Facts about seeds
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- University of California Museum of Paleontology – Seed plants: Fossil Record
- UNESCO World Heritage Centre – Bordeaux, France
- 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.
Did you know? Seeds
The function of all flowering plants is to produce seeds. Once the seeds have grown and ripened, the plant has to get them to somewhere that they will be happy growing. Find out how they do this.
On this page
- How plants spread seeds
- Interesting facts
How plants spread seeds
Plants have many ways of spreading or dispersing their seeds.
- gravity – heavy seeds will just fall off the plant.
- wind – very fine seeds will blow away on the wind. Some seeds have special parachutes or wings to help them fly, for example, dandelions.
- hooks – the seeds are covered with hooks which catch on to a passing animals’ fur; they then catch a free ride to another place where they are rubbed off later.
- animals – the seeds look like tasty treats for the animals to eat, but they pass undigested through the animal. Animals, including birds and insects, sometimes bury the seeds.
- pepperpot – the seed-pod is like a little pepperpot and sprinkles the seeds over quite a wide area.
- exploding – the seed-pod bursts suddenly, throwing all the seeds out over a large area.
- floating – some seeds grow with air trapped in them, so they can float away from the parent plant.
- The largest seed in the world is the double coconut. It can measure up to 50cm (1.6ft) around the middle! Coconuts have a fibrous coating and an air space inside them, because they need to be able to float to a new home. Some coconuts have floated 2,000km over the sea before they find dry land!
- Seeds provide the world’s daily food. Your breakfast cereal and toast, your pasta or pizza lunch and your rice dish for tea all started life as seeds from different grasses. Read more about cereals in our Did you know? article.
- Some orchid seed-pods hold 3 million seeds.
- Kapok is soft, fluffy stuff that comes from a seed-case. Years ago it was used to fill life jackets, because it is light, strong and waterproof. Nowadays, modern plastics have replaced it.
- Some seeds found in frozen soil in Canada were grown and produced flowers – the seeds were thought to be more than 10,000 years old!
- There are some very dangerous seeds, such as those that come from deadly nightshade; two berries could kill you. Even more dangerous are the seeds from the Castor-oil plant – one bean will kill an adult.
- Most oak trees don’t grow acorns until they are at least 50 years old. If you have planted an acorn, how old will you be before you can plant its acorns?
Try growing an oak tree from an acorn in our home-grown project.
Discover interesting facts about seeds Gardening with Children from BBC Gardening