Although the idea of magical and possibly animate trees does exist in mythology and folklore, the idea of a race of such beings, in the form of ents, seems to be original to Tolkein. He took the name from a variant form of "ettin", the original English word for what we now call a "giant", and not from any pre-existing tree-person. (Other giants exist in Tolkien's world, but they are very much bit players unlike, say, the giants of Narnia). D&D 0E included ents in its original rulebook, but legal problems led to them being renamed as "treants" from 1E onwards. For similar copyright reasons, it's this newer name that has stuck in fantasy games in general, even though, really, there isn't much difference between the D&D version and their entish inspiration.
1E
We're told that the 1E treant is indistinguishable from a tree until it moves, but this is hard to credit from the picture. Granted, if you're not looking too closely at the trees you're passing, and you're in a heavily forested environment, perhaps even in dim light... I mean, sure, you might not notice, because they're certainly camouflaged. But "indistinguishable" is a stretch, and more so for this version than any other.
Rather than having a single trunk, for instance, the lower part of the treant's body is divided into two sturdy legs, each ending in three long and splayed toes. The arms are positioned roughly where they would be on a humanoid, and of a similar shape, with clear elbows, wrists and so on. The hands have three fingers and a thumb. There is no neck, but the region above the "shoulders" includes a pair of human-like eyes and a long nose. It's unclear if there's intended to be a mouth, and no explicit statement that treants can speak, although it's what we'd expect if they're based on ents.
All of this indicates a certain fixity of form, the sort of defined shape that (most) animals have and that trees and most other plants tend not to. The only semi-random, plantlike, part of the body form is the collection of short leafy branches arising from the upper body, creating a far smaller and shorter crown than we'd expect for a tree with a "trunk" so thick. A leafy sheet also covers the body below the nose, presumably as an allusion to Treebeard in
Lord of the Rings.
Treants are remarkably tough and, while we can't compare their strength with that of giants in this edition, they're clearly intended to be in the same range. The fact that we're specifically told they can destroy buildings is surely another nod to their literary inspiration. Their armour class is better than steel, which bark and even the hardest of woods aren't. This may be intended to reflect their lack of vital organs and the general thickness of the wood compared to steel armour. Still, you'd think it would be easier to hack a branch off with an axe than it would be to damage an iron golem - and it isn't.
Whether or not they can speak, treants are as intelligent as humans. They live in groups of up to twenty, but there's no indication of any social structure, implying a very egalitarian and loose society, which fits with their stated alignment. Oddly, we're told that they live in caves - albeit caves in woodland - which is a rather un-treelike thing to do.
While the treant in 2E is still distinguishable from the average tree, it's notably less so than its predecessor. It doesn't seem to have legs at all, although there are feet projecting from the base of the trunk, with woody rootlike toes. The arms don't seem to arise opposite each other, instead taking the form of a pair of lower branches that happen to end in a splayed set of elongated finger-like twigs. These, and the toes, have the sort of irregular shape we'd expect of the branches of a tree, and the creature is entirely treelike above the face.
The face is much lower down on the body, proportionately speaking, enhancing the non-humanoid look. The eyes are less obviously humanlike, too, and the "nose" is actually a branch, complete with twigs and leaves. They obviously do have a mouth this time, and we're told that they have their own language and have some ability to converse in others. There is also considerably more information about their biology and lifecycle... and no mention that they lair in caves.
3E
The 3E version is intermediate between the two previous ones in form. The body is more humanoid than in 2E, with identifiable legs and hands with short, leafless, fingers, rather than twiggy branches. On the other hand, the limbs do have a more plantlike appearance, with an irregular number of joints and digits, and the "toes" being particularly twisted and rootlike. There are narrow branches jutting off from the arms a well as the larger ones on the crown, but nothing like the leaf-bearing "nose" of 2E. Most significantly, however, this treant has a neck and an approximately human-like head and face - although it does seem to lack visible ears.
The more detailed stats in this edition allow us to see that the treant is as strong as a frost giant - although it's twice as tall, and almost twice as heavy. The fact that it now has a damage reduction rating (except against things like axes) makes it harder to argue that the bark isn't really as strong as steel, since any lack of vital organs is surely covered by the reduction. One can only assume that the bark is in some way magical, at least so long as it is part of the living treant.
Treants live in much smaller communities than before, but their alignment has shifted towards something we'd expect to imply more of a social structure. Perhaps because of those smaller communities, however, we don't see that - possibly a low reproduction rate may force this on them, or their biology otherwise makes it difficult for too many of them to survive in a given area of forest.
5E
There's something a further shift back towards the 1E look in this edition. The limbs and body are more regular in form, and the neck has disappeared again. The most notable difference is perhaps the number of arms; it has four, but they're irregularly arranged, with the implication that a specific number might not be standard. The fingers are claw-like, rather than the blunt structures of 1E and 3E or the slender twigs of 2E. Small leafy branches also seem to project from the trunk, although the main leaf-bearing branches sprout from above and behind the head as before.
The strength remains on a par with a frost giant, but the bark is no longer more impressive than steel - if probably still rather tougher than we might expect regular bark to be. The alignment has changed back to its original descriptor and, if anything, it's implied that treant society is even more nebulous and solitary than it was before. Treants speak a number of languages in this edition, but we're not told which is the one that they use amongst themselves - either Druidic or Sylvan seems likely but would presumably have to be instinctive since it's not obvious how they would be taught it, given the changes in their lifecycle from 2E.
Whereas even demons and efreet appear to be, at least superficial, members of the Animal Kingdom, the treant is indisputably a plant. This has significant implications for how it might work biologically, although the answer to this is largely that, in the real world, it wouldn't.
The primary issue here is that real plants lack the ability to be fully active to the extent that treants are supposed to be. It's not that they can't move, since creepers and so on can feel their way about - albeit very slowly. Some plants can even move rapidly, with the Venus flytrap probably being the next known example, but sustained motion and complex coordinated movement are a different matter. Even then, the movement is achieved in soft tissue whereas the treant is largely composed of wood. Presumably, the xylem vessels contract like muscle fibres, which would require some rather radical molecular and biochemical rearrangement of their structure to be at all plausible if it's more than just "magic". However it's done, it's likely that these woody fibres form structures similar to humanoid muscles, since they have to move in the same way, and most illustrations show some hint of joints in the appropriate places.
Getting the energy and metabolism to move these pseudo-muscles is also difficult to resolve. Treants have noses, so it's at least possible that they have lungs, composed of some soft spongy plant material full of air spaces, with which to absorb appropriate quantities of oxygen. But it's perhaps more likely that they breathe, as plants do, through their leaves, probably with some magical boosting of the effectiveness of their chlorophyll. This would leave the nose as a purely sensory organ.
There's nothing to say that treants don't have a sense of smell, and it seems reasonable that the nose isn't purely aesthetic. The idea of the nose as a chemosensory pit with no connection to a respiratory system isn't even a unique one, since it's pretty much how fish work and, for that matter, some invertebrates.
3E states that vegetable creatures, such as treants, "eat" and if that's literally true, there has to be a digestive system of some kind. But this is more ambiguous in other systems; while the tendriculos of 3E undoubtedly eats, it's less clear that treants do so, unless we count merely absorbing nutrients from the soil. If this is so, the treant's mouth must be purely used for communication, perhaps attached to a simple set of bellows in the trunk to allow air to form sounds, but no stomach or intestines. Additional nutrients are obtained from the soil, since at least 2E and 5E agree that treants can temporarily let down roots and anchor themselves to the ground.
Once we've got rid of both the digestive and respiratory systems, many other organs also cease to be required. We can probably do without dedicated excretory organs, with the leaves, and possibly roots/toes, taking on this function too. We certainly don't need a liver or pancreas and there's clearly no skeleton.
Whether we'd need a circulatory system is perhaps more ambiguous, given the need to transport oxygen and so forth round the body rather more urgently than sessile plant needs to. The circulatory fluid would be sap, since that's largely what sap is for in plants. and to get it around such a large body rapidly, a proper system of arteries and veins would realistically be required, which, in turn, implies a woody heart somewhere inside the trunk. But, if this is the case, sap would spray out of a treant like blood from an artery when it is cut, which just seems odd. So, again, we may have to rely on magic to explain the level of effectiveness that the sap has, and, if there's no high "blood" pressure then we can also do without the heart.
One thing we probably do need is a nervous system. We can say this mainly because treants are consistently shown as having eyes, and even if these aren't composed of what we'd expect at a cellular level, they are complex structures that it's hard to imagine aren't attached to something. These would have to be modified xylem or (perhaps more likely) phloem cells and spread throughout the body. Somewhere there must be a dense mass of such cells forming the treant's brain, and it's probably just behind the eyes. On the other hand, this need not be at all structured like an animal brain, and it could be somewhat decentralised, emphasising the general lack of vital organs that make a treant so difficult to kill with spears or maces.
The treant's signature power is the ability to animate other trees, rather like the huorns of Lord of the Rings. This is clearly a magical power, so there isn't much to say about it specifically. But we are left with the issue of treant reproduction.
We're given two different versions of this, in 2E and 5E. The latter states that treants are literally born as trees, and only later transform into the mobile, thinking, version - retaining the ability to revert if necessary. Although it's not explicitly states, the obvious implication is that reproduction occurs when they are in tree form, using buds, flowers, seeds, and so on and that, until they begin to transform, there isn't any real difference between them and other trees.
This limits the need for "entwives" since the great majority of tree species produce both pollen and seeds and are thus (from an animal perspective) hermaphrodites. Nonetheless, some trees, such as willow and teak do have separate sexes - a given plant is either male or female - so this might be true of the relevant kinds of treant, too.
Indeed, if they are originally regular trees, it follows that all treants have an identifiable species; there are oak-treants, beech-treants, and so on. There does seem to be some limitation here as to which species are able to become treants. All the examples we see in the core rulebooks are broadleaved hardwoods and not, say, conifers. (Saying a tree is "hardwood", incidentally, really just means that it uses flowers for reproduction; the wood is usually harder than "softwood"... but it doesn't have to be and there are some notable exceptions). There's also, perhaps unsurprisingly, an apparent minimum size, since there's no indication of woody shrub treants (azaleas, say) but the typical stats imply a maximum as well. A sequoia-treant would surely be different from what's described!
2E, however, states that treants are a distinct type of being that may end their lives by transforming into a regular tree but don't start out that way. Distinct males and females exist, with the latter bearing young by forming a stalk from the side of their trunk which eventually splits away - something not unlike pregnancy. We don't know how they mate, although pollination seems the most likely explanation.
This means that a treant does not necessarily have to mirror a particular tree species, although they may well do so anyway. The fact that they are all broadleaved trees here may therefore be less significant, just a description of how they look. We're told that, at least in temperate climes, their leaves turn golden in the autumn, but that they don't fall out - or, if they do, they're replaced as they go, rather like a mammal moulting. If these are the organs through which a treant obtains its oxygen and most of its nutrition, it makes sense that it wouldn't lose them, although turning yellow does rather imply a lack of chlorophyll which, by rights, should have the same effect as losing the leaf. This should force the treant into hibernation through the winter, which is effectively what real-world deciduous trees are doing... but apparently, it doesn't.
So, magic again.
