Hypogeal

Hypogeal means "underground".

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.^[1] The converse, where the cotyledons expand, throw off the seed shell and become photosynthetic above the ground, is epigeal.
In water purification works, the hypogeal (or Schmutzdecke) layer is a biological film just below the surface of slow sand filters. It contains microorganisms that remove bacteria and trap contaminant particles.

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