cowpea vs leguminous plant

cowpea and leguminous plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
cowpea	Yes	No	No	No
leguminous plant	Yes	No	No	No

As nouns, leguminous plant is a hypernym of cowpea; that is, leguminous plant is a word with a broader meaning than cowpea:

cowpea: sprawling Old World annual cultivated especially in southern United States for food and forage and green manure
leguminous plant: an erect or climbing bean or pea plant of the family Leguminosae

Other hypernyms of cowpea include legume.

cowpea (noun)	leguminous plant (noun)
eaten fresh as shell beans or dried	an erect or climbing bean or pea plant of the family Leguminosae
sprawling Old World annual cultivated especially in southern United States for food and forage and green manure
fruit or seed of the cowpea plant