capers growing in the cracks of a centuries' old wall

So that's what they look like! Only seen them in jars.

I was attracted to the placques on the wall... My brother in law's family came to Canada from Puglia...On a return trip to Italy recently he found a placque commemorating an ancestor with his exact name on a wall such as this...aparently the deceased stay in the ground only 30-40 years and then their marker is lifted and placed on the wall and someone else is buried in the plot!! Is that the case with these markers?

No, Lori, these markers are "thank you" placques for graces/favours received in the sanctuary.

And yes you are right about the deceased being removed from big graves after a few decades - they are then put into smaller space-saving ossuaries.