AskDefine | Define cotyledons

Extensive Definition

A cotyledon (or "seed leaf") (Greek: κοτυληδών) is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants (angiosperms). Species with one cotyledon are called monocotyledonous (or, "monocots") and placed in the Class Liliopsida. Plants with two embryonic leaves are termed dicotyledonous ("dicots") and placed in the Class Magnoliopsida(Jackson, 08).
The cotyledon of grasses and many other monocotyledons is highly modified leaf composed of a scutellum and a coleoptile. The scutellum is a tissue within the seed that is specialized to absorb stored food from the adjacent endosperm. The coleoptile is a protective cap that covers the plumule (precursor to the stem and leaves of the plant).
Gymnosperm seedlings also have cotyledons, and these are often variable in number (multicotyledonous), with from 2 to 24 cotyledons forming a whorl at the top of the hypocotyl (the embryonic stem) surrounding the plumule. Within each species, there is often still some variation in cotyledon numbers, e.g. Monterey Pine (Pinus radiata) seedlings have 5–9, and Jeffrey Pine (Pinus jeffreyi) 7–13 (Mirov 1967), but other species are more fixed, with e.g. Mediterranean Cypress always having just two cotyledons. The highest number reported is for Big-cone Pinyon (Pinus maximartinezii), with 24 (Farjon & Styles 1997). The cotyledons may be ephemeral, lasting only days after emergence, or persistent, enduring a year or more on the plant. The cotyledons contain (or in the case of gymnosperms and monocotyledons, have access to) the stored food reserves of the seed. As these reserves are used up, the cotyledons may turn green and begin photosynthesis, or may wither as the first true leaves take over food production for the seedling.
Cotyledons may be either epigeal, expanding on the germination of the seed, throwing off the seed shell and become photosynthetic above the ground; or hypogeal, not expanding, remaining below ground and not becoming photosynthetic. The latter is typically the case where the cotyledons act as a storage organ, as in many nuts and acorns.

References

cotyledons in Catalan: Cotiledó
cotyledons in Czech: Děložní lístek
cotyledons in Danish: Kimblad
cotyledons in German: Kotyledon
cotyledons in Spanish: Cotiledón
cotyledons in Esperanto: Kotiledono
cotyledons in French: Cotylédon
cotyledons in Galician: Cotiledón
cotyledons in Croatian: Supka
cotyledons in Italian: Cotiledone
cotyledons in Georgian: ლებანი
cotyledons in Malay (macrolanguage): Kotiledon
cotyledons in Dutch: Zaadlob
cotyledons in Polish: Liścień
cotyledons in Portuguese: Cotilédone
cotyledons in Finnish: Sirkkalehti
cotyledons in Swedish: Hjärtblad
cotyledons in Chinese: 子叶
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