AGRONOMY ENGINEERING ENGLISH I LCDA. ARACELIS TORRES
INTRODUCTION
Reproductive structure in plants that consists of a plant embryo, usually accompanied by a supply of food (endosperm, which is produced during fertilization) and enclosed in a protective coat. Seed embryos contain one or more cotyledons. In typical flowering plants, seed production follows pollination and fertilization. As seeds mature, the ovary that enclosed the ovules develops into a fruit containing the seeds. Most seeds are small, weighing less than a gram; the smallest contain no food reserve. At the opposite extreme, the seed of the double coconut palm may weigh up to about 60 lb. (27 kg). Seeds are highly adapted to transportation by animals, wind, and water. When circumstances are favorable, water and oxygen penetrate the seed coat, and the new plant begins to grow (see germination). The longevity of seeds varies widely: some remain viable for only about a week; others have been known to germinate after hundreds or even thousands of years.
DEFINITION A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant. The formation of the seed completes the process of reproduction in seed plants (started with the development of flowers and pollination), with the embryo developed from the zygote and the seed coat from the integuments of the ovule. Seeds have been an important development in the reproduction and spread of flowering plants, relative to more primitive plants such as mosses, ferns and liverworts, which do not have seeds and use other means to propagate themselves. This can be seen by the success of seed plants (both gymnosperms and angiosperms) in dominating biological niches on land, from forests to grasslands both in hot and cold climates. The term "seed" also has a general meaning that antedates the above — anything that can be sown, e.g. "seed" potatoes, "seeds" of corn or sunflower "seeds". In the case of sunflower and corn "seeds", what is sown is the seed enclosed in a shell or husk, whereas the potato is a tuber.
Ontogeny of the Seed-Coat The bitegmic, anatropous ovule develops into an exalbuminous, partially pachychalazal and endotegmic seed. In the mature seed-coat the extensive chalazae with associated tanniniferous hypostases sensu lato manifests externally as a characteristic brown patch. The walls of the cells of the hypostases are impregnated with callose and lipidic substances, which most probably represent cutin. Ultimately the outer integument and outer parts of the inner integument are more or less squashed. However, the cell walls of the inner epidermis of the inner integument show distinct secondary thickening and lignifications. The pachychalazal seed with undifferentiated seed-coat characterizes not only a number of the genera of the tribe Anacardieae, but also occurs in Heeria of the tribe Rhoeae. A number of genera of the tribe Spondiadeae have a partially pachychalazal seed. The seed-coat of the latter shows varying degrees of traces of an exo-, meso- and/or endotestal lignifications. The seed of certain genera of the Rhoeae is partially pachychalazal and endotegmic, or probably only endotegmic.
Origin The seed is a reproductive unit complex, characteristic of higher vascular plants, which is formed from the egg plant, usually after fertilization. It is found in flowering plants (angiosperms) and gymnosperms. In angiosperms the eggs develop within an ovary, while in gymnosperms, the structure that contains them is very different, as not a real flower, but the structure of the seeds of these plants is basically similar to that of flowering plants.
Function Seeds serve several functions for the plants that produce them. Key among these functions is 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.
DOUBLE FERTILIZATION Double fertilization is a complex fertilization mechanism that has evolved in flowering plants (angiosperms). This process involves the joining of a female gametophyte (mega gametophyte, also called the embryo sac) with two male gametes (sperm). It begins when a pollen grain adheres to the stigma of the carpel, the female reproductive structure of a flower. The pollen grain then takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary through the style. The tip of the pollen tube then enters the
ovary and penetrates through the micropyle opening in the ovule. The pollen tube proceeds to release the two sperm in the mega gametophyte.
STRUCTURE OF EXTERNAL AND INTERNALOF THE SEED OF MONOCOTYLEDONS AND DICOTYLEDONS A seed consists of three parts: a Dormant Embryo, a Storage Tissue, and a Seed Coat. Not every seed that has evolved on this planet has precisely the same structure. In some seeds, the endosperm is retained as the storage tissue. In other seeds, the endosperm is more or less used up to put storage chemicals into the embryo itself (commonly in the cotyledons). Below is a diagram of two hypothetical seeds. The upper seed shows a dicot that lacks endosperm; its storage material (blue) is held in the cotyledons, the lower seed shows a monocot that has a welldeveloped endosperm (also blue). Both of these examples qualify as true seeds because they possess all three parts needed to make a true seed.
Below is a photomicrograph of a longitudinal section of a Capsella seed. The seed coat is the three-layered outer covering. The outermost and innermost layers of the seed coat pick up a red dye used in the preparation of this sample. That dye is preferentially held by parts of the specimen that have waxy or brittle biomolecules. Cutin, sabering, lignin are three components of plant cells that pick up the red dye. This leads us to the idea that the integument has a waterproofing (cutin/sobering) function and also possibly a mechanical strengthening (lignin) function that could impede herbivore. Looking toward the inside of the seed coat, there is a bent embryo (dark looking) surrounded by some loose and light colored cells of the remaining endosperm? Indeed the paucity of endosperm leads us to the conclusion that much of the storage material has been moved to the embryo itself...perhaps much of that in the cotyledons. However, this seed is clearly somewhere between the two extremes diagrammed above. The embryo has an axis (on the left) with a downward-pointing radicle including a root apex. The radicle ultimately penetrates the seed coat in seed germination, branches profusely, and becomes the primary root system. Toward the top of the embryonic axis, the radicle becomes hypocotyl...transitioning from root-type anatomy into stem-type anatomy as we shall see later. In seeds with hypogenous germination, this hypocotyl is too short and does not grow above the soil. In species with epigeous germination, the hypocotyl elongates rapidly, lifting the cotyledon(s) out of the soil and into the air. The axis has two appendages, the cotyledons; as there are two cotyledons, Capsella is a dicotyledonous plant. There are two classes of flowering plants, the dicots and the monocots, and they are distinguished (in part) on the basis of the number of cotyledons found on their dormant
embryos. The part of the axis where the cotyledons attach is called a node because this zone of the axis does not elongate. At the extreme top of the axis is a shoot apex; after germination this shoot apex will elongate, and make appendages to produce the collection of stems and leaves that constitute the plant shoot. MORPHOLOGICAL DIFFERENCES BETWEEN SEEDS OF ANGIOSPERMSPES AND GYMNOSPERMS Gymnosperms A gymnosperm (Greek for "naked seed") is a vascular plant that produces seeds that are not protected by fruit but are hidden in a woody cone. They do not have flowers, but most retain their leaves year round. Gymnosperms include over 600 species in four divisions: Conifers, Cycads, Ginkgoes, and Gnetophytes. Most of today's gymnosperms belong to the conifer division. Gymnosperms are found in most of the world's regions and take most of the credit for timber and paper products.
Angiosperms Angiosperms are vascular plants that produce flowers and fruit with one or more seeds. Angiosperms make up two classes: monocotyledonous and dicotyledonous plants. Angiosperms make up more than 80% of all plant species, ranging from roses to palm trees.
Gymnosperm - 'naked seed' The endosperm is (n haploid) Reproduction is achieved through male and female cones Mainly represented by tree species Phylem coniferophyta Phylem Gnetophytes Phylem Cycadophyta Phylem Ginkgophyta
Angiosperm - flowering plant
The endosperm is (3n triploid) Male pollen and female ovules are produced Fruits and flowers Phylem anthophyta
Read more: http://wiki.answers.com/Q/What_is_the_difference_between_angiosperm s_and_gymnosperms#ixzz21Mg5yjuo TYPES AND MODIFICATION OF THE SEEDS We have seen the basic features that are common to most types of seeds. However, different plant species have their own modifications. In stone fruits, for example, the seed is surrounded by a hardened fruit layer, known as the endocarp. More broadly, the seeds of angiosperm plants differ from those produced by gymnosperm plants. ‘Angiosperm’ literally means ‘enclosed seed’, and gymnosperm ‘naked seed’. Angiosperm seeds are enclosed by a hard or fleshy fruit, whereas gymnosperm seeds are usually exposed to the outside world, for example on the cone of a conifer.
Seeds also vary a lot in size. Orchid seeds look like tiny dust particles, whereas other seeds are much larger, such as the giant (over 20kg) seeds produced by the coco of palm tree. As a rule, larger seeds are produced in smaller quantities, as more energy goes into their production.
ADAPTATION OF THE SEEDS TO SPREAD OR DISPERSION For plant species to survive, it's critical that its seeds are spread over the landscape. Otherwise, the seeds would drop in a clump at the base of the plant. To grow, the seeds would then have to compete with the mature plant and all the other seeds for water, sunlight and nutrients. To avoid this fate, plants have developed seed adaptations. Wind Some plants depend on the wind to spread their seeds. In these cases, the seeds are often attached to fine, fluffy threads that catch the breeze, enabling them to float to a new patch of land. Examples of plants with these seed adaptation are dandelions and cattails. Some tree seeds, such as those from maples and ashes, also depend on the wind. These seeds will often have a propeller shape.
Water Plants also use water to transport their seeds to new locations. Coconuts -- which are a seed -- are an example. The thick shells keep the salt water out and the buoyant coconuts can float on the sea for hundreds of miles without being damaged.
Animals Animals are another way seeds are dispersed. Plants that use this method have seeds that attract animals, such as berries. After the seeds have passed through an animal's digestive tract, they can take root at the site of the animal dropping containing the seed. Other seeds are adapted to stick to animal fur. Such seeds, often known to humans as burrs, eventually fall off and take root. Germination Some seeds have extra adaptations that help control when the seeds are germinated no matter how the seeds were dispersed. For example, some seeds will not germinate -- or begin to grow -- until after a fire. Those seeds depend on the heat to crack the shell or open the pine cone, allowing the seeds to drop to the ground. Other seeds depend on the wind to scratch their casings, allowing water to seep into the seed and begin the germination process. Read more: What Are Three Seed Adaptations? | eHow.com http://www.ehow.com/info_8494069_three-seed adaptations.html#ixzz21MtpS4pH
http://www.ehow.com/info_8494069_three-seedadaptations.html#ixzz21MuQOkyY http://www.ehow.com/info_8494069_three-seedadaptations.html#ixzz21MvZQlni http://www.exploringnature.org/db/detail.php?dbID=45&detID=2792 IMPORTANT AGRICULTURAL SEED LEVEL Seed production is an essential process in agriculture. Thanks to it, peasants have been domesticated plant species we consume today, creating a huge variety within each species to be adapted to the different environmental conditions and cultural needs. This process has continued in some regions for at least ten thousand years. From the twentieth century, there is a current tech industrial production and seed varieties designed to create work with chemicals and mass production conditions. At the beginning of XXI century, this trend has shifted in many regions of the planet to the traditional production of seeds. The industry argues that the industrial seeds are more productive and free of pests. Peasant movements like Via Campesina argue that such production depends on the application of agrochemicals and overall energy subsidy on oil and traditional seeds that are more robust and suitable for local food. The marketing of seed industry is one of the causes of genetic erosion.
BIBLIOGRAPHY
PRISCILA ANDRESSA CORTEZ1 and SANDRA MARIA CARMELLO-GUERREIRO 1,2(received: November 10, 2006; accepted: January 17, 2008) Revista Brasil. Bot., V.31, n.1, p.71-79, jan.mar. 2008 Ontogeny and structure of the pericarp and the seed coat of Miconia albicans (Sw.) Triana (Melastomataceae) from “cerrado”, Brazil http://www.scielo.br/pdf/rbb/v31n1/a08v31n1.pdf Website Gerhard Leubner Lab Royal Holloway, University of London 2005. The Seed Biology Place http://www.seedbiology.de/structure.asp The World of the Plants http://www.botanical-online.com/llavorangles.htm Catherine Steinbauer, eHow Contributor Adaptations?
What Are Three Seed
http://www.ehow.com/info_8494069_three-seed-adaptations.html Amsel, Sheri. “Adaptations.” Pollination and Seed Dispersal Adaptations. Exploring Nature Educational Resource. © 2005 - 2012. July 22, 2012. http://exploringnature.org/db/detail.php?dbID=45&detID=2792