|from the June 27, 2010 Newsletter issued from Hacienda
Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
HYPOGEAL & EPIGEAL SEEDLINGS
Seeds contain stored energy to help the future plant, at germination, get a start in life. When you crack open a corn grain, the white, starchy material making up most of the grain contains that stored energy. When you split a bean, most of the starchy and/or waxy material also contain stored energy for the future plant.
Upon germination, in some plants the energy-bearing part or parts remains in place belowground, but in other plants the energy-bearing material is actually hauled to above the soil's surface, since it's stored in the seedling's first two leaves, the cotyledons.
When the starchy/waxy storage unit stays belowground as roots grow downward from it and stems and leaves grow upward from it, we speak of HYPOGEAL germination. When the energy-holding starchy/waxy stuff is stored in the cotyledons and is raised above the ground's surface, we speak of EPIGEAL germination.
While weeding the organic garden I pulled up two good examples of hypogeal and epigeal germination. You can see them, a hypogeal morning-glory seedling at the left and an epigeal Piich-tree sapling (Enterlobium cyclocarpum of the Bean Family) on the right in the above photo.
Notice the Piic-tree's thick cotyledons, while the morning-glory's first two leaves are thin and veiny like regular leaves.
from the February 17, 2013 Newsletter issued from the
valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the
southern border of the Edwards Plateau, USA
In that picture the seedling on the left is emerging as it should, but the one on the right has a problem, a fairly common one with red-pepper seedlings. The problem is that its first two leaves, or cotyledons, are curled up inside the tan-colored seed coat, which should have stayed in the soil as the cotyledons were withdrawn. Now the leaves can't expand as they need to. Usually this happens because the seed coat for some reason dries out and becomes too stiff to yield to the leaves' efforts to withdraw. Sometimes you can help by pulling the seed coat off the imprisoned leaves. Other times while trying to do this you break off the leaves, not helping matters.
Jalapeño seedlings provide a good example of "epigeal germination" -- their cotyledons are raised above the ground and serve as the plants' first two leaves. The other main form of germination is "hypogeal," in which cotyledons remain inside the seed coat below the ground. It happens that I'm sprouting lentil seeds, which are hypogeal. You can see two hypogeal lentil seeds in my hand showing roots growing downward from the seed and sprouts growing upward, with the two cotyledons remaining within each seed coat below:
Cotyledons of hypogeal seeds like the lentil often provide stored energy for the seeds' growing roots and stems during their early part of their growth before the seedlings begin photosynthesizing their own food.
Most gymnosperms and dicots are epigeal like our jalapeños while most monocots are hypogeal. Earlier it was thought that hypogeal seeds were the most evolutionarily advanced, but in fact both germination strategies can be found within the same plant groups, so that assumption can't be made. For example, both germination types occur among the plums, genus Prunus.