The Angiosperm Seed Develops from the ovule as a result of double fertilisation. Contains the embryo and stored nutrients, and is protected by the seed coat (testa). The seed coat is derived from the integuments. Endosperm: derived from the fused polar and sperm nuclei in the central cell of the embryo sac, stores food external to the embryo. Perisperm: sporophyte nucellar tissue, stores food external to the embryo. In many dicots, the endosperm and perisperm are transient and are absorbed by the developing embryo before the seed becomes dormant. The mature seed then stores its food in the cotyledons which become fleshy. The micropyle may remain as an occluded pore or disappear. The funiculus abscises from the ovule, leaving a scar (hilum). In anatropous ovules part of the funiculus remains as a longitudinal ridge, the raphe. Aril: a funiculus outgrowth. Caruncle: an integumentary protuberance near the micropyle. Elaiosome: an oily appendage used as food by ants. Seed Development Double fertilisation initialises seed development. The ovule and endosperm grow and the embryo sac enlarges to accommodate the endosperm. Once the endosperm reaches maximum volume, the embryo starts to grow rapidly, initially by cell division and then by cell enlargement. In the pea (Pisum sativum), the embryo almost fills the embryo sac as it grows at the expense of the endoderm. (Organic C is tranferred from the endosperm to the embryo). Finally the seed becomes dormant (at this stage in the pea the suspensor disintegrates). The growing embryo sac is a sink, absorbing water and soluble materials from the surrounding ovular tissues (which are digested) which are supplied by the vessels in the funiculus. In the pea, the growth of pod and seeds is under the control of gibberellins, auxin and abscisic acid. Hormones mobilised synthesised by the growing ovules, following fertilisation, stimulate growth of the pod. Albuminous seeds: store food in the endosperm (monocotyledons) or perisperm (Amaranthaceae, Chenopodiaceae, Polygonaceae). Exalbuminous seeds: food is stored in the embryo; no endosperm or perisperm when mature. Most seeds show a combination of storage tissue types. Mature Embryo Dicotyledons: the two cotyledons arise as lateral organs at the apex of the embryo axis and are in the same position, relative to the meristem, as foliage leaves. Monocotyledons: one cotyledon, displaces apical meristem to a lateral position. Embryo: upright, bent or curved. Breaking Seed Dormancy Seed coat (testa) STARCH Endosperm SUGAR Cotyledon (scutellum) Aleurone layer amylase Storage protein Epicotyl RNA Amino acids growth GA3 DNA 660 nm Pfr Pr Radicle 730 nm H2O Dark Imbibition The diagram above shows the breaking of seed dormancy (in a generalised grass/cereal seed). All seeds must absorb (imbibe) water to become metabolically active, but not all require a red light stimulus to stimulate phytochrome, e.g. lettuce seeds require both the red light stimulus and gibberellic acid activation by imbibition, whereas barley seeds require only gibberellic acid activation by imbibition. Q.1 At what two stages on the above diagram is hydrolysis occurring? Q.2 What is the function of the scutellum in grass seeds? Q.3 What is the advantage of requiring a red light stimulus to break dormancy in certain plants? Some plants require a cold period to break seed dormancy, and others require damage to the seed coat to allow water to enter. In the walnut (Juglans regia) the nut has to decompose on the forest floor for two years, before its hard seed coat is weakened enough to allow water in and the germinating embryo out! Endosperm Formation Endosperm is characteristic of angiosperm seeds. There are three modes of endosperm formation: 1. Nuclear: many nuclei are formed by nuclear divisions whilst the endosperm remains noncellular or develops cell walls later. The centre of the cell is occupied by a large vacuole and the nuclei are parietal. In Capsella, endosperm nuclei accumulate at the two ends of the embryo sac and the endosperm at the chalazal end digests nucellar cells in front of the advancing embryo sac. The nucellar cells hypertrophy before digestion and become rich in protein and nucleic acids. Their digested remains are seen inside vesicles in large vacuoles in the endosperm cells. In Eranthis hiemalis (Ranunculaceae) the endosperm digests adjacent integumentary cells. Integumentary cell nuclei migrate into the endosperm and are broken down. This reduces the integumentary cell layers from 11 to 7 by the time the endodermis becomes cellular. The cell walls form from phragmoplasts or by wall ingrowth. In Pisum, an endosperm cell wall and middle lamella joins the endosperm from the outer walls of the embryo. The outermost layer becomes the aleurone layer and endosperm cells eventually take up the central vacuolar space. In Cocos nucifera, the central cavity does not fill with cells, but contains coconut milk. 2. Cellular: cell wall formation begins with the first mitosis and continues until the endosperm stops growing. 3. Helobial: the embryo sac is divided into two unequal cells, the larger chalazal cell usually develops noncellularly whilst the smaller micropylar cell develops in either manner. Occurs mainly in the monocotyledons. Food stores contain mainly carbohydrate, protein and lipid. Cereals: 70-80% dry weight is starch, peas and beans 50% starch. Zea contains mostly starch in the endosperm, but the embryo is 50% oil. Rape and mustard (Brassicaceae) are 40% oil and 30% protein; soya (Fabaceae) seeds are 20% oil and 40% protein. The starchy endosperm may be living, with starch grains inside amyloplasts, or nonliving with free starch grains. Carbohydrates may also be stored in thickened cell walls (of endosperm or cotyledons) composed mostly of hemicelluloses. Protein is stored in granules enclosed in membrane derived from the tonoplast and may consist of globulins. Oil is stored as triglyceride in cytoplasmic granules, which may be bound by a unit membrane or a phospholipid monolayer (?). Seed Coat The seed coat is dry in most angiosperms, but may have fleshy appendages, e.g. elaiosomes, or juicy layers such as the fleshy epidermis of pomegranate (Punica) seeds. Many gymnosperms have fleshy seed coats. Cuticular layers and phenolic compounds restrict water entry into the seed. The seed coat and associated coverings (pericarp, other floret parts in grasses) may contain germination inhibitors and only when these are removed will the seed germinate. Seeds that rely on animal ingestion to disperse them, have testa that are resitant to digestive processes. The epidermis of some seeds secretes mucilage and this may cause them to stick to animals and be dispersed, or may prevent desiccation or prevent germination in excessive moisture by swelling and impeding oxygen diffusion into the seed.
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