Seed and seeding

 

I.             Generalized seed structure

                A.            Where the seed comes from

                                1.             Double fertilization

                                                A.            Fusion of egg and sperm (zygote)

                                                B.            Fusion of two polar nuclei with second sperm (endosperm)

                B.            Parts of the seed

                                1.             Embryo (from zygote)

                                2.             Endosperm (may or may not be present in mature seed)

                                3.             Seed coat (from integuments)

II.            Kinds of seeds

                A.            Dicots - endosperm is partially or completely absorbed by the embryo by cotyledons

                                1.             Seed coat (external structure)

                                                A.            Hilum - place where seed was attached to plant

                                                B.            Micropyle - small opening in the integuments

                                2.             Embryo

                                                A.            Embryo axis

                                                                1.             Plumule (shoot apex, first foliage leaves, and epicotyl)

                                                                2.             Epicotyl (portion beneath leaves)

                                                                3.             Hypocotyl (portion beneath cotyledonary node) - think of it as the root/shoot junction

                                                B.            Cotyledons

                                                C.            Radicle (primary root)

                                3.             Stored food and mineral reserves (in cotyledons and other tissues)

                                4.             Enzymes and hormones (found everywhere)

                B.            Monocots - endosperm is a discrete, major structural seed unit

                                1.             Seed coat (external structure)

                                                A.            Pericarp - fruit wall developed from the ovary wall

                                                B.            Aleurone layer - protein-rich layer which encases the endosperm

                                2.             Embryo

                                                A.            Embryo axis

                                                                1.             Plumule (shoot apex, first foliage leaves, and epicotyl)

                                                                2.             Coleoptile - protective sheath around shoot apex

                                                                3.             Epicotyl and hypocotyl (similar to dicots)

                                                B.            Scutellum - one cotyledon

                                                C.            Radicle (primary root)

                                                                1.             Coleorhiza - protective sheath around root apex

                                3.             Stored food and mineral reserves (in starchy endosperm, cotyledons, and other tissues)

                                4.             Enzymes and hormones (found everywhere)

III.          Germination

                A.            Process imbibition and absorption of water, hydration of tissues, absorption of oxygen, activation of enzymes and digestion, transport of nutrients to embryo axis, increase in respiration, cell division & growth, and embryo emergence

                B.            Hormones - gibberellins (activate digestion), cytokinins (stimulate cell division), and auxins (cell enlargement)

                C.            Epigeal emergence (bean) - elongation of the hypocotyl (cotyledons push above soil surface)

                D.            Hypogeal emergence (grass) - elongation of the epicotyl (cotyledons remain below soil surface)