It's still an adjective. The use of 'the' is not because it becomes a noun but rather because it is part of the superlative form of the adjective.
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AnonymousTrees are known as [the largest of all plants].No, largest is not a noun. Sometimes, what we call 'fusion' occurs between a modifier and the head of a noun phrase and that's what's happening here: the adjective 'largest' serves as a modifier and as head of the bracketed noun phrase at the same time (i.e. they're 'fused'), so the