Bumelia lanuginosa vs buckthorn

Bumelia lanuginosa and buckthorn both are nouns.

Word	Noun	Adjective	Verb	Adverb
Bumelia lanuginosa	Yes	No	No	No
buckthorn	Yes	No	No	No

As nouns, buckthorn is a hypernym of Bumelia lanuginosa; that is, buckthorn is a word with a broader meaning than Bumelia lanuginosa:

Bumelia lanuginosa (noun)	buckthorn (noun)
deciduous tree of southeastern United States and Mexico	any shrub or small tree of the genus Bumelia
	a shrub or shrubby tree of the genus Rhamnus; fruits are source of yellow dyes or pigments
	an Old World plantain with long narrow ribbed leaves widely established in temperate regions