According to my Peterson trees and shrubs field guide for the northeast, hawthorns are the only simple-leaved plants with non-leafy thorns mostly over 1" long.
It goes on to say that these plants "are virtually indistinguishable as a species to all except the few botanists who have given the genus special study" since they are individually variable and frequently hybridize. "The number of species of hawthorns in this country has been variously determined as over 1000 and as less than 100".
