Critters II!

[Metastachydium]

yup, I do 5e stuff too

yup, I do 5e stuff too

Cool. Let's stick to 3.5 here, though.
Here's my thing:
Thamnads are basically shrubs, as in literally. Nice, thick thornless evergreen shrubs up to 7' tall and as wide as a Medium Plant can get with nice little oval-shaped leafs (think of something like those of a Japanese barberry). Thamnads are, however, sapient shrubs and capable of assuming a humanoid form via rearranging their tissues into such a shape (this means that the mass and composition (as far as tissue types are concerned) of a thamnad always remains unchanged). This is supposed to make them more pleasant to the humanoid eye and to help them blend in better with bipedal folks, but it doesn't really work, mostly since most humanoids find them at least somewhat creepy for obvious reasons. The human form of a thamnad tends to resemble a slender human late teenage girl (roughly 4'10 to 5'2 tall) with a smooth green skin, small, weirdly round earlobes and soft, longish almost vinelike little branches with leaves for hair (tends to here means `in more than 80% of all cases`; less than 15% of all thamnads looks more like a weird androgynous child of the same height, and less than 5% of all thamnads looks like an overgrown long branch-haired leafy-bearded gnome instead; regardless of their appearences, all thamnads are sexless). Thamnads have ears, eyes and a nose, but don't actually need them. They have cells sensitive to light, vibration and various chemicals all around their green parts (mostly in their „skin” and „hair”), so they kind of see, hear and smell with (largely) their entire body. While this gives them a 360° vision, among other things, they are deuteranopic (fun fact: actual plants actually don't tend to be sensitive to medium vawelength light) and their vision and hearing have a limited range as compared to that of other creatures. This also means that while they can blink, they have an unsettling tendency to forget that they should be doing that.
Their equivalent of a nervous system (a web of electric, hormonal and otherwise chemical signaling pathways) is likewise massively decentralised, which means that lopping off, say, their head will not achieve much (the worst that or losing any larger chunk of their body can do is a partial memory loss) (I guess no criticals covers this one). They cannot bleed to death, either (special ability that automatically stabilizes them when they sink below 0 hp?). Nevertheless, they will die if they get burned or otherwise fried, digested (or otherwise exposed to a bit too much corrosive stuff), devoured by the horde flock (they are pathologically afraid of herbivores, and especially goats, not to mention goat people), exposed to extreme temperatures and the like. Hanging or crucifixion will likewise often kill them through dehydration.
Thamnads are autotrophous and only need light, water and minerals to survive. They take up water and minerals through their roots; when in human form, their roots are located in a cavity within their torso, well-protected and invisible from the outside. Their „mouth” leads into this cavity. When in humanoid form, they will, therefore swallow all manners of weird decaying matter, including manure and pieces of rotten carcasses if available, washing these down with copious amounts of water. Those travelling with a thamnad tend to learn quite quickly to stay away from a thamnad's bottles and waterskins.
Since they need as much of their bodies to be exposed to light as possible, while they do wear clothes when in humanoid form, these tend to be none too heavy, sleeveless and knee-length at most.
Thamnads do not have reproductive organs and cannot hybridise with other species. They reproduce asexually: when thamnads wish to reproduce themselves, they switch to stationary form and extend up to four branches of their roots away from themselves under the ground. They then redirect nutrients into these roots and eventually a new stem starts to develop from these. The offsprings of a thamnad are basically clones, and they oftentimes retain some of the mother plant's memories. These offsprings remain attached to the mother plant until they grow large enough to sustain themselves and until the mother plant teaches them, among other things, how to recognize the specific patterns of vibration that constitute the languages the thamnad knows (thamnads are otherwise surprisingly quick learners and they can store quite a lot of data in their body). Once that is said and done, the mother plant severs the connection, switches to humanoid form and leaves the offsprings to their own devices (thamnads do not build societies of their own).
It takes roughly two years for a thamnad to reach full maturity (and gain the ability to rearrange themselves). They can live up to 50 years.
Thamnads are largely harmless. They are ridiculously slow (low base speed) and perfectly useless in physical combat. When feeling in danger, they will often instinctively revert to stationary form to stand out less (this should be a standard action). They are inherently magical, but most thamnads are not capable of doing much else with their magic other than rearranging themselves (i.e. switching between stationary and humanoid form) and producing speech (they lack speech organs) which sounds like relatively high-pitched, strangely loud whispering (joke level spell-like abilities?). This is also the main reason why they want to blend in with humanoids: they often seek to learn how to use stronger magic (arcane or divine) from them and they hope to acquire faithful traveling companions who can protect them from goats when they roam about the lands, which they do a lot (partly since they don't like extreme temperature and tend to migrate towards the closest pole to avoid hot summers, and flee towards the equator from colder seasons). To help with that, they tend to exhibit an exceedingly lawful behaviour when around others.
While they don't always get along with humanoids (their utter lack of social skills might be the reason why), they like other plants and other plants like them back. They can communicate with any other plant (including plant creatures) and they do not even need magic for that (they use chemicals and electric impulses; (so this is more of an extraordinary material than Speak with Plants at will)), but only when in stationary form (in which form they can also regrow lost „body parts” and heal faster).
I'd like them to be written up as a playable race as well, if that's not a problem. I think I'd give them a high STR penalty (they are shrubs), a DEX bonus (plants are flexible), a high INT bonus (they are smart shrubs quick learners) and a CHA penalty (they behave… weirdly), no weapon and armour proficiencies and as many plant immunities as you can fit. Favoured class could be Wizard. No level adjustment, please.