Epigeal germination is a type of germination whereby the seed leaves or the cotyledons are brought on to the surface or above the soil along with the shoot during germination. This is usually due to rapid elongation and curved-like shape of the hypocotyl. This change in the shape or curvature of the hypocotyl allows the seed leaves or the cotyledons to come above the surface of the soil. The hypocotyl is the part of the stem of an embryo plant beneath the stalk of the cotyledons. After the cotyledons have come above the soil surface, the hypocotyl straightens resulting to falling off, of seed coat and with time the appearance of the cotyledons change to green.
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Angiosperm seedlings fall into two main classes with respect to the fates of their cotyledons during the growth of the axis. Seedlings that raise their cotyledons above the soil surface are said to be epigeal, while those whose cotyledons remain in the soil are termed hypogeal. The position of the elongating axis relative to the site of attachment of the cotyledons generally determines whether the seedling will be epigeal or hypogeal.
For example, the initial elongating region of the seedling shoot axis in garden bean Phaseolus , castor bean Ricinus , and onion Allium is the hypocotyl, located below the cotyledonary node Web Figure Thus, when the hypocotyl elongates it lifts the cotyledon s above the soil. The internode immediately above the cotyledonary node is called the epicotyl. In most epigeal seedlings both the hypocotyl and epicotyl elongate. In contrast, only the epicotyl elongates in hypogeal seedlings, such as in pea Pisum.
The seeds of members of the grass family are also hypogeal, although the anatomy of the shoot axis is somewhat different. For example, in maize Zea the axis directly above the modified cotyledon, or scutellum, is called the mesocotyl see Web Figure A, part e. For deeply planted seeds, the mesocotyl helps to raise the shoot closer to the soil surface. Web Figure A—C: epigeal germination; D—E: hypogeal germination.
An example of a plant with epigeal germination is the common bean Phaseolus vulgaris. The opposite of epigeal is hypogeal underground germination. Epigeal is also not the same as hypogeal germination; both epigeal and hypogeal plants will grow differently. Epigeal germination implies that the cotyledons are pushed above ground. The hypocotyl elongates while the epicotyl remains the same in length. In this way, the hypocotyl pushes the cotyledon upward.
An example of a plant with hypogeal germination is the pea Pisum sativum. The opposite of hypogeal is epigeal above-ground germination. Hypogeal germination implies that the cotyledons stay below the ground. The epicotyl part of the stem above the cotyledon grows, while the hypocotyl part of the stem below the cotyledon remains the same in length. In this way, the epicotyl pushes the plumule above the ground.
10 Difference between Epigeal And Hypogeal Germination (With Comparison Chart)