"It is only natural, Sir, that you are surprised by the color of my face."
-- Luigi Ugolini, “The Vegetable Man”
I'm going to go out on a limb here and call Ugolini's story science fiction. Upon finishing "The Vegetable Man" I was reminded of Wyndham's triffids, as well as Wells' "The Food of the Gods and How It Came to Earth". You might think these to be obvious connections, considering the subject matter of the story, but there's more to it. Ugolini's narrative has a sparsity to it that, while certainly weird, is definitely reminiscent of the pulp sf narratives circa the 1910-1920.
That said, "The Vegetable Man" gives us a narrator's attempt at explaining events via science that, arguably, fails to do so. A sense of distrust about our own bodies lies at the core of the story, coupled with notions about unexplored territory (at the time, the Amazon jungle in particular). The notion of invasion is very much front and centre; perhaps an anxiety about what may be found in places untouched by "civilised man", and how such discoveries may alter our perceptions of ourselves. Or perhaps Ugolini is simply writing about our own sense of being, halted and trapped within a perishable body when he writes, "Do you feel strong and fearless enough to endure the sight of something truly terrible?"
Reviewed from The Weird: A Compendium of Strange and Dark Stories, Eds. Anne and Jeff Vandermeer