Seed is a mature ovule comprising an embryo or miniature undeveloped plant and food reserves enclosed within a protective outer covering, known as a seed coat (testa). Open pollinated plants such as heirloom varieties usually grow true to type from seed.
Seeds disperse in many ways. Some have wings or barbs that attach to animal fur or feathers, and others float in water. Others have hard or thick seed coats that require physical or chemical stimulus to break a physical or chemical dormancy.
Definition
The seed of a plant is its embryonic beginning, enclosed within a protective coating and with some stored food materials. Seeds are the means of reproduction for all flowering plants. Seeds are also important as sources of many foods, including wheat, corn, beans and peanuts.
The embryo is surrounded by a nutrient tissue, called endosperm and a protective seed coat. The nutrient tissue is often composed of starch and proteins.
Seeds vary in size, shape and color. They may be discoid (having thick sides and rounded corners), ellipsoid, lenticular or ovoid. Some seeds are striped or patterned with lines or ridges. All seeds can grow into a new plant given the right conditions. This is known as germination. The first part of the new plant that emerges is called the radicle and is followed by a stem, then leaves called cotyledons.
Origin
Seeds represent one of the most complex and evolutionary successful methods of sexual reproduction in vascular plants. They evolved in the gymnosperms and angiosperms (which account for most biodiversity today). Other plants such as ferns, mosses and liverworts do not produce seeds but use water-dependent means to propagate themselves.
A seed consists of an embryo with stored food and a protective covering, called the integument. The integument may consist of one or two cotyledons, or in the case of some flowering plants, of a tissue derived from the nucellus called perisperm.
In a seed plant the male gametophyte is hidden in the pollen grain and the female gametophyte in the ovule. This arrangement is known as heterospory. Experiments have shown that a 2m:1p genomic ratio in the endosperm is critical for normal seed development.
Structure
Seeds may differ from one another in shape, size and surface but they have similar structure. They contain a fertilized mature ovule (embryonic plant) inside a protective coating called a seed coat.
The embryo develops into a new plant when the seed grows. This is aided by the food stored in the seed known as endosperm. The embryo may have only one cotyledon or seed leaf (Monocotyledons) or two cotyledons (Dicotyledons). There is a root part called radicle and a prospective shoot called plumule at either end of the embryonal axis.
There is also a seed coat, which can be smooth, wrinkled, ribbed or striate. The seed coat protects the embryo from microbes and the environment. It has a scar on the top called a hilum and a small pore called a micropyle.
Function
Seeds have an extraordinary ability to wait, often for a long time, until external conditions are suitable for them to sprout into new plants. Besides their genetic information, seeds also contain food stored in the endosperm or cotyledons.
The seed coat, a protective cuticle, may also play an important role in seed viability. It is a physical barrier against pathogen penetration and provides the seed with a degree of protection against harsh environments.
In some cases, the seed coat is colored or contains antifungal compounds, for example glucosinolates in brassicaceous crops. Studies of mutants impaired in the production of these seed coat pigments have revealed that genes involved are involved in synthesis and compartmentation of PA flavonoid compounds. Moreover, it is likely that the activity of seed DNA ligases is essential to maintenance of seed viability during storage and germination.
Dispersal
Some plants use animal dispersal to get their seeds to new places. Fleshy fruit or seedpods often have burrs or barbs that catch on animals’ fur, feathers or digestive tracts (endozoochory). Others have wings for wind dispersal.
Other fruits crack open when ripe and shoot their seeds out into the environment. Examples include apples, figs and gorse. This is known as ephemeral dispersal.
Many tree species that produce fleshy fruits rely on hitchhiking animals as their free “bus” for seed dispersal. All major groups of vertebrates including mammals, birds and reptiles act as animal dispersers. Even large carnivorous frugivores like our lions and jaguar can disperse tree seeds by simply brushing against them.
