Minecraft Seeds planned patterns?

I noticed some pattern while planning on my server. I can't plant anything exactly in those places. I gave a friend a few seeds but he couldn't plant anything either. Picture follows:

I want to plant my seeds and I'm curious. Anyone with an idea?

Could be a problem with the respective light level. Maybe just hang up a few more torches

You build underground. That means you need more light.

If you don't have enough light, you won't be able to place seeds either.

There's not enough light in those places to plant anything. Seeds require a certain level of light.

Light spreads in a diamond-shaped pattern, hence the pattern.

Put water in the middle where you have the wood right now and then the earth will get wet and your seeds will grow faster. And what is still important place a couple of torches in this dark room and you have already solved the problem with planting

These are very likely just wooden slabs with water underneath.