hypocotyl Syllabification: (hy·po·cot·yl)Pronunciation: /ˈhīpəˌkätl, ˌhīpəˈkätl/Definition of hypocotyl noun Botany the part of the stem of an embryo plant beneath the stalks of the seed leaves, or cotyledons, and directly above the root. hypocotyl in other Oxford dictionaries Definition of hypocotyl in the British & World English dictionary