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 (spermatophytes). 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 (such as the peach) have a hardened fruit layer (the endocarp) 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 (416 million to 358 million years ago). From these early gymnosperms, seed ferns evolved during the Carboniferous period (359 to 299 million years ago); 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'' (1832, v. 2, pp. 618–626, Paris); (translated title, "Plant physiology, or Exposition of the vital forces and functions of plants")
* 1846 viability studies by Augustin de Candolle, published in "Sur la durée relative de la faculté de germer des graines appartenant à diverses familles" ('' Annales des Sciences Naturelles; Botanique'', 1846, III 6: 373–382); (translated title, "On the relative duration of the ability to germinate seeds belonging to various families")
* 1897 seed hygrometric studies by Victor Jodin (''Annales Agronomiques'', October 1897)
* 1912's Henry B. Guppy's 528 page "''Studies in Seeds and Fruits- An Investigation with the Balance''" (1912, London, England); subsequently reviewed in ''Science'' (June 1914, Washington, D.C.)

Development

Angiosperm 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 (flowering plants) seeds consist of three genetically distinct constituents: (1) the embryo formed from the zygote, (2) the endosperm, which is normally triploid, (3) 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 (''funiculus, funiculi'') 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 (the funicle or funiculus).

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

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