hypocotyl
noun
hy·po·cot·yl ˈhī-pə-ˌkä-tᵊl 
: the part of the axis of a plant embryo or seedling below the cotyledon see seedling illustration
Word History
Etymology
short for hypocotyledonous (stem), from hypo- + cotyledon + -ous
First Known Use
1880, in the meaning defined above