The Plant Men are a species of vegetable-based creatures with a peculiar appearance. They are native to the fields and woodlands which flourish in the mystical Valley Dor, on the planet Barsoom, and are described as mindless beasts by the First Born. They are terribly aggressive killers and seem to display no signs of fear or even self-preservation during fights, as they tend to launch savagely at their opponents and will stop at nothing until either party is killed.
Their appearance can be described as quasi-humanoid, although not quite, and is physically hideous, according to John Carter. Their skin is blue and mossy, and their mouthless heads are dominated by a single pupil-less white eye, below which there is a single, asymmetrical nostril. The head is covered by hundreds of long hair-like tendrils which move constantly, sensing the environment around them. The tallest members of the species can easily reach on three or even four meters in height. Their blood is described as a viscous dark-green fluid.
On the torso they have two extendable, tentacle-like arms, each terminating on a circular mouth. The legs are almost human, although with enormous feet (described as 90 cm long, wide and flattened) which allow them to jump over long distances. They also have a 2 meters long, robust tail which flattens vertically at the tip and is used as their main weapon during attacks: as they leap wildly into the air, slashing the tail at their victims. They can easily bash a humanoid skull in this fashion.
After the killing, their two mouths are used to suck the body fluids out of a victim, leaving a bloodless corpse behind which is then consumed by the White Apes. The two species appear to have a sort of symbiosis developed, as they will often join forces against an opponent and share the bounty as detailed above. Plant Men are actually omnivorous: besides feeding on blood, they also spend lots of time grazing small plants with their two mouths. They are gregarious creatures which move around in small familial groups, their offspring budding from the adults' armpits.
The Therns place great respect at these creatures, as well as the savage White Apes which are also denizens of the Valley. This is because Therns believe that when one of them dies before reaching a thousand years of age (Martian years), their soul will be transferred to the body of a Plant Man. If the Plant Man too dies before that time, the soul will be transferred to a White Ape; and if the White Ape too meets its demise too soon, then the Thern soul passes into a Silian and is lost forever.
- The Gods of Mars, by Edgar Rice Burroughs (1918) (First appearance)