Hypogeal germination refers to germination in many monocot seeds (e.g maize) and few dicot (e.g broad bean). In this form of germination, the the hypogeal remain short and remain underground, cotyledons give up stored food gradually to the developed plumuleband radicle.  On the other hand, epigeal germination refers to germination in most dicot seed e.g sunflower, cowpea, etc. The cotyledons come above ground to form the first green foilage leaves; leaves start to photosynthesise and supply food to growth region.

hypogeal and epigeal germination
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