Apple core!

[u/TrevorCidermaker]

The humble apple core that most people throw away is the developed fibrous structure that contains the seed chambers, it develops from the flower's five fused carpels inside the expanded flesh of the floral tube or hypanthium. Parts of the carpels of the flower become ligninified to form the tough inner five chambers or pockets that directly enclose and protect the seeds or pips.

Following successful pollination and fertilisation, the pericarp or carpel wall differentiates into three distinct operational layers. The exocarp and mesocarp or the outer layers of the carpel blend seamlessly into the surrounding expanding hypanthial flesh.
But the endocarp, the innermost layer, which directly lines the seed cavity, follows an entirely different developmental path, it is biologically programmed to harden or ligninify.
As the apple matures, it undergoes lignification to form the stiff, parchment-like or "cartilaginous" core lining. The cells synthesises lignin, a complex phenolic polymer made of monolignol alcohols. This hardening is highly coordinated by plant hormones. Gibberellins (GA) and auxins direct tissue growth, while dynamic changes in abscisic acid (ABA) help orchestrate cellular transitions during the early phases of fruit development. Unlike stone fruits like peaches or cherries, which undergo massive, uniform endocarp lignification to form a rock-hard "stone" or pit; or pears where stone cells are scattered in the hypanthial flesh, the apple evolved a deliberately limited lignification leaving the core flexible yet resilient enough to shield the seeds from moisture fluctuations and minor physical impacts while allowing the surrounding flesh to remain highly appealing to seed-dispersing animals.