The acorn seed is enclosed in a leathery, bullet-shaped casing that emerges from a shingled cap. Each oak species has an acorn cap with a slightly different design, their own signature beanie or architectural roofline. The cap depth can vary from shallow to deep and the shape of the shingles covering the cap can be distinctly flat or warty depending on the species of oak.
Both the walnut hull and the acorn cap derive from bracts originally enclosing the flower buds. These plant parts modify their function through the reproductive stages, first protecting the developing flower (and perhaps controlling the timing of wind pollination in a way that may increase the genetic diversity or food quality of the seed crop) and then protecting the fertilized seed as it swells with food transported from the mother tree and stored in the developing nut - future nourishment once the nut breaks away and starts its separate life or becomes a menu item.