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Seedling
From Wikipedia, the free encyclopedia
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For other uses, see
Seedling (disambiguation).
Monocot (left) and dicot (right) seedlings
A seedling is a young plant
sporophyte developing out of a plant embryo from a
seed. Seedling development starts with
germination of the seed. A typical young seedling consists of three main parts: the
radicle (embryonic root), the
hypocotyl (embryonic shoot), and the
cotyledons (seed leaves). The two classes of
flowering plants are distinguished by their numbers of seed leaves:
Monocotyledons (monocots) have one blade-shaped cotyledon, whereas
dicotyledons (dicots) possess two round cotyledons.
Gymnosperms are more varied. For example,
pine seedlings have up to eight cotyledons. The seedlings of some flowering plants have no cotyledons at all. These are said to be
acotyledons.
Contents
Germination and early seedling development
Development of an
angiosperm (
maple) seedling
Development of a
gymnosperm (
Douglas Fir) seedling
Main article:
Germination
During germination, the young plant emerges from its protective seed coat with its radicle first, followed by the cotyledons. The radicle orients towards gravity, while the hypocotyl orients away from gravity and elongates through cell expansion to push the cotyledons out of the ground.
Photomorphogenesis and etiolation
Main articles:
Photomorphogenesis and
Etiolation
Dicot seedlings grown in the light develop short hypocotyls and open cotyledons exposing the
epicotyl. This is also referred to as
photomorphogenesis. In contrast, seedlings grown in the dark develop long
hypocotyls and their cotyledons remain closed around the epicotyl in an apical hook. This is referred to as skotomorphogenesis or
etiolation. Etiolated seedlings are yellowish in color as
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