In the <answer>diffuse-porous</answer> 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.
What kind of wood often has ring divisions that can't even be seen by looking at it?
diffuse-porous