[s1e9] The Anatomy Lesson -

He is a mass of swamp vegetation that "dreamed" it was Alec after absorbing the scientist's memories and DNA following the lab explosion.

Watch a deep-dive discussion and review of this pivotal episode here: [S1E9] The Anatomy Lesson

To confirm this, Swamp Thing later retrieves Alec Holland’s actual decayed corpse from the water, realizing he is merely a plant mimicry. Subplots & Transformations He is a mass of swamp vegetation that