Editing Seed (section)

== Structure ==
[[File:Dycotyledon seed diagram-en.svg|thumb|The parts of a [[bean]] seed (a [[dicot]]), showing the seed coat and [[embryo]]]]
[[File:Budowa nasienia-dwuliscienne.svg|thumb|upright|Diagram of the internal structure of a [[dicot]] seed and embryo: (a) seed coat, (b) [[endosperm]], (c) [[cotyledon]], (d) [[hypocotyl]]]]

A typical seed includes two basic parts:
# an [[embryo]];
# a seed coat.

In addition, the '''[[endosperm]]''' forms a supply of nutrients for the embryo in most monocotyledons and the endospermic dicotyledons.

=== Seed types ===

Seeds have been considered to occur in many structurally different types (Martin 1946).<ref name="Gerhard Leubner-2015">{{citation |url=http://www.seedbiology.de/evolution2.asp |title=The Seed Biology Place |publisher=Gerhard Leubner Lab, Royal Holloway, University of London |access-date=13 October 2015 |url-status=live |archive-url=https://web.archive.org/web/20150924100016/http://www.seedbiology.de/evolution2.asp |archive-date=24 September 2015 }}</ref> These are based on a number of criteria, of which the dominant one is the embryo-to-seed size ratio. This reflects the degree to which the developing cotyledons absorb the nutrients of the endosperm, and thus obliterate it.<ref name="Gerhard Leubner-2015" />

Six types occur amongst the monocotyledons, ten in the dicotyledons, and two in the gymnosperms (linear and spatulate).<ref>{{cite book|url=https://books.google.com/books?id=uGJL_Ys6wlQC|title=Carol C. Baskin, Jerry M. Baskin. Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination. Elsevier, 2001|isbn= 978-0-12-080263-0 |page=27|via=google.ca|last1=Baskin|first1=Carol C.|last2=Baskin|first2=Jerry M.|year=2001|publisher=Elsevier }}</ref> This classification is based on three characteristics: embryo morphology, amount of endosperm and the position of the embryo relative to the endosperm.

[[File:Monocot dicot seed.svg|thumb|Diagram of a generalized dicot seed (1) versus a generalized monocot seed (2). A. [[Scutellum (botany)|Scutellum]] B. [[Cotyledon]] C. Hilum D. Plumule E. Radicle F. [[Endosperm]]]]
[[File:Comparison of Monocotyledons and Dicotyledons.png|thumb|Comparison of [[monocotyledons]] and [[dicotyledons]]]]

=== Embryo ===
In endospermic seeds, there are two distinct regions inside the seed coat, an upper and larger endosperm and a lower smaller embryo. The [[embryo]] is the fertilised ovule, an immature [[plant]] from which a new plant will grow under proper conditions. The embryo has one [[cotyledon]] or seed leaf in [[monocotyledon]]s, two cotyledons in almost all [[dicotyledon]]s and two or more in gymnosperms. In the fruit of [[cereal|grains]] (caryopses) the single monocotyledon is shield shaped and hence called a '''[[scutellum (botany)|scutellum]]'''. The scutellum is pressed closely against the endosperm from which it absorbs food and passes it to the growing parts. Embryo descriptors include small, straight, bent, curved, and curled.

=== Nutrient storage ===
<!-- Deleted image removed: [[File:Seed layers.jpg|thumb|Layers within an endospermic maize seed]] -->
Within the seed, there usually is a store of [[nutrient]]s for the [[seedling]] that will grow from the embryo. The form of the stored nutrition varies depending on the kind of plant. 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]]. It is usually [[triploid]], and is rich in [[Vegetable oil|oil]] or [[starch]], and [[protein]]. In gymnosperms, such as [[Pinophyta|conifers]], the food storage tissue (also called endosperm) is part of the female [[gametophyte]], a [[haploid]] tissue. The endosperm is surrounded by the [[aleurone]] layer (peripheral endosperm), filled with proteinaceous aleurone grains.

Originally, by analogy with the animal [[ovum]], the outer nucellus layer ([[perisperm]]) was referred to as [[albumen]], and the inner endosperm layer as vitellus. Although misleading, the term began to be applied to all the nutrient matter. This terminology persists in referring to endospermic seeds as "albuminous". The nature of this material is used in both describing and classifying seeds, in addition to the embryo to endosperm size ratio. 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 (plant)|Croton]]'' and [[Poppy]]. The endosperm is called "horny" when the cell walls are thicker such as [[Date palm|date]] and [[Coffee bean|coffee]], or "ruminated" if mottled, as in [[nutmeg]], [[Palm (plant)|palms]] and [[Annonaceae]].<ref>{{cite web|url=https://books.google.com/books?id=waAMAAAAYAAJ&pg=PA155|title=The Encyclopædia Britannica, 9th ed. (1888) vol. 4 |page=155|work=google.ca|year=1888 }}</ref>

In most monocotyledons (such as [[Poaceae|grasses]] and [[Arecaceae|palms]]) and some ('''endospermic''' or '''albuminous''') dicotyledons (such as [[castor bean]]s) the embryo is embedded in the endosperm (and nucellus), which the seedling will use upon [[germination]]. In the '''non-endospermic''' dicotyledons the endosperm is absorbed by the embryo as the latter grows within the developing seed, and the cotyledons of the embryo become filled with stored food. At maturity, seeds of these species have no endosperm and are also referred to as '''exalbuminous''' seeds. The exalbuminous seeds include the [[legumes]] (such as [[bean]]s and [[pea]]s), trees such as the [[oak]] and [[walnut]], vegetables such as [[Squash (vegetable)|squash]] and [[radish]], and [[sunflower]]s. According to Bewley and Black (1978), Brazil nut storage is in hypocotyl and this place of storage is uncommon among seeds.<ref>Bewley & Black (1978) Physiology and Biochemistry of Seeds in Relation to Germination, pag.11</ref> All gymnosperm seeds are albuminous.

=== Seed coat ===
{{see also|Scarification (botany)}}
[[File:Pomegranate arils.jpg|thumb|Seed coat of pomegranate]]

The seed coat develops from the maternal tissue, the [[integument]]s, originally surrounding the ovule. The seed coat in the mature seed can be a paper-thin layer (e.g. [[peanut]]) or something more substantial (e.g. thick and hard in [[honey locust]] and [[coconut]]), or fleshy as in the [[sarcotesta]] of [[pomegranate]]. The seed coat helps protect the embryo from mechanical injury, predators, and drying out. Depending on its development, the seed coat is either '''bitegmic''' or '''unitegmic'''. Bitegmic seeds form a testa from the outer integument and a tegmen from the inner integument while unitegmic seeds have only one integument. Usually, parts of the testa or tegmen form a hard protective mechanical layer. The mechanical layer may prevent water penetration and germination. Amongst the barriers may be the presence of [[lignified]] [[sclereids]].<ref>{{cite web|url=http://5e.plantphys.net/article.php?ch=23&id=8&search=seed|title=Sinauer Associates, Inc., Publishers|website=5e.plantphys.net|access-date=7 May 2018|url-status=live|archive-url=http://archive.wikiwix.com/cache/20140122033254/http://5e.plantphys.net/article.php?ch=23&id=8&search=seed|archive-date=22 January 2014}}</ref>

The outer integument has a number of layers, generally between four and eight organised into three layers: (a) outer epidermis, (b) outer pigmented zone of two to five layers containing [[tannin]] and starch, and (c) inner epidermis. The '''endotegmen''' is derived from the inner epidermis of the inner integument, the '''exotegmen''' from the outer surface of the inner integument. The '''endotesta''' is derived from the inner epidermis of the outer integument, and the outer layer of the testa from the outer surface of the outer integument is referred to as the '''exotesta'''. If the exotesta is also the mechanical layer, this is called an exotestal seed, but if the mechanical layer is the endotegmen, then the seed is endotestal. The exotesta may consist of one or more rows of cells that are elongated and pallisade like (e.g. [[Fabaceae]]), hence 'palisade exotesta'.<ref>{{cite web|url=http://archive.gramene.org/db/ontology/search?id=PO:0006048|title=plant_anatomy Term "seed coat epidermis" (PO:0006048)|work=gramene.org|url-status=live|archive-url=https://web.archive.org/web/20140203143705/http://archive.gramene.org/db/ontology/search?id=PO:0006048|archive-date=2014-02-03}}</ref><ref>{{cite book|title=6 – Seed and fruit – University Publishing Online – Paula J. Rudall. Anatomy of Flowering Plants: An Introduction to Structure and Development. Third edition|publisher=Cambridge University Press |date=2007|isbn=978-0-521-69245-8|doi=10.1017/CBO9780511801709|last1=Rudall|first1=Paula J.}}</ref>

In addition to the three basic seed parts, some seeds have an appendage, an '''[[aril]]''', a fleshy outgrowth of the funicle ([[ovule|funiculus]]), (as in [[Taxus|yew]] and [[nutmeg]]) or an oily appendage, an '''[[elaiosome]]''' (as in ''[[Corydalis]]''), or hairs (trichomes). In the latter example these hairs are the source of the textile crop [[cotton]]. 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.

A scar also may remain on the seed coat, called the '''[[Hilum (biology)|hilum]]''', where the seed was attached to the ovary wall by the funicle. Just below it is a small pore, representing the micropyle of the ovule.

=== Size and seed set ===
[[File:Seed variety.jpg|thumb|A collection of various vegetable and herb seeds]]
Seeds are very diverse in size. The dust-like orchid seeds are the smallest, with about one million seeds per gram; they are often embryonic seeds with immature embryos and no significant energy reserves. Orchids and a few other groups of plants are [[myco-heterotrophy|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.<ref>Smith, Welby R. 1993. ''Orchids of Minnesota''. Minneapolis: University of Minnesota Press. p. 8.</ref> At up to 55 pounds (25 kilograms) the largest seed is the ''[[coco de mer]]''(Lodoicea maldivica).<ref>{{cite journal |last=Blackmore |first=Stephen |author2=See-Chung Chin |author3=Lindsay Chong Seng |author4=Frieda Christie |author5=Fiona Inches |author6=Putri Winda Utami |author7=Neil Watherston |author8=Alexandra H. Wortley |date=2012 |title=Observations on the Morphology, Pollination and Cultivation of Coco de Mer (''Lodoicea maldivica'' (J F Gmel.) Pers., Palmae) |journal=Journal of Botany |volume=2012 |pages=1–13 |doi=10.1155/2012/687832 |doi-access=free}}</ref> This indicates a 25 Billion fold difference in seed weight. Plants that produce smaller seeds can generate many more seeds per flower, while plants with larger seeds invest more resources into those seeds and normally produce fewer seeds. Small seeds are quicker to ripen and can be dispersed sooner, so autumnal blooming plants often have small seeds. 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. Herbaceous perennials and woody plants often have larger seeds; they can produce seeds over many years, and larger seeds have more energy reserves for germination and seedling growth and produce larger, more established seedlings after germination.<ref>{{cite journal|journal=Plant Ecology|volume=194|issue=2|pages=149–156|last=Kosinki|first=Igor|title=Long-term variability in seed size and seedling establishment of ''Maianthemum bifolium''|doi=10.1007/s11258-007-9281-1|year=2007|bibcode=2007PlEco.194..149K |s2cid=31774027}}</ref><ref>{{cite journal |author1=Shannon DA |author2=Isaac L |author3=Brockman FE |title=Assessment of hedgerow species for seed size, stand establishment and seedling height |journal=Agroforestry Systems |volume=35 |issue=1 |pages=95–110 |date=February 1996 |doi=10.1007/BF02345331 |bibcode=1996AgrSy..35...95S |s2cid=2328584 }}</ref>