Seed

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.

Seed plants now dominate biological niches on land, from forests to grasslands both in hot and cold climates.

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 seeds of corn are constructed with these structures; pericarp, scutellum (single large cotyledon) 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.

(The seed coats of some monocotyledon plants, such as the grasses, are not distinct structures, but are fused with the fruit wall to form a pericarp.)

Anatropous ovules have a portion of the funiculus that is adnate (fused to the seed coat), and which forms a longitudinal ridge, or raphe, just above the hilum.

In angiosperms, the stored food begins as a tissue called the endosperm, which is derived from the mother plant and the pollen via double fertilization.

The endosperm may be considered to be farinaceous (or mealy) in which the cells are filled with starch, as for instance cereal grains, or not (non-farinaceous).

The endosperm may also be referred to as "fleshy" or "cartilaginous" with thicker soft cells such as coconut, but may also be oily as in Ricinus (castor oil), Croton and Poppy.

The endosperm is called "horny" when the cell walls are thicker such as date and coffee, or "ruminated" if mottled, as in nutmeg, palms and Annonaceae.

The exalbuminous seeds include the legumes (such as beans and peas), trees such as the oak and walnut, vegetables such as squash and radish, and sunflowers.

Other seed appendages include the raphe (a ridge), wings, caruncles (a soft spongy outgrowth from the outer integument in the vicinity of the micropyle), spines, or tubercles.

Orchids and a few other groups of plants are mycoheterotrophs which depend on mycorrhizal fungi for nutrition during germination and the early growth of the seedling.

Some terrestrial orchid seedlings, in fact, spend the first few years of their lives deriving energy from the fungi and do not produce green leaves.

Many annual plants produce great quantities of smaller seeds; this helps to ensure at least a few will end in a favorable place for growth.

They usually give a seedling a faster start than a sporeling from a spore, because of the larger food reserves in the seed and the multicellularity of the enclosed embryo.

Foraging ants disperse seeds which have appendages called elaiosomes[30] (e.g. bloodroot, trilliums, acacias, and many species of Proteaceae).

[32] Seed dormancy has two main functions: the first is synchronizing germination with the optimal conditions for survival of the resulting seedling; the second is spreading germination of a batch of seeds over time so a catastrophe (e.g. late frosts, drought, herbivory) does not result in the death of all offspring of a plant (bet-hedging).

It involves the reactivation of the metabolic pathways that lead to growth and the emergence of the radicle or seed root and plumule or shoot.

However, the nature of the seed coat determines how rapidly water can penetrate and subsequently initiate germination.

[52] Also, in Arabidopsis seeds, the activities of the DNA repair enzymes Poly ADP ribose polymerases (PARP) are likely needed for successful germination.

Stratification, also called moist-chilling, breaks down physiological dormancy, and involves the addition of moisture to the seeds so they absorb water, and they are then subjected to a period of moist chilling to after-ripen the embryo.

Other methods used to assist in the germination of seeds that have dormancy include prechilling, predrying, daily alternation of temperature, light exposure, potassium nitrate, the use of plant growth regulators, such as gibberellins, cytokinins, ethylene, thiourea, sodium hypochlorite, and others.

[55] Seeds may be sterile for few reasons: they may have been irradiated, unpollinated, cells lived past expectancy, or bred for the purpose.

[60] Over this period, there were six bumper, five poor, and nine good seed crops, when evaluated for production of adequate seedlings for natural forest reproduction.

For example, the gluten of wheat, important in providing the elastic property to bread dough is strictly an endosperm protein.

Particularly in developing countries, a major constraint faced is the inadequacy of the marketing channels to get the seed to poor farmers.

[67] In addition, seeds containing amygdalin – apple, apricot, bitter almond,[68] peach, plum, cherry, quince, and others – when consumed in sufficient amounts, may cause cyanide poisoning.

[68][69] Other seeds that contain poisons include annona, cotton, custard apple, datura, uncooked durian, golden chain, horse-chestnut, larkspur, locoweed, lychee, nectarine, rambutan, rosary pea, sour sop, sugar apple, wisteria, and yew.