mon·o·po·di·um 
(mŏn′ə-pōdē-əm)
(mŏn′ə-pōdē-əm)n. pl. mon·o·po·di·a (-dē-ə)
A main axis of a plant, such as the trunk of a spruce, that maintains a single line of growth, giving off lateral branches.
[New Latin, from Late Latin monopodius, one-footed, from Greek monopous : mono-, mono- + pous, pod-, foot; see ped- in the Appendix of Indo-European roots.]
mon′o·podi·al (-dē-əl) adj.