Seed structure
The parts of an avocado seed (a dicot), showing the seed coat, endosperm, and embryo.
A typical seed includes three basic parts: (1) an embryo, (2) a supply of nutrients for the embryo, and (3) a seed coat.
The embryo is 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.
Within the seed, there usually is a store of nutrients for the seedling that will grow from the embryo. The form of the stored nutrition varies depending on the kind of plant. In angiosperms, the stored food begins as a tissue called the endosperm, which is derived from the parent plant via double fertilization. The usually triploid endosperm is rich in oil or starch and protein. In gymnosperms, such as conifers, the food storage tissue is part of the female gametophyte, a haploid tissue. In some species, the embryo is embedded in the endosperm or female gametophyte, which the seedling will use upon germination. In others, the endosperm is absorbed by the embryo as the latter grows within the developing seed, and the cotyledons of the embryo become filled with this stored food. At maturity, seeds of these species have no endosperm and are termed exalbuminous seeds. Some exalbuminous seeds are bean, pea, oak, walnut, squash, sunflower, and radish. Seeds with an endosperm at maturity are termed albuminous seeds. Most monocots (e.g. grasses and palms) and many dicots (e.g. brazil nut and castor bean) have albuminous seeds. All gymnosperm seeds are albuminous.
The seed coat (or testa) develops from the tissue, the integument, originally surrounding the ovule. The seed coat in the mature seed can be a paper-thin layer (e.g. peanut) or something more substantial (e.g. thick and hard in honey locust and coconut). The seed coat helps protect the embryo from mechanical injury and from drying out.
In addition to the three basic seed parts, some seeds have an appendage on the seed coat such an aril (as in yew and nutmeg) or an elaiosome (as in Corydalis) or hairs (as in cotton). There may also be a scar on the seed coat, called the hilum; it is where the seed was attached to the ovary wall by the funiculus.
Seed production
Immature Elm seeds.
Seeds are 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" on the bracts of cones. However, the seeds do become covered by the cone scales as they develop in some species of conifer.
Kinds of seeds
There are a number of modifications to seeds by different groups of plants. One example is that of the so-called stone fruits (such as the peach), where a hardened fruit layer ( the endocarp) surrounds the actual seed and is fused to it.
Many structures commonly referred to as "seeds" are actually dry fruits. Sunflower seeds are sold commercially while still enclosed within the hard wall of the fruit, which must be split open to reach the seed.
Seed development
The inside of a Ginkgo seed, showing a well-developed embryo, nutritive tissue (megagametophyte), and a bit of the surrounding seed coat.
Diagram of the internal structure of a dicot seed and embryo. (a) seed coat, (b) endosperm, (c) cotyledon, (d) hypocotyl.
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. Angiosperm seeds consist of three genetically distinct constituents: (1) the embryo formed from the zygote, (2) the endosperm, which is normally triploid, (3) the seed coat from tissue derived from the maternal tissue of the ovule. In angiosperms, the process of seed development begins with double fertilization and involves the fusion of the egg and sperm nuclei into a zygote. The second part of this process is the fusion of the polar nuclei with a second sperm cell nucleus, thus forming a primary endosperm. Right after fertilization the zygote is mostly inactive but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food that the young plant will consume until the roots have developed after germination or it develops into a hard seed coat. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant, the inner integument forms the tegmen and the outer forms the testa. When the seed coat forms from only one layer it is also called the testa, though not all such testa are homologous from one species to the next.
In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization but one sperm nucleus unites with the egg nucleus and the other sperm is not used.[1] Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development.[2] The seed is composed of the embryo (the result of fertilization) and tissue from the mother plant, which also form a cone around the seed in coniferous plants like Pine and Spruce.
The ovules after fertilization develop into the seeds; the main parts of the ovule are the funicle; which attaches the ovule to the placenta, the nucellus; the main region of the ovule were the embryo sac develops, the micropyle; A small pore or opening in the ovule where the pollen tube usually enters during the process of fertilization, and the chalaza; the base of the ovule opposite the micropyle, where integument and nucellus are joined together.[3]
The shape of the ovules as they develop often affects the finale shape of the seeds. Plants generally produce ovules of four shapes: the most common shape is called anatropous, with a curved shape. Orthotropous ovules are straight with all the parts of the ovule lined up in a long row producing an uncurved seed. Campylotropous ovules have a curved embryo sac often giving the seed a tight “c” shape. The last ovule shape is called amphitropous, where the ovule is partly inverted and turned back 90 degrees on its stalk or funicle.
In the majority of flowering plants, the zygote's first division is transversely oriented in regards to the long axis, and this establishes the polarity of the embryo. The upper or chalazal pole becomes the main area of growth of the embryo, while the lower or micropylar pole produces the stalk-like suspensor that attaches to the micropyle. The suspensor absorbs and manufacturers nutrients from the endosperm that are utilized during the embryos growth.[4]
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. Monocotyledonous plants like corn, have other structures; instead of the hypocotyle-epicotyle, it has a coleoptile that forms the first leaf and connects to the coleorhiza that connects to the primary root and adventitious roots form from the sides. The seeds of corn are constructed with these structures; pericarp, scutellum (single large cotyledon) that absorbs nutrients from the endosperm, endosperm, plumule, radicle, coleoptile and coleorhiza - these last two structures are sheath-like and enclose the plumule and radicle, acting as a protective covering. The testa or seed coats of both monocots and dicots are often marked with patterns and textured markings, or have wings or tufts of hair.
Seed size and seed set
Seeds are very diverse in size. The dust-like orchid seeds are the smallest with about one million seeds per gram, they are often embryonic seeds with immature embryos and no significant energy reserves. Orchids and a few other groups of plants are myco-heterotrophs which depend on mycorrhizal fungi for nutrition during germination and the early growth of the seedling. Some terrestrial Orchid seedlings, in fact, spend the first few years of their life deriving energy from the fungus and do not produce green leaves.[5] At over 20 kg, the largest seed is the coco de mer. 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. Small seeds are quicker to ripen and can be dispersed sooner, so fall blooming plants often have small seeds. Many annual plants produce great quantities of smaller seeds; this helps to ensure that at least a few will end in a favorable place for growth. Herbaceous perennials and woody plants often have larger seeds, they can produce seeds over many years, and larger seeds have more energy reserves for germination and seedling growth and produce larger, more established seedlings after germination.[6][7]
Seed functions
Seeds serve 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. Seeds fundamentally are a means of reproduction and most seeds are the product of sexual reproduction which produces a remixing of genetic material and phenotype variability that natural selection acts on.
Thursday, December 18, 2008
Monday, December 15, 2008
AGRICULTURE IN NEPAL
Agriculture in Nepal has long been based on subsistence farming, particularly in the hilly regions where peasants derive their living from fragmented plots of land cultivated in difficult conditions. Government programs to introduce irrigation facilities and fertilizers have proved inadequate, their delivery hampered by the mountainous terrain. Population increases and environmental degradation have ensured that the minimal gains in agricultural production, owing more to the extension of arable land than to improvements in farming practices, have been cancelled out. Once an exporter of rice, Nepal now has a food deficit.
Over 80 percent of the population is involved in agriculture, which constitutes 41 percent of GDP. The seasonal nature of farming leads to widespread underemployment, but programs to grow cash crops and encourage cottage industries have had some success over the years. Two-sevenths of the total land is cultivated, of which 1.5 million hectares produced 3.7 million metric tons of the staple crop of rice in 1999. Wheat and maize together take up a similar portion of the available land, with harvests of 1 million metric tons and 1.5 million metric tons, respectively, in 1999. Production of cash crops increased substantially in the 1970s, and sugarcane, oilseed, tobacco, and potatoes (a staple food in some areas) were the major crops. Agricultural production accounted for about three-fourths of total exports in the late 1980s. As noted earlier, most exports consist of primary agricultural produce which goes to India. In general the majority of Nepalese farmers are subsistence farmers and do not export surplus; this does not prevent a minority in the fertile southern Tarai region from being able to do so. Most of the country is mountainous, and there are pockets of food-deficit areas. The difficulties of transportation make it far easier to export across the border to India than to transport surplus to remote mountain regions within Nepal. A considerable livestock population of cattle, goats, and poultry exists, but the quality is poor and produces insufficient food for local needs.
Government efforts to boost the agricultural economy have focused on easing dependence on weather conditions, increasing productivity, and diversifying the range of crops for local consumption, export, and industrial inputs. Solutions have included the deployment of irrigation, chemical fertilizers, and improved seed varieties, together with credit provision, technical advice, and limited mechanization. This has had some effect. Land under irrigation increased from 6,200 hectares in 1956 to 583,000 hectares in 1990. The use of chemical fertilizers, introduced in the 1950s, climbed to about 47,000 metric tons by 1998. Still, the weather continues to determine good and bad years for the average farmer. On a national scale, while production of both food and cash crops grew annually by 2.4 percent from 1974 to 1989, population increased at a rate of 2.6 percent over the same period.
Increased agricultural activity has placed tremendous stress on the fragile ecosystems of the mountains, with severe deforestation leading to erosion and flooding that threatens the livelihoods of farmers throughout the country. In the rush to open up arable land in the early years of development, Nepal lost half its forest cover in the space of 3 decades. Government plans to maintain cover at 37 percent depend on the success of community forestry programs, which merge traditional and modern agro-forestry and conservation practices. Responsibility is placed in the hands of Forest User Groups, which included almost 800,000 households in 1999.
A potent issue is that of land reform. Before 1950, a feudal system held sway. Land ownership was concentrated in the hands of landlords who contracted out to tenant farmers. Increased productivity may have been suppressed by such a system. Even though the legal mechanisms for land reform (such as placing limits on the amount of land owned) do exist, in practice most farmers still have pitifully small holdings. Predictably, land reform has been the mandate of every political party in Nepal, particularly the communists.
Over 80 percent of the population is involved in agriculture, which constitutes 41 percent of GDP. The seasonal nature of farming leads to widespread underemployment, but programs to grow cash crops and encourage cottage industries have had some success over the years. Two-sevenths of the total land is cultivated, of which 1.5 million hectares produced 3.7 million metric tons of the staple crop of rice in 1999. Wheat and maize together take up a similar portion of the available land, with harvests of 1 million metric tons and 1.5 million metric tons, respectively, in 1999. Production of cash crops increased substantially in the 1970s, and sugarcane, oilseed, tobacco, and potatoes (a staple food in some areas) were the major crops. Agricultural production accounted for about three-fourths of total exports in the late 1980s. As noted earlier, most exports consist of primary agricultural produce which goes to India. In general the majority of Nepalese farmers are subsistence farmers and do not export surplus; this does not prevent a minority in the fertile southern Tarai region from being able to do so. Most of the country is mountainous, and there are pockets of food-deficit areas. The difficulties of transportation make it far easier to export across the border to India than to transport surplus to remote mountain regions within Nepal. A considerable livestock population of cattle, goats, and poultry exists, but the quality is poor and produces insufficient food for local needs.
Government efforts to boost the agricultural economy have focused on easing dependence on weather conditions, increasing productivity, and diversifying the range of crops for local consumption, export, and industrial inputs. Solutions have included the deployment of irrigation, chemical fertilizers, and improved seed varieties, together with credit provision, technical advice, and limited mechanization. This has had some effect. Land under irrigation increased from 6,200 hectares in 1956 to 583,000 hectares in 1990. The use of chemical fertilizers, introduced in the 1950s, climbed to about 47,000 metric tons by 1998. Still, the weather continues to determine good and bad years for the average farmer. On a national scale, while production of both food and cash crops grew annually by 2.4 percent from 1974 to 1989, population increased at a rate of 2.6 percent over the same period.
Increased agricultural activity has placed tremendous stress on the fragile ecosystems of the mountains, with severe deforestation leading to erosion and flooding that threatens the livelihoods of farmers throughout the country. In the rush to open up arable land in the early years of development, Nepal lost half its forest cover in the space of 3 decades. Government plans to maintain cover at 37 percent depend on the success of community forestry programs, which merge traditional and modern agro-forestry and conservation practices. Responsibility is placed in the hands of Forest User Groups, which included almost 800,000 households in 1999.
A potent issue is that of land reform. Before 1950, a feudal system held sway. Land ownership was concentrated in the hands of landlords who contracted out to tenant farmers. Increased productivity may have been suppressed by such a system. Even though the legal mechanisms for land reform (such as placing limits on the amount of land owned) do exist, in practice most farmers still have pitifully small holdings. Predictably, land reform has been the mandate of every political party in Nepal, particularly the communists.
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