For those who are aficionados of such things, the "Holy Trinity" of Weird Tales authors are generally reckoned as H.P. Lovecraft, Robert E. Howard, and Clark Ashton Smith. Beginning his career as a poet with a macabre and fantastic bent, Smith was encouraged to turn his hand to prose fiction by Lovecraft, with whom he maintained an extensive correspondence over the years (though to be fair, everyone Lovecraft corresponded with ultimately ended up with an enormous volume of letters having been sent back and forth). Smith's work lacks the grim goriness of Howard or the bleak cosmic outlook of Lovecraft, and instead brings a touch of fantasy or whimsy to the darkness; while many of his tales were fairly straight forward stories of ghouls, ghosts and extraterrestrials, quite often his stories would take unusual turns or contain hidden jokes (for example, in "The Seven Geases," the subhuman Voormis dwell in caves on Mount Voormithadreth - which sounds an awful lot like a lisped 'Voormis' address'). Today's story is more of a straightforward piece of macabre adventure, and can be read here.
James Falmer and Roderick Thone, professional orchid hunters, decide to try their hand at another profession -- treasure hunters. Hearing rumors of a crumbling ruin raised by forgotten hands, deep in the jungles of Venezuela, in which unimaginable quantities of gold and silver have been buried, they set off in search with two local guides. Thone is laid up with fever a day's trip from the ruin, and Falmer presses on ahead. He returns withdrawn and taciturn, initially only saying that he'd found the ruin but that legends of treasure were false.
As a fever grips him, Falmer becomes more talkative; he explains, frenzied and horrified, that in the great pit that served the people of that ruin as an ossuary, he encountered the dried remains of some monstrous plant, its roots and vines threaded through multiple human skeletons, apparently having sprouted from the bodies of the dead; brushing against it, he got a face full of some grayish powder, like spores of some sort. And now the pain in his head is becoming unbearable, like something inside trying to get out...
At first glance, this is a fairly straightforward Weird Menace story with human protagonists struggling to survive against a killer plant. But looking beneath the surface suggests that there's a lot more going on here.
First off, it's kind of really left to the reader to decide if there actually is a killer plant that roots itself in the human brain and then threads its vines through the entire body. Both men are stricken with tropical fever during the story, and the killer plant could very well be the result of delirium and hallucinations brought on by disease, dehydration and who knows what else. Is the plant rooting its way through Falmer or is he slowly dying of some nameless disease up a forgotten tributary of the Orinoco? Thone is our viewpoint character here, and he's bedridden with fever from the very beginning of the story - hardly the most reliable of narrators.
But, assuming the plant is real, Smith gives us some tantalizing hints about its nature - and then leaves the reader to draw their own conclusions. Falmer describes the ruins as resembling no civilization's he's seen, and as being like something from another world. Perhaps they are. Perhaps they were raised by inhuman hands in some unimaginably remote epoch of Earth's past, and the plant was something that came with them from some other world to Earth.
Or maybe they're not alien; maybe it's some strange development, some quirk of evolution deep in the jungle that had its chance, didn't quite make it, but left a viable seed pod, buried in the ossuary, waiting to be disturbed and dispersed to try again. In a world where there are plants that smell like rotting meat to attract flies as pollinators and plants that trick prey into climbing into a vat of digestive fluids, whose to say a plant that survives by rooting itself in animal tissue and feeding on that is unrealistic? And after all, there's no reference to the bones the plant had been rooted in being non-human...but, to play devil's advocate, there's also no firm evidence to suggest the people buried in the ruined city are its original builders.
Now I'm just going back and forth and it's making my head hurt.