Hypogeal
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In botany, a seed is described as hypogeal when the cotyledons of the germinating seed remain non-photosynthetic, inside the seed shell, and below ground.
The converse, where the cotyledons expand, throw off the seed shell and become photosynthetic above the ground, is epigeal.
Also a layer just below the surface such as the biological film in slow sand filter beds used in water purification works.