AskDefine | Define cotyledon

Dictionary Definition

cotyledon n : embryonic leaf in seed-bearing plants [syn: seed leaf]

User Contributed Dictionary

English

Pronunciation

  • IPA: /kɑt.l.id.n/

Noun

  1. The leaf of the embryo of a seed-bearing plant; after germination it becomes the first leaves of the seedling.

Synonyms

Derived terms

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

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