Here is what you see if you stripe the bark off a limb: dense
and long wood ridges (striae) are formed on the wood underneath
the bark. It is not that wood ridges form bark bumps, but rather the
ridges on the wood correspond to depressions on the bark surface. The
smooth wavy bark is diagnostic for rusty and gray willow in New England; the
related American species don't have the striae pronounced as much.