Spruce wood in a scanning electron microscope (view perpendicular to the trunk diameter). At high resolution, the circular pits are easy to see. Pits are thinner portions of the cell wall that allowing mass transfer between neighboring cells.
See also Youtube:
Image width: 215µm
Dictionary: xylotomy (wood anatomy) describes macro- and microstructures of different wood species. Based on their characteristic microstructures, wood species can be identified.