The chambers in the middle, seen here in my sample comprise what is known as a “chambered pith” (a pith being the center of a tree/branch/twig). Here's how it happens (in elaborate, if brief science talk). When it comes to chambered piths, it seems the only two choices spoken of online are black walnut (Juglans nigra), and butternut (Juglans cinerea), aka “white walnut.” Here a dichotomous tree terminates in those choices after choosing “chambered pith.”
Wikipedia’s entry on the walnut tree features this image of the chambered pith of a black walnut. I’m a little iffy on it being a black walnut, however, as some shots, like those found on this page show that while the chambered pith is similar, the wood in that shot seems more woody, and the twig shots seem hairy, whereas my limb was smooth and hairless. However, at this great page on butternut, and its sister page on black walnut, there’s a comparison shot of the twigs of both which suggests that if either, I have black walnut. The buds on butternut are pointier and longer, and the leaf scars much larger, with a shape that’s elongated more down the stem. Even the author of those pages implies it’s not always easy to tell them apart for a novice like me.
There are some more good shots of young butternut twigs here, and they certainly do show some similarities, like the bark color and texture, and the distribution of the little white dots, or “lenticels” (pronounced LEN-tih-kuls), which act as pores to bring oxygen into the growing limb, similar to those found on my silver birch. For reference again, here are my twigs: one, two, and three, with clearer views of the lenticels here and here. In that last one the lenticels are fading, and the trunk right between my fingers seems to be gearing up to split into the more textured, woody bark of older trees.
Here’s a shot of black walnut on Flickr that shows more of the transition from young to mature tree. Its buds do look a lot like those on my samples. There’s a bit more on butternut, black walnut, and hickory (also in the Juglandaceae Family) here. Making things more difficult is the fact that there are 8 genera in Juglandaceae, and its genus Juglans alone is divided into 4 sections. The Japanese Walnut (J. ailantifolia) isn't too dissimilar, telling me that I’d probably have to run through all of the Juglandaceae Family to know for sure what I have.
For now, I’m going to say it’s a black walnut, and be happy :) I suppose I could always send my Flickr set to The Walnut Council and see what they have to say about it. There’s a council for everything! Anyway, pretty neat to get a chance to work in walnut this young, though I wouldn’t go chopping one down on my own for the opportunity.
-- Gary, Los Angeles, video game animator