How are seeds made
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- seed – Children’s Encyclopedia (Ages 8-11)
- seed – Student Encyclopedia (Ages 11 and up)
Seed, the characteristic reproductive body of both angiosperms (flowering plants) and gymnosperms (e.g., conifers, cycads, and ginkgos). Essentially, a seed consists of a miniature undeveloped plant (the embryo), which, alone or in the company of stored food for its early development after germination, is surrounded by a protective coat (the testa). Frequently small in size and making negligible demands upon their environment, seeds are eminently suited to perform a wide variety of functions the relationships of which are not always obvious: multiplication, perennation (surviving seasons of stress such as winter), dormancy (a state of arrested development), and dispersal. Pollination and the “seed habit” are considered the most important factors responsible for the overwhelming evolutionary success of the flowering plants, which number more than 300,000 species.
The superiority of dispersal by means of seeds over the more primitive method involving single-celled spores, lies mainly in two factors: the stored reserve of nutrient material that gives the new generation an excellent growing start and the seed’s multicellular structure. The latter factor provides ample opportunity for the development of adaptations for dispersal, such as plumes for wind dispersal, barbs, and others.
Economically, seeds are important primarily because they are sources of a variety of foods—for example, the cereal grains, such as wheat, rice, and corn (maize); the seeds of beans, peas, peanuts, soybeans, almonds, sunflowers, hazelnuts, walnuts, pecans, and Brazil nuts. Other useful products provided by seeds are abundant. Oils for cooking, margarine production, painting, and lubrication are available from the seeds of flax, rape, cotton, soybean, poppy, castor bean, coconut, sesame, safflower, sunflower, and various cereal grains. Essential oils are obtained from such sources as juniper “berries,” used in gin manufacture. Stimulants are obtained from such sources as the seeds of coffee, kola, guarana, and cocoa. Spices—from mustard and nutmeg seeds; from the aril (“mace”) covering the nutmeg seed; from the seeds and fruits of anise, cumin, caraway, dill, vanilla, black pepper, allspice, and others—form a large group of economic products.
The nature of seeds
In the typical flowering plant, or angiosperm, seeds are formed from bodies called ovules contained in the ovary, or basal part of the female plant structure, the pistil. The mature ovule contains in its central part a region called the nucellus that in turn contains an embryo sac with eight nuclei, each with one set of chromosomes (i.e., they are haploid nuclei). The two nuclei near the centre are referred to as polar nuclei; the egg cell, or oosphere, is situated near the micropylar (“open”) end of the ovule.
With very few exceptions (e.g., the dandelion), development of the ovule into a seed is dependent upon fertilization, which in turn follows pollination. Pollen grains that land on the receptive upper surface (stigma) of the pistil will germinate, if they are of the same species, and produce pollen tubes, each of which grows down within the style (the upper part of the pistil) toward an ovule. The pollen tube has three haploid nuclei, one of them, the so-called vegetative, or tube, nucleus seems to direct the operations of the growing structure. The other two, the generative nuclei, can be thought of as nonmotile sperm cells. After reaching an ovule and breaking out of the pollen tube tip, one generative nucleus unites with the egg cell to form a diploid zygote (i.e., a fertilized egg with two complete sets of chromosomes, one from each parent). The zygote undergoes a limited number of divisions and gives rise to an embryo. The other generative nucleus fuses with the two polar nuclei to produce a triploid (three sets of chromosomes) nucleus, which divides repeatedly before cell-wall formation occurs. This process gives rise to the triploid endosperm, a nutrient tissue that contains a variety of storage materials—such as starch, sugars, fats, proteins, hemicelluloses, and phytate (a phosphate reserve).
The events just described constitute what is called the double-fertilization process, one of the characteristic features of all flowering plants. In the orchids and in some other plants with minute seeds that contain no reserve materials, endosperm formation is completely suppressed. In other cases it is greatly reduced, but the reserve materials are present elsewhere—e.g., in the cotyledons, or seed leaves, of the embryo, as in beans, lettuce, and peanuts, or in a tissue derived from the nucellus, the perisperm, as in coffee. Other seeds, such as those of beets, contain both perisperm and endosperm. The seed coat, or testa, is derived from the one or two protective integuments of the ovule. The ovary, in the simplest case, develops into a fruit. In many plants, such as grasses and lettuce, the outer integument and ovary wall are completely fused, so seed and fruit form one entity; such seeds and fruits can logically be described together as “dispersal units,” or diaspores. More often, however, the seeds are discrete units attached to the placenta on the inside of the fruit wall through a stalk, or funiculus.
The hilum of a liberated seed is a small scar marking its former place of attachment. The short ridge (raphe) that sometimes leads away from the hilum is formed by the fusion of seed stalk and testa. In many seeds, the micropyle of the ovule also persists as a small opening in the seed coat. The embryo, variously located in the seed, may be very small (as in buttercups) or may fill the seed almost completely (as in roses and plants of the mustard family). It consists of a root part, or radicle, a prospective shoot (plumule or epicotyl), one or more cotyledons (one or two in flowering plants, several in Pinus and other gymnosperms), and a hypocotyl, which is a region that connects radicle and plumule. A classification of seeds can be based on size and position of the embryo and on the proportion of embryo to storage tissue; the possession of either one or two cotyledons is considered crucial in recognizing two main groups of flowering plants, the monocotyledons and the eudicotyledons.
Seed, the characteristic reproductive body of both angiosperms and gymnosperms. Essentially, a seed consists of a miniature undeveloped plant (the embryo), which, alone or in the company of stored food, is surrounded by a protective coat. Learn more about seed characteristics, dispersal, and germination.
What Is A Seed – A Guide To The Seed Life Cycle And Its Purpose
Most organic plant life starts out as a seed. What is a seed? It is technically described as a ripened ovule, but it is so much more than that. Seeds house an embryo, the new plant, nourish and protect it. All types of seeds fulfill this purpose, but what do seeds do for us outside of growing new plants? Seeds may be used as food for humans or animals, spices, beverages and are even used as industrial products. Not all seeds fill all of these needs and, in fact, some are poisonous.
What is a Seed?
Plant life starts with seeds unless the plant reproduces by spores or vegetatively. Where do seeds come from? They are the byproduct of a flower or flower-like structure. Sometimes seeds are encased in fruits, but not always. Seeds are the primary method of propagation in most plant families. The seed life cycle starts with the flower and ends with a seedling, but many steps in between vary from plant to plant.
Seeds vary in their size, dispersal method, germination, photo response, need for certain stimuli, and many other complicating factors. For instance, look at the seed of the coconut palm and compare it to the minute seeds of an orchid and you will get some idea of the vast variety in sizes. Each of these also has a different method of dispersal and has certain germination requirements that are only found in their natural environments.
The seed life cycle can also vary from just a few days of viability to up to 2,000 years. No matter the size or life span, a seed contains all the information necessary to produce a new plant. It is about as perfect a situation as nature has devised.
Where Do Seeds Come From?
The simple answer to this questions is from a flower or fruit, but it is more complex than that. The seeds of conifers, such as pine trees, are contained in scales inside the cone. The seeds of a maple tree are inside the little helicopters or samaras. The seed of a sunflower is contained in its large flower, familiar to most of us because they are also a popular snack food. The large pit of a peach contains a seed inside the hull or endocarp.
In angiosperms, seeds are covered while in gymnosperms, seeds are naked. Most types of seeds have a similar structure. They have an embryo, cotyledons, a hypocotyl, and a radicle. There is also an endosperm, which is the food that sustains the embryo as it begins to sprout and a seed coat of some sort.
Types of Seeds
The appearance of seeds of different varieties varies greatly. Some of the grain seeds we commonly grow are corn , wheat and rice . Each has a different appearance and the seed is the primary part of the plant we eat.
Peas , beans and other legumes grow from seeds found in their pods. Peanut seeds are another example of a seed that we eat. The huge coconut contains a seed inside the hull, much like a peach.
Some seeds are grown just for their edible seeds, like sesame seeds. Others are made into beverages as in the case of coffee . Coriander and clove are seeds used as spices. Many seeds have a powerful commercial oil value too, such as canola .
The uses of seeds are as diverse as the seeds themselves. In cultivation, there are open pollinated, hybrid, GMO and heirloom seeds just to add to the confusion. Modern cultivation has manipulated many seeds, but the basic make up is still the same – the seed houses the embryo, its initial food source and some sort of protective cover.
What is a seed? It is technically described as a ripened ovule, but it is so much more than that. Seeds house an embryo, the new plant, nourish and protect it. All types of seeds fulfill this purpose, but what do seeds do for us outside of growing new plants? Find out here.