Frutescens means shrubby, bushy [Stearn] and Gleditsia frutescens is probably a shrubby variety, possibly a nursery name for a garden variety of one of the commonly grown species described here.  Perhaps the most likely is Gleditsia sinensis Lam. var. nana, syn. Gleditsia nana Hort. ex Dippel, a dwarf form probably introduced to cultivation between 1840, when Paxton’s Dictionary was originally published, and 1849 when a second edition was published with a supplement.