I literally can't keep my eyes off it (pun intended). I think you did a great job with this piece, and the fact you drew it with charcoal pencils and had it redrawn so that it could be put up on the portal is in itself a daunting task.
So, let's see! (Okay, I'll stop)
The tree itself is very detailed, with the individual leaves being shown clearly without any gaps or breaks, yet all of them can be seen without any being hidden almost fully behind the other. The varying colors show a single light source, probably from the top left corner, as leaves are brightest there and follow a gradient color to the right where it is a little darker.
The trunk of the tree, and the branches are all drawn very nicely, each of them wrapping the lower half on all parts of the eyeball, almost as if to show the blood vessels at the back of the eye, and the progression of a tree to an eye. (Is there some sort of subtle meaning here that I don't get, the conversion of energy from a tree which eventually passes to an eye?) The branches are mostly the same color, but some parts are colored abnormally, as if to show mutation, and the roots the "blood vessels" of the eye. Good job, I always found drawing a tree difficult.
The vitreous humor of the eye is faintly tinted with red on a normal white, presumably to show signs of life and/or strain. The rest of the eye is drawn nicely, but there's a, kind of "problem" with the cornea. I wouldn't exactly call it a problem per se, since I've found that this is entirely normal in eyes, but it's odd because this isn't what I normally see in eye diagrams. The "problem": the cornea's transparent, with a slight outline. Again, this ain't a problem, but it's just odd to me. The lens of the eye's also good, except if you're aiming for a touch of realism like you did with the cornea, then wouldn't the lens also be transparent? Just saying.
The background could use some work though, as others pointed out -- it's just a gradient background fading to black from white. Perhaps a skybox to just show that the eyeball tree helix is "floating" in the air? Or how about a cross-section of the soil, with the eye "planted" deep in the soil? There are many combinations to choose from, but this...rather dull background isn't one of the good ones, I must say.
All in all, a good piece of art, although I don't think I'd like it enough to get it tattooed onto my skin forever, like XsakuX said. Either way, a brilliant way to come back from your hiatus, and I hope you have many more dry spells, so that we can see more art like this :P
~~ Summary ~~
~~ Good points ~~
1) Nice detailing of tree, eye
2) Subtle detailing of leaves & branches
3) Nice touch of realism in the drawing
~~ Bad points ~~
1) Dull background
2) [If aiming for realism] Lens should be transparent
3) [If not aiming for realism] Cornea's a bit strange
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