Cephalocereus senilis cristata.

Description: The standard columnar Cephalocereus senilis (commonly known as the old man cactus) is very popular and widely grown for its woolly appearance, but this beautiful crested form is still rare in cactus collection. The most striking feature is the long, greyish-white hairs it possesses, which serve to protect it from the harsh sun common to its native habitat in eastern Hidalgo, Mexico. More precisely the long white hairs are actually long bristles that benefit the plant by reducing sun exposure and by trapping a layer of air next to the cactus body. This helps to cool the plant in the summer and keep it warm in the winter. The coat of hair makes many plants appear almost snow-white (however, the hairs conceal numerous sharp yellow spines).

Cephalocereus senilis forma cristata (cactus-art.biz)