By the time we encounter them in 'the two towers,' the ents of Fangorn are the last of their species, and are for the most part old even by the measure of their kind. Their march on Isengard, in an earlier age, would have likely been a task for the young and strong among them. As it happens, it falls to the last of them still 'awake.' As Treebeard tells us, many of his people have 'gone sleepy' and no longer move or speak but stand inert deep in their forests, indistinguishable from the trees they shepherded to all but the most discerning eyes. This is likely the natural end of an ent's life, only really lamentable in that with them, one by one, go the last presence of the great giants that once roamed all the forests of the world, soon to dwindle forever into the scattered, confused legends of men.
It's difficult to conceive of a consistent physiology for ents since, as Pippin observes, they vary widely from one another in appearance - much more so than would be expected of members of the same species - and the variety likely stems from effects of the conditions of their environments on their physical bodies over the course of their long, long lives. I can picture them aging quite differently from one another based - for example - on the amount of rainfall in their environments, with some, like those in green Ossiriand of old, being really mossy, their bodies and hair plastered with lichen, mosses and even mushrooms, whereas ents from a dryer, higher altitude environment like the pine forests of Dorthonion would be really straight and tall, and their skin very craggy and relatively bare of smaller plant/fungal growth. My thinking on their skin (seen more clearly on the figure on the left) is that, as with many animals (including us) the skin on their torsos, particularly in high motion joint areas, moves more than the skin over their limbs, and so becomes cracked and thickened into these bark like structures, while the limbs remain smoother, as Tolkien describes for Treebeard (We are never told if those basic physical descriptions we are offered for Treebeard himself - tall head, short neck, rough, bark-like torso, smoother arms - hold true for the others or not, but it seems likely to me that ents come into the world looking not so different from each other) The one on the right has become covered in forest growth; I imagine he'd live in a wetter, temperate rainforest environment, the craggy skin on his torso (and of course his hair and beard as well) providing more opportunities for things like moss or fungi to "catch" and grow on him until he appears roughly clothed in the stuff. Branches get tangled in their hair overtime, becoming meshed into their matted locks until they finally rot (entish hair would probably end up as a plant nursery, especially in wetter environments) I also kind of like the idea that many older ents would grow to be slightly asymmetrical, the result of standing in one place for a long time on uneven ground or in uneven sunlight, their bodies responding like trees to their conditions over long periods of inertia.
Ultimately I expect ents are - in their early youth and in Yavanna's original conception of them - composed very much like humans/elves but, on a cellular level, all their bodily systems (circulatory, nervous, skin, digestive, ect) are built and function a little more like woody plants than ours do, resulting in them being quasi-photosynthetic and growing in strange, un-animal-like ways over time. Like the dwarves they are made in imitation of the "children of illuvatar" and I would even venture to say - again like dwarves who are mortal - they are "second theme" creatures, made closer in imitation of humans than of elves, but they're a lot farther afield than dwarves; Yavanna is the mother of all biological life on earth (including, it should be assumed, the physical bodies made of cells and carbon base used to house the souls of elves and men) and so, in designing a humanoid of her own, she can mix and match characteristics from the different kingdoms of life with more confidence and originality than Aule; ents therefore are humanoids more than plant-creatures (and indeed they seem to count themselves this way in their ancient "lists") but they're such a weird one that they strain the definition between plant and animal life; real-life "green men," remembered to us in those stylized medieval keystone carvings, or in figures like the Green Knight of the arthurian cycles (or the Jolly Green Giant brand mascot)
part of the Tolkien Sketchblog (cant really call it 'weekly' anymore, but hopefully that'll change soon