Zizyphus Spina-Christi

from The Collaborative International Dictionary of English v.0.48

Christ's-thorn \Christ's-thorn`\, n. (Bot.)
   One of several prickly or thorny shrubs found in Palestine,
   especially the {Paliurus aculeatus}, {Zizyphus
   Spina-Christi}, and {Zizyphus vulgaris}. The last bears the
   fruit called {jujube}, and may be considered to have been the
   most readily obtainable for the Crown of Thorns.
   [1913 Webster]