Everything about Cotyledon totally explained
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's 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's often still some variation in cotyledon numbers, for example
Monterey Pine (
Pinus radiata) seedlings have 5–9, and
Jeffrey Pine (
Pinus jeffreyi) 7–13 (Mirov 1967), but other species are more fixed, with for example
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.
Further Information
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