senna vs shrub

senna and shrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
senna	Yes	No	No	No
shrub	Yes	No	No	No

As nouns, shrub is a hypernym of senna; that is, shrub is a word with a broader meaning than senna:

senna: any of various plants of the genus Senna having pinnately compound leaves and showy usually yellow flowers; many are used medicinally
shrub: a low woody perennial plant usually having several major stems

Other hypernyms of senna include bush.

senna (noun)	shrub (noun)
any of various plants of the genus Senna having pinnately compound leaves and showy usually yellow flowers; many are used medicinally	a low woody perennial plant usually having several major stems