SEEDS, GARMINATION AND TROPISMS

 

A seed is an embryonic plant enclosed in a protective outer covering. Defin By Wikipedia

SEED STRUCTURE

A seed develops from an ovule after fertilization. It consists of a tough coat or testa enclosıng an embryo which is made up of a plumule, a radicle and one or two cotyledons. In favourable Conditions the seed can grow and become a fully independent plant, bearing flowers and seeds during its life cycle. In the embryo of the seed are all the potentialities of development and growth to a mature plant resembling other members of its Species in almost every detail of leaf shape, cell distribution and flower colour and structure.

  The testa. The integuments round the ovule form the testa, a tough, hard coat which protects the seed from fungi, bacteria and insects. It has to be split open by the radicle before germination can proceed.

   The hilum is a scar left by the stalk which attached the ovule to the ovary wall.

   The micropyle is the opening in the integuments through which the pollen tube entered at fertilization. It remains as a tiny pore in the testa opposite the tip of the radicle and admits water to the embryo before germination.

   The radicle is the embryonic root which grows and develops into the root system of the plant.

   The plumule is the leafy part of the embryonic shoot. These leaves are attached to the embryonic stem, of which the part above the attachment of the cotyledons is called the epicotyl and the part below, the hypocotyl.

Cotyledons. Monocotyledons such as grasses and cereals have seeds with only one cotyledon. The other flowering plants, the dicotyledons, have two cotyledons in their seeds. These cotyledons are modified leaves attached to the epicotyl and hypocotyl by short stalks and they often contain food reserves which are used in the early stages of germination. In most dicotyledonous plants the cotyledons are brought out of the testa and above the ground where they become green and make food by photosynthesis. The cotyledons eventually fall off, usually after the first foliage leaves have been formed. The cotyledon leaves bear no resemblance to the ordinary foliage leaf, the shape of which is first apparent when the plumule leaves open and grow.


THE COURSE OF GERMINATION

Vigna unguiculata: black-eye bean or cowpea. The seed absorbs water and swells. After two to three days, the radicle grows and bursts through the testa. It grows down be- tween the soil particles, its tip protected by a root cap.

Root hairs appear in the region where elongation has ceased.

Water and salts from the soil are absorbed by the root hairs on the radicle and pass to the rest of the seedling. Later, lateral roots develop from the radicle. Above these, and continuous with the radicle, is a part of the stem called the hypocotyl (regions of stem below cotyledons). The rapid elongation of this hypocotyl results in its arching up because the lower part of it where it joins the radicle is firmly anchored in the soil. The arched hypocotyl draws the cotyledons from the testa and they are dragged backwards out of the soil. The plumule is between them, and in this position is well protected from damage by the soil particles. Sometimes the testa, still partly enclosing the cotyledons, is brought above the soil and is pushed off later as the cotyledons separate.


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   Once the cotyledons are above soil level the hypocotyl straightens and the cotyledons having become green, spread out and begin to manufacture food. The plumule between them grows and produces the first foliage leaves. The cotyledons shrivel and drop off leaving a scar on the stem. Germination which results in the cotyledons being brought above ground is called epigeal. The later leaves that develop are trifoliate (divided into three).

   In the early stages of germination, the food reserves in the cotyledons, mostly starch and protein, have been acted upon by enzymes and converted to soluble products which pass to, and are used by, the actively growing regions where new cells and new protoplasm are being made, and energy for these processes is being released. Glucose is formed from the stored starch, being utilized in various ways. Some is built up into cellulose and incorporated into new cell walls, and part is Oxidized by respiration releasing energy which is used in the many chemical activities taking place in the growing regions.

The conversion of starch to glucose (a sugar) also results in a decrease of osmotic potential, which may assist the seedlings to take in water and their newly formed cells to extend during growth. When the first foliage leaves are above soil and their chlorophyll properly developed, the seedling can make its own food and is independent of the cotyledons.

Maize. The maize grain is really a fruit containing one seed; the thin ovary wall is fused to the testa. The fruit absorbs water, swells, and a radicle bursts through the coleorhiza and fruit wall. Root hairS grow on the upper regions of the radicle. The plumule groWs straight up and through the fruit wall, but the growing point and first leaves are protected by a sheath, the coleoptile, with a hard, pointed tip.

From the base of the plumule grow adventitious roots. Once above the soil, the first leaves burst out of the coleoptile which remains as a sheath round the leaf bases. The cotyledon remains below the soil, absorbing food from the endosperm and transmitting it to the growing root and shoot. Eventually, both the cotyledon and the exhausted endosperm rot away.

   The germmination of a seedling such as maize, which leaves its cotyledons below the ground, is called hypogeal.

SEEDS, GARMINATION AND TROPISMS SEEDS, GARMINATION AND TROPISMS Reviewed by Admin on January 17, 2022 Rating: 5

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