lengthen vs elongate

lengthen is not an adjective while elongate is an adjective.

Word	Noun	Adjective	Verb	Adverb
lengthen	No	No	Yes	No
elongate	No	Yes	Yes	No

As verbs, elongate is a hyponym of lengthen; that is, elongate is a word with a more specific, narrower meaning than lengthen:

Other hyponyms of lengthen include draw out, extend, prolong, protract, stretch.

lengthen (adjective)	elongate (adjective)
	having notably more length than width; being long and slender
	(of a leaf shape) long and narrow

lengthen (verb)	elongate (verb)
become long or longer	make long or longer by pulling and stretching
make longer