Please answer a question about the following article about Wood:

In the diffuse-porous woods, the demarcation between rings is not always so clear and in some cases is almost (if not entirely) invisible to the unaided eye. Conversely, when there is a clear demarcation there may not be a noticeable difference in structure within the growth ring.

In diffuse-porous woods, if the demarcation between rings is obvious, what type of difference within the ring might not be?
structure