Wood:

The structure of hardwoods is more complex. The water conducting capability is mostly taken care of by vessels: in some cases (oak, chestnut, ash) these are quite large and distinct, in others (buckeye, poplar, willow) too small to be seen without a hand lens. In discussing such woods it is customary to divide them into two large classes, ring-porous and diffuse-porous.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". In wood from a willow tree, what would you need to use to see the tiny vessels?
a hand lens