class Dicotyledones vs dicotyledon

class Dicotyledones and dicotyledon both are nouns.

Word	Noun	Adjective	Verb	Adverb
class Dicotyledones	Yes	No	No	No
dicotyledon	Yes	No	No	No

class Dicotyledones (noun)	dicotyledon (noun)
comprising seed plants that produce an embryo with paired cotyledons and net-veined leaves; divided into six (not always well distinguished) subclasses (or superorders): Magnoliidae and Hamamelidae (considered primitive), Caryophyllidae (an early and distinctive offshoot), and three more or less advanced groups: Dilleniidae, Rosidae, Asteridae	flowering plant with two cotyledons; the stem grows by deposit on its outside