In this interesting close-up by Dietmut Teijgeman-Hansen, we see what remains of a leaf that was likely consumed by an insect and is now slowly decomposing. According to reddit’s resident biologist Unidan:

“The reason this skeletal like pattern remains in the leaves is due to higher concentrations of lignin (a strong carbon structural molecule that is what makes wood woody) in the tracheid xylem cells. The more easily decomposed cells will rot away (or be eaten away, as it is more nutritious and more easily digested by insects), leaving the tougher skeletal lignin frame” [Source]