This question deals with the magic explained in this question. It is also explained below.
In my fantasy novel, magic is a natural part of the world. It is not some mystical force shrouded in mystery, but rather backed by science (though only I, the author, know all of it's workings).
In my world, magic is a force that by its nature changes living cells. It is similar to radiation, but different in the respect that it changes what the cell does, usually in a beneficial way. For example, if the cells of an eye were exposed to magic, the magic might make the eye also see infrared light.
There are those in my world who can control the change worked by the magic. (They can force the magic to make the eye see infrared light. They can also use magic to make that same eye go blind.)
I've already ascertained that magic-wielders would be able to grow trees into nearly any shape they wanted, including houses (see the linked question). What I am now curious about, is how the magic-wielders would go about making 'windows' in those tree houses. I am not certain how a tree could be altered to produce something relatively small, thin, and transparent, and yet sturdy enough to keep rain and cold out, and heat in. Currently, I've gotten as far as some sort of transparent leaf, but aside from not being sure if a leaf would work grown over a gap in the trunk, I'm not sure if it can be made transparent in the first place.
How could a tree be altered to grow something functionally similar to a window? I don't necessarily need the window to be open-able, though that would be a nice side-benefit if possible.
Note for clarification: This question cannot be answered simply by 'it's magic!' The magic cannot do anything. It can alter living cells to do things they normally wouldn't, but are still possible within the natural world.