When you were out collecting twigs to force buds, you may have noticed that some twigs were stout while some twigs were skinny. And maybe when you cut the twig you looked at the cut surface and noticed white or orange spongy stuff in the heart of the twig. The spongy stuff is pith, also called medulla (which means middle), and is essentially the undifferentiated, highly moldable ingredient of new growth in a twig. It also stores starches and water, but sadly as the twig ages, the pith’s utility dries up and the cells die leaving behind a spongy tube. Trees with compound leaves tend to have larger piths (though there are, of course, exceptions). Within the maples, for example, boxelder has the stoutest twigs and widest diameter of pith. It is also the only maple with compound leaves (boxelder is also called ash-leaf maple).
Cross section of mountain maple twig, showing orange, spongy, 2 year old pith