Cephalia are these weird things some cactuses get where a gross column starts growing out of them at a certain age:
They are pretty much for flowers. Here is how one of the authors of A Cactus Odyssey describes them:
When a plant of Melocactus undergoes its transition into making a cephalium, it is like a young person going through puberty and developing adult characters like a beard. When a plant makes a lateral cephalium, however, it is as if only one narrow strip of a maturing plant undergoes puberty while the rest of him or her continues to be a child, albeit a giant version of that child. Maybe it is best not to think about it this way after all.