Seed

Seed

• 1.
  Seed  A seedis a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food.  The seed performs the essential functions of:  REPRODUCTION. Normally, most flowering plants reproduce through seeds, except those that are vegetatively propagated.  CONTAINER OF EMBRYO. The seed is a container in which the embryo develops and reaches maturity.  STORAGE OF FOOD. The food material for the growing embryo is stored in the cotyledons or in the endosperm, especially in monocotyledonous plants.
• 2.
  Seed structure  Atypical seed includes three basic parts: (1) an embryo, (2) a supply of nutrients for the embryo, and (3) a seed coat.
• 3.
  Embryo  The embryois an immature plant from which a new plant will grow under proper conditions. The embryo has one cotyledon or seed leaf in monocotyledons, two cotyledons in almost all dicotyledons and two or more in gymnosperms. The radicle is the embryonic root. The plumule is the embryonic shoot. The embryonic stem above the point of attachment of the cotyledon(s) is the epicotyl. The embryonic stem below the point of attachment is the hypocotyl.
• 4.
  Seed production  Seedsare produced in several related groups of plants, and their manner of production distinguishes the angiosperms ("enclosed seeds") from the gymnosperms ("naked seeds"). Angiosperm seeds are produced in a hard or fleshy structure called a fruit that encloses the seeds, hence the name. (Some fruits have layers of both hard and fleshy material). In gymnosperms, no special structure develops to enclose the seeds, which begin their development "naked" .
• 5.
  Seed development The seed,which is an embryo with two points of growth (one of which forms the stems the other the roots) is enclosed in a seed coat with some food reserves.  The embryo is composed of different parts; The epicotyle will grow into the shoot. The radicle grows into the primary root. The hypocotyl connects the epicotyle and the radicle. The cotyledons form the seed leaves. The testa or seed coat forms the outer covering of the seed.
• 6.
  Seed size  Seedsare very diverse in size. The dust-like orchid seeds are the smallest with about one million seeds per gram.  Plants that produce smaller seeds can generate many more seeds per flower, while plants with larger seeds invest more resources into those seeds and normally produce fewer seeds.
• 7.
  Seed functions  Seedsserve several functions for the plants that produce them. Key among these functions are nourishment of the embryo, dispersal to a new location, and dormancy during unfavorable conditions.  Embryo nourishment Seeds protect and nourish the embryo or young plant. Seeds usually give a seedling a faster start than a sporeling from a spore, because of the larger food reserves in the seed.
• 8.
  Seed dispersal  Biologicaldispersal  By wind (anemo chory)  By water (hydrochory)  By animals (zoochory)
• 9.
  Seed dormancy  Seeddormancy is a condition of plant seeds that prevents germinating under optimal environmental conditions. Living, non dormant seeds germinate when soil temperatures and moisture conditions are suited for cellular processes and division; dormant seeds do not.
• 10.
   Physical dormancyor hard seed coats occurs when seeds are impermeable to water. At dormancy break a specialized structure, the ‘water gap’, is disrupted in response to environmental cues, especially temperature, so that water can enter the seed and germination can occur.  Mechanical dormancy Mechanical dormancy occurs when seed coats or other coverings are too hard to allow the embryo to expand during germination.
• 11.
   Chemical dormancy Includes growth regulators etc, that are present in the coverings around the embryo. They may be leached out of the tissues by washing or soaking the seed, or deactivated by other means. Other chemicals that prevent germination are washed out of the seeds by rainwater or snow melt.  Photodormancy or light sensitivity affects germination of some seeds. These photoblastic seeds need a period of darkness or light to germinate.  Thermodormancy is seed sensitivity to heat or cold.
• 12.
  Techniques to BreakDormancy  Seed Scarification -- Seed coat (external dormancy) results from a seed's hard seed coat that is impervious to water and gases. Any process of breaking, scratching, or mechanically altering the seed coat to make it permeable to water and gases is known as scarification.  Methods include: Physically break the hard seed coats or soften them by chemicals. Means of scarification include soaking in hot water or poking holes in the seed with a pin or rubbing them on sandpaper or cracking with a press or hammer. Soaking the seeds in solvents or acids is also effective for many seeds.  Sometimes fruits are harvested while the seeds are still immature and the seed coat is not fully developed and sown right away before the seed coat become impermeable.
• 13.
   Under naturalconditions seed coats are worn down by rodents chewing on the seed, the seeds rubbing against rocks (seeds are moved by the wind or water currents), by undergoing freezing and thawing of surface water, or passing through an animal's digestive tract  Microorganisms are often effective in breaking down hard seed coats and are sometimes used by people as a treatment, the seeds are stored in a moist warm sandy medium for several months under non-sterile conditions.
• 14.
   Seed StratificationThis dormancy prevents seed of many species from germinating when environmental conditions are not favorable for survival of the seedlings.  Moist chilling is a method to break down physiological dormancy and involves the addition of moisture to the seeds so they imbibe water and then the seeds are subject to a period of moist chilling to after-ripen the embryo.  Sowing outside in late summer and fall and allowing to overwinter outside under cool conditions is an effective way to stratify seeds, some seeds respond more favorably to periods of oscillating temperatures which are part of the natural environment.
• 15.
   Leaching orthe soaking in water removes chemical inhibitors in some seeds that prevent germination.  Rain and melting snow naturally accomplish this task. For seeds planted in gardens, running water is best - if soaked in a container, 12 to 24 hours of soaking is sufficient. Soaking longer, especially in stagnant water that is not changed, can result in oxygen starvation and seed death.  Seeds with hard seed coats can be soaked in hot water to break open the impermeable cell layers that prevent water intake.
• 16.
   Other methodsused to assist in the germination of seeds that have dormancy include prechilling, predrying, daily alternation of temperature, light exposure, potassium nitrate, the use of plant growth regulators like gibberellins, cytokinins, ethylene, thiourea, sodium hypochlorite plus others.[32]  Some seeds germinate best after a fire, for some seeds fire cracks hard seed coats while in other seeds chemical dormancy is broken in reaction to the presence of smoke, liquid smoke is often used by gardeners to assist in the germination of these species.[33]