monocotyledon vs monocotyledonous

monocotyledon is a noun but monocotyledonous is not a noun.

Word	Noun	Adjective	Verb	Adverb
monocotyledon	Yes	No	No	No
monocotyledonous	No	Yes	No	No

monocotyledon (noun)	monocotyledonous (noun)
a monocotyledonous flowering plant; the stem grows by deposits on its inside

monocotyledon (adjective)	monocotyledonous (adjective)
	(of a flowering plant) having a single cotyledon in the seed as in grasses and lilies