of starch. reason why Soybean is such an Macrosclereids. One of these develops layer of Macrosclereids is clearly important crop. from the Funiculus. visible in the Testa (T)