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Seed dormancy 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.
One important function of most seeds is delayed germination, which allows time for dispersal and prevents germination of all the seeds at same time. The staggering of germination safeguards some seeds and seedlings from suffering damage or death from short periods of bad weather or from transient herbivores; it also allows some seeds to germinate when competition from other plants for light and water might be less intense. Another form of delayed seed germination is seed quiescence, which is different than true seed dormancy and occurs when a seed fails to germinate because the external environmental conditions are too dry or warm or cold for germination. Many species of plants have seeds that delay germination for many months or years, and some seeds can remain in the soil seed bank for more than 50 years before germination. Some seeds have a very long viability period, and the oldest documented germinating seed was nearly 2000 years old based on radiocarbon dating.
True dormancy or innate dormancy is caused by conditions within the seed that prevent germination under normally ideal conditions. Often seed dormancy is divided into two major categories based on what part of the seed produces dormancy: exogenous and endogenous. There are three types of dormancy based on their mode of action: physical, physiological and morphological.
There have been a number of classification schemes developed to group different dormant seeds, but none have gained universal usage. Dormancy occurs because of a wide range of reasons that often overlap, producing conditions in which definitive categorization is not clear. Compounding this problem is that the same seed that is dormant for one reason at a given point may be dormant because of another reason at a later point. Some seeds fluctuate from periods of dormancy to non dormancy, and despite the fact that a dormant seed appears to be static or inert, in reality they are still receiving and responding to environmental cues.
Exogenous dormancy is caused by conditions outside the embryo and is often broken down into three subgroups:
Which occurs when seeds are impermeable to water or the exchange of gases. Legumes are typical examples of physically dormant seeds; they have low moisture content and are prevented from imbibing water by the seed coat. Chipping or cracking of the seed coat or any other coverings allows water intake. Impermeability is often caused by an outer cell layer which is composed of macrosclereid cells or the outer layer is composed of a mucilaginous cell layer. The third cause of seed coat impermeability is a hardened endocarp. Seed coats that are impermeable to water and gases form during the last stages of seed development.
Mechanical dormancy occurs when seed coats or other coverings are too hard to allow the embryo to expand during germination. In the past this mechanism of dormancy was ascribed to a number of species that have been found to have endogenous factors for their dormancy instead. These endogenous facts include physiologically dormancy cased by low embryo growth potential
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.
Endogenous dormancy is caused by conditions within the embryo itself, and it is also often broken down into three subgroups: physiological dormancy, morphological dormancy and combined dormancy, each of these groups may also have subgroups.
Physiological dormancy prevents embryo growth and seed germination until chemical changes occur. These chemicals include inhibitors that often retard embryo growth to the point where it is not strong enough to break through the seed coat or other tissues. Physiological dormancy is indicated when an increase in germination rate occurs after an application of gibberellic acid (GA3) or after Dry after-ripening or dry storage. It is also indicated when dormant seed embryos are excised and produce healthy seedlings: or when up to 3 months of cold (0-10Â°C) or warm (=15Â°C) stratification increases germination: or when dry after-ripening shortens the cold stratification period required. In some seeds physiological dormancy is indicated when scarification increases germination.
Physiological dormancy is broken when inhibiting chemicals are broken down or are no longer produced by the seed; often by a period of cool moist conditions, normally below (+4C) 39F, or in the case of many species in Ranunculaceaeand a few others,(-5C) 24F.Abscisic acid is usually the growth inhibitor in seeds and its production can be affected by light. Some plants like Peony species have multiple types of physiological dormancy, one affects radicle (root) growth while the other affects plumule (shoot) growth. Seeds with physiological dormancy most often do not germinate even after the seed coat or other structures that interfere with embryo growth are removed. Conditions that affect physiological dormancy of seeds include:
- Drying; some plants including a number of grasses and those from seasonally arid regions need a period of drying before they will germinate, the seeds are released but need to have a lower moister content before germination can begin. If the seeds remain moist after dispersal, germination can be delayed for many months or even years. Many herbaceous plants from temperate climate zones have physiological dormancy that disappears with drying of the seeds. Other species will germinate after dispersal only under very narrow temperature ranges, but as the seeds dry they are able to germinate over a wider temperature range.
- Photodormancy or light sensitivity affects germination of some seeds. These photoblastic seeds need a period of darkness or light to germinate. In species with thin seed coats, light may be able to penetrate into the dormant embryo. The presence of light or the absence of light may trigger the germination process, inhibiting germination in some seeds buried too deeply or in others not buried in the soil.
- Thermodormancy is seed sensitivity to heat or cold. Some seeds including cocklebur and amaranth germinate only at high temperatures (30C or 86F) many plants that have seed that germinate in early to mid summer have thermodormancy and germinate only when the soil temperature is warm. Other seeds need cool soils to germinate, while others like celery are inhibited when soil temperatures are too warm. O
A cotyledon (ËŒkoÊŠtÉ›lÉªdÉ”Ë�n; "seed leaf" from Greek: ÎºÎ¿Ï„Ï…Î»Î·Î´ÏŽÎ½ kotylÄ“dÅ�n, gen.: ÎºÎ¿Ï„Ï…Î»Î·Î´ÏŒÎ½Î¿Ï‚ kotylÄ“donos, from ÎºÎ¿Ï„Ï�Î»Î· kotÃ½lÄ“ "cup, bowl"), is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants (angiosperms). Species with one cotyledon are called monocotyledonous (or, "monocots") and placed in the class Liliopsida. Plants with two embryonic leaves are termed dicotyledonous ("dicots") and placed in the class Magnoliopsida.
In the case of dicot seedlings whose cotyledons are photosynthetic, the cotyledons are functionally similar to leaves. However, true leaves and cotyledons are developmentally distinct. Cotyledons are formed during embryogenesis, along with the root and shoot meristems, and are therefore present in the seed prior to germination. True leaves, however, are formed post-embryonically (i.e. after germination) from the shoot apical meristem, which is responsible for generating subsequent aerial portions of the plant.
The cotyledon of grasses and many other monocotyledons is a highly modified leaf composed of a scutellumand acoleoptile. The scutellum is a tissue within the seed that is specialized to absorb stored food from the adjacentendosperm. The coleoptile is a protective cap that covers the plumule (precursor to the stem and leaves of the plant).
Gymnosperm seedlings also have cotyledons, and these are often variable in number (multicotyledonous), with from 2 to 24 cotyledons forming a whorl at the top of the hypocotyl (the embryonic stem) surrounding the plumule. Within each species, there is often still some variation in cotyledon numbers, e.g. Monterey Pine (Pinus radiata) seedlings have 5â€“9, and Jeffrey Pine (Pinus jeffreyi) 7â€“13 (Mirov 1967), but other species are more fixed, with e.g. Mediterranean Cypress always having just two cotyledons. The highest number reported is for Big-cone Pinyon (Pinus maximartinezii), with 24 (Farjon & Styles 1997).
The cotyledons may be ephemeral, lasting only days after emergence, or persistent, enduring a year or more on the plant. The cotyledons contain (or in the case of gymnosperms and monocotyledons, have access to) the stored food reserves of the seed. As these reserves are used up, the cotyledons may turn green and begin photosynthesis, or may wither as the first true leaves take over food production for the seedling.
Epigeal versus hypogeal development
Cotyledons may be either epigeal, expanding on the germination of the seed, throwing off the seed shell, rising above the ground, and perhaps becoming photosynthetic; or hypogeal, not expanding, remaining below ground and not becoming photosynthetic. The latter is typically the case where the cotyledons act as a storage organ, as in many nuts and acorns.
Hypogeal plants have (on average) significantly larger seeds than epigeal ones. They also are capable of surviving if the seedling is clipped off, as meristem buds remain underground (with epigeal plants, the meristem is clipped off if the seedling is grazed). The tradeoff is whether the plant should produce a large number of small seeds, or a smaller number of seeds which are more likely to survive.
Related plants show a mixture of hypogeal and epigeal development, even within the same plant family. Groups which contain both hypogeal and epigeal species include, for example, the Araucariaceae family of Southern Hemisphere conifers, the Fabaceae (pea family), and the genus Lilium(seeLily seed germination types).
The term cotyledon was coined by Marcello Malpighi. John Ray was the first botanist to recognise that some plants have two and others only one, and eventually the first to recognise the immense importance of this fact to systematics.
The embryo, contained within the seed, is the next generation of plant. Thus successful seed germination is vital for a species to perpetuate itself. By definition, germination commences when the dry seed, shed from its parent plant, takes up water (imbibition), and is completed when the embryonic root visibly emerges through the outer structures of the seed (usually the seed or fruit coat). Thereafter, there is seedling establishment, utilizing reserves stored within the seed, followed by vegetative and reproductive growth of the plant, supported by photosynthesis. The seed is metabolically inactive (quiescent) in the mature, dry state and can withstand extremes of drought and cold. For example, dry seeds can be stored over liquid nitrogen at -150 degrees Celsius (-238 degrees Fahrenheit) for many years without harm. Upon hydration of a seed, metabolism commences as water enters its cells, using enzymes and structural components present when the seed was dry. Respiration to provide energy has been observed within minutes of water uptake. Mitochondria that were stored in the dry seed are involved, although initially they are somewhat inefficient because of damage sustained during drying and rehydration. During germination they are repaired and also new organelles are synthesized. Protein synthesis also commences rapidly in the imbibing seed. Early during germination, stored messenger ribonucleic acids (mRNAs) are used as templates for protein synthesis, but later in germination these are replaced with newly transcribed messages, some of which code for a different set of proteins. Although the pattern of seed protein synthesis changes during germination, no proteins have been identified as being essential for this event to be completed. Elongation of cells of the radicle (embryonic root) is responsible for its emergence from the seed. This is a turgor -driven process and is achieved through increased elasticity of the radicle cell walls, by a process which is not known. Cell division and deoxyribonucleic acid (DNA) replication occur after germination, as the radicle grows, and reserves of protein, carbohydrate, and oil, stored in the dry seed, are used to support seedling growth. Mature seeds of some species are incapable of germinating, even under ideal conditions of temperature and hydration, unless they receive certain environmental stimuli; such seeds are dormant. Breaking of this dormancy may be achieved in several ways, depending upon the species. Frequently, dormancy is lost from seeds as they are stored in the dry state for several weeks to years, a phenomenon called dry after-ripening. But many seeds remain dormant in a fully imbibed state; they are as metabolically active as nondormant seeds, but yet fail to complete germination. Dormancy of these seeds may be broken by one or more of the following: (1) light, sunlight being the most effective; (2) low temperatures (1 to 5 degrees Celsius [33.8 to 41 degrees Fahrenheit]) for several weeks; (3) day/night fluctuating temperatures of 1 to 10 degrees Celsius (41 to 50 degrees Fahrenheit); (4) chemicals, such as nitrate in the soil, or applied hormones (gibberellins) in the laboratory; and (5) fire. Dormancy mechanism operate to control the germination of seeds in their natural environment and to optimize the conditions under which the resultant seedling can become established. Dormant seeds that require light will not germinate unless they are close to the soil surface; hence germinated seeds will not expend their stored reserves before they can reach the surface and become photosynthetically independent seedlings. This is particularly important for small, wind-dispersed weed seeds. The light-perception mechanism in light-requiring seeds involves a receptor protein, phytochrome, which is activated by red wavelengths of light and inactivated by far-red (near-infrared). Far-red light from sunlight penetrates farther into soil than does red, but also light penetrating through a leaf canopy is richer in farred than red, since the latter is absorbed by photosynthetic pigments in the leaf. Hence, germination of light-sensitive seeds is advantageously inhibited under a leaf canopy and helps explain why germination and subsequent plant growth is so profuse in forest clearings. Seeds that need a period of low temperature cannot germinate immediately after dispersal in the summer or early autumn but will do so after being subjected to the cold of winter, conditions that may cause the parent plant to die, and thus remove competition for space in the spring. The requirement for alternating temperatures will prevent germination of seeds beneath dense vegetation because the latter dampens the day/night temperature fluctuations; these seeds will germinate only when there is little vegetation cover, again reducing competition with established plants. Seed dormancy is also important in relation to agricultural and horticultural crops. Its presence causes delayed and sporadic germination, which is undesirable. On the other hand, the absence of dormancy from cereals, for example, can result in germination of the seed on the ear, causing spoilage of the crop. Thus having mild dormancy to prevent this, which is lost during storage of the seed (dry after-ripening), is desirable. see also Fire Ecology; Reproduction in Plants; Seeds J. Derek Bewley Bewley, J. Derek. "Seed Germination and Dormancy." Plant Cell 9 (1997): 1055â€“1066. â€”â€”, and Michael Black. Seeds: Physiology of Development and Germination, 2nd ed. New York: Plenum Press, 1994.
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Answers:Grass is the classic example of a monocotyledon plant. The first shoot sent up on germination is a single leaf. A pea is a good example of a dicotyledon. Cotyledon is the first leaf that appears on germination. 'Mono' means one, 'di' means two. The traditional differences between monocots and dicots are: Flowers: In monocots, flowers are trimerous (number of flower parts in a whorl in threes) while in dicots the flowers are tetramerous or pentamerous (flower parts are in fours or fives). Pollen: In monocots, pollen has one furrow or pore while dicots have three. Seeds: In monocots, the embryo has one cotyledon while the embryo of the dicot has two. Stems: In monocots, vascular bundles in the stem are scattered, in dicots arranged in a ring. Roots: In monocots, roots are adventitious, while in dicots they develop from the radicle. slice of onion, showing parallel veins in cross section slice of onion, showing parallel veins in cross section Leaves: In monocots, the major leaf veins are parallel, while in dicots they are reticulate. Not all of these, though, are necessarily definitive. The leaves of most pine trees (which are multicotyledinous) have parallel veins, for example. There is a good picture of a monocot and a dicot seedling side by side here: http://en.wikipedia.org/wiki/Cotyledon
Answers:1) fruit (although this depends on your definition of fruit) 2) large gametophyte 3) white pine 4) flower are to angiosperms 5) wind and the pines 6) number of cotyledons 7) Seeds = 2 Cones = 1 Fruits = 3 Flowers = 4 Hope this helps! :)
Answers:There's a very easy way to tell whether a plant is dicot or monocot. Look at the veins in the leaf. If they branch out like a tree (dendritic pattern) they are dicots. If the veins are parallel, like the veins in a blade of grass or a corn leaf, it's a monocot. The first link is a pic of an oak leaf, which is a dicot. The second link is a pic of the parallel veins of a corn leaf. Also, dicot flowers tend to have flower parts in multiples of four or five (petals, stamens, sepals). Monocot flowers tend to have flower parts in multiples of threes.