Thin layers or laminae that make up a single summer band in tree ring, what are they?
Not as a confident answer but as a comment... to provide image.

The wood TS looks like ring-porous, Oak (Quercus sp.) wood (Not as identification but anatomical features are comparable).

The dark bands (radial here) seems to be xylem fibres, and bright bands (radial here), seems to be xylem parenchyma.

This is a magnified image of wood of post oak (Quercus stellata). .
Source, URL http://www.wood-database.com/wp-content/uploads/post-oak-endgrain-zoom.jpg

where it seems to me



If there is anything wrong in my knowledge, inform me and feel free to downvote this answer. If any better answer comes, I'll delete this answer.