Seed


In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a testa. More generally, the term seed means anything that can be sown, which may include seed and husk or tuber. Seeds are the product of the ripened ovule, after the embryo sac is fertilized by sperm from pollen, forming a zygote. The embryo within a seed develops from the zygote and grows within the mother plant to a certain size before growth is halted.
The formation of the seed is the defining part of the process of reproduction in seed plants. Other plants such as ferns, mosses and liverworts, do not have seeds and use water-dependent means to propagate themselves. Seed plants now dominate biological niches on land, from forests to grasslands both in hot and cold climates.
In the flowering plants, the ovary ripens into a fruit which contains the seed and serves to disseminate it. Many structures commonly referred to as "seeds" are actually dry fruits. Sunflower seeds are sometimes sold commercially while still enclosed within the hard wall of the fruit, which must be split open to reach the seed. Different groups of plants have other modifications, the so-called stone fruits have a hardened fruit layer fused to and surrounding the actual seed. Nuts are the one-seeded, hard-shelled fruit of some plants with an indehiscent seed, such as an acorn or hazelnut.

History

The first land plants evolved around 468 million years ago, and reproduced using spores. The earliest seed bearing plants to appear were the gymnosperms, which have no ovaries to contain the seeds. They arose during the late Devonian period. From these early gymnosperms, seed ferns evolved during the Carboniferous period ; they had ovules that were borne in a cupule, which consisted of groups of enclosing branches likely used to protect the developing seed.
Published literature about seed storage, viability and its hygrometric dependence began in the early 19th century, influential works being:
  • 1832 seed storage guide in Augustin Pyramus de Candolle's Conservation des Graines, part of his 3-volume Physiologie végétale, ou Exposition des forces et des fonctions vitales des végétaux ;
  • 1846 viability studies by Augustin de Candolle, published in "Sur la durée relative de la faculté de germer des graines appartenant à diverses familles" ;
  • 1897 seed hygrometric studies by Victor Jodin
  • 1912's Henry B. Guppy's 528 page "Studies in Seeds and Fruits- An Investigation with the Balance" ; subsequently reviewed in ''Science''

    Development

seeds are "enclosed seeds", produced in a hard or fleshy structure called a fruit that encloses them for protection. 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.
Angiosperm seeds consist of three genetically distinct constituents: the embryo formed from the zygote, the endosperm, which is normally triploid, the seed coat from tissue derived from the maternal tissue of the ovule. In angiosperms, the process of seed development begins with double fertilization, which involves the fusion of two male gametes with the egg cell and the central cell to form the primary endosperm and the zygote. Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food the young plant will consume until the roots have developed after germination.

Ovule

After fertilization, the ovules develop into the seeds. The ovule consists of a number of components:
  • The funicle or seed stalk which attaches the ovule to the placenta and hence ovary or fruit wall, at the pericarp.
  • The nucellus, the remnant of the megasporangium and main region of the ovule where the megagametophyte develops.
  • The micropyle, a small pore or opening in the apex of the integument of the ovule where the pollen tube usually enters during the process of fertilization.
  • The chalaza, the base of the ovule opposite the micropyle, where integument and nucellus are joined.
The shape of the ovules as they develop often affects the final 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 megagametophyte 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.
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 manufactures nutrients from the endosperm that are used during the embryo's growth.

Embryo

The main components of the embryo are:
  • The cotyledons, the seed leaves, attached to the embryonic axis. There may be one, or two. The cotyledons are also the source of nutrients in the non-endospermic dicotyledons, in which case they replace the endosperm, and are thick and leathery. In endospermic seeds, the cotyledons are thin and papery. Dicotyledons have the point of attachment opposite one another on the axis.
  • The epicotyl, the embryonic axis above the point of attachment of the cotyledon.
  • The plumule, the tip of the epicotyl, and has a feathery appearance due to the presence of young leaf primordia at the apex, and will become the shoot upon germination.
  • The hypocotyl, the embryonic axis below the point of attachment of the cotyledon, connecting the epicotyl and the radicle, being the stem-root transition zone.
  • The radicle, the basal tip of the hypocotyl, grows into the primary root.
Monocotyledonous plants have two additional structures in the form of sheaths. The plumule is covered with a coleoptile that forms the first leaf while the radicle is covered with a coleorhiza that connects to the primary root and adventitious roots form the sides. Here the hypocotyl is a rudimentary axis between radicle and plumule. The seeds of corn are constructed with these structures; pericarp, scutellum that absorbs nutrients from the endosperm, plumule, radicle, coleoptile, and coleorhiza – these last two structures are sheath-like and enclose the plumule and radicle, acting as a protective covering.

Seed coat

The maturing ovule undergoes marked changes in the integuments, generally a reduction and disorganization but occasionally a thickening. 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. The testae of both monocots and dicots are often marked with patterns and textured markings, or have wings or tufts of hair. When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next. The funiculus abscisses, the scar forming an oval depression, the hilum. Anatropous ovules have a portion of the funiculus that is adnate, and which forms a longitudinal ridge, or raphe, just above the hilum. In bitegmic ovules both inner and outer integuments contribute to the seed coat formation. With continuing maturation the cells enlarge in the outer integument. While the inner epidermis may remain a single layer, it may also divide to produce two to three layers and accumulates starch, and is referred to as the colourless layer. By contrast, the outer epidermis becomes tanniferous. The inner integument may consist of eight to fifteen layers.
As the cells enlarge, and starch is deposited in the outer layers of the pigmented zone below the outer epidermis, this zone begins to lignify, while the cells of the outer epidermis enlarge radially and their walls thicken, with nucleus and cytoplasm compressed into the outer layer. these cells which are broader on their inner surface are called palisade cells. In the inner epidermis, the cells also enlarge radially with plate like thickening of the walls. The mature inner integument has a palisade layer, a pigmented zone with 15–20 layers, while the innermost layer is known as the fringe layer.

Gymnosperms

In gymnosperms, which do not form ovaries, the ovules and hence the seeds are exposed. This is the basis for their nomenclature – naked seeded plants. Two sperm cells transferred from the pollen do not develop the seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development. The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce.

Shape and appearance

Seeds are very diverse, and as such there are many terms are used to describe them.

Terms to describe shape

  • Bean-shaped – resembling a kidney, with lobed ends on either side of the hilum
  • Square or Oblong – angular, with all sides being either equal, or longer-than-wide
  • Triangular – three-sided, broadest below the middle
  • Elliptic or Ovate or Obovate – rounded at both ends, or egg shaped, being rounded but either symmetrical about the middle, or broader below the middle, or broader above the middle
  • Discoid – resembling a disc or plate, having both thickness and parallel faces and with a rounded margin)
  • Ellipsoid
  • Globose – spherical
  • Subglobose
  • Lenticular
  • Ovoid
  • Sectoroid
  • Other common descriptors for seeds focus on color, texture, and form. Striate seeds are striped with parallel, longitudinal lines or ridges. The most common colours are brown and black, with other colours appearing less frequently. The surface texture varies from highly polished to considerably roughened. The surface may also have a variety of appendages, and be described by terms such as papillate or digitiform. A seed coat with the consistency of cork is referred to as suberose. Other terms include crustaceous.