The plant seed is a structure that is formed by the maturation of the ovule within the ovary of the angiosperms. It is often described as a “mature ovule”. It has multiple uses to man such as source of food (e.g. cereals, grain legumes, seed vegetables), beverages (e.g. coffee, cacao, juice from young coconut), and spices (e.g. anise, nutmeg, mustard, sesame). Many plant seeds also provide raw materials for the industrial processing of various products such as vegetable oil, starch, biofuel, lubricants, and fiber.
The angiospermous seed is an organ found in plant shoot, attached to the stem, and originating from a flower. Its primary function is reproduction in which plants perpetuate themselves, mainly sexually. The seed is widely used in the deliberate production of seedlings, known as plant propagation.
In addition, this organ also serves as a diaspore or dispersal unit of many plants. Many seeds are equipped with adaptations which ensure or enhance dispersal such as dust-like and balloon seeds, wing-like appendages, hairs, parachutes, feathers and hooks, and water repellant surfaces.
The development of the plant seed follows pollination, the transfer of pollen from the anther to the stigma, and double fertilization. Double fertilization is a unique process in the angiosperms in which two fertilizations occur, one leading to the formation of a diploid embryo and another which is responsible for the triploid endosperm.
But there is an exception to the natural process of seed formation. In apomixis, the seed is developed without double fertilization and the resulting embryo is of maternal origin. Consequently, the plant that is propagated from apomictic seed possesses the gene constitution that dictates the characters of the parent plant from which the seed is sourced just like in asexual or vegetative propagation. This is the reason why Gregor Mendel failed to duplicate his results on garden pea. He wanted to show that his observations also apply to other plants but, unfortunately, he chose hawkweed (Hieracium sp.) which is an apomict, or apomictic species.
Apomixis occurs mostly in three plant families: Poaceae or Gramineae (grass family), Rosaceae (rose family), and Asteraceae (sunflower family). Apomictic crops include the Citrus species, mangosteen (Garcinia mangostana), lanzones (Lansium domesticum), and blackberries (Rubus fruticosus).
There are other more types of plant seeds.
Angiosperms or flowering plants have seeds enclosed within fruits, as contrasted to the gymnosperms in which seeds lie exposed on the cone or similar structures. The term angiosperm in fact means “covered seeds” or “enclosed seeds.”
Seeds vary in size, form, shape, texture, color, and chemical composition. Size varies from the extremely small, dust-like seeds of orchids to the giant seeds of the palm Seychelles nut or coco de mer or double coconut (Lodoicea maldivica).
The smallest seeds that are found in orchids can be 0.11 mm long, each weighing less than 0.5 μm. One gram may contain seeds in excess of two million and one capsule (a fruit type) can contain up to 4 million seeds. In contrast, the largest seed, which belongs to the Seychelles nut (actually a one-seeded fruit), can be 50 cm long and weigh up to 20 kg, but takes 7-10 years to mature (Kesseler and Stuppy 2009).
Cereal seeds are rich in carbohydrate; legume seeds in protein. There are seeds also that are rich in oil such as castor bean, jatropha and sunflower. The seeds of the desert shrub jojoba (Simmondsia chinensis) contain a wax that is used as a substitute for sperm whale oil in the production of lotions, shampoos and other cosmetic products, and even machine oil (Postlethwait and Hopson 1989).
Plant seeds can also be classified as to longevity and germination based on their tolerance to drying or dessication and low-temperature storage. Orthodox seeds are tolerant to both drying and low temperature while recalcitrant seeds are sensitive. Those which can be dried to minimal levels of moisture content without significant adverse effect on germination but are sensitive to low temperature storage are called intermediate seeds (click to read Sexual Propagation: Orthodox vs. Recalcitrant Seeds).
(Ben G. Bareja. edited Nov. 2011)
Click to continue reading: The Parts of a Seed and Their Functions