Astrophytum ornatum (Monk's Hood Cactus)

Native to northeastern Mexico, this solitary cactus begins with a rounded stem and becomes columnar with age. Its pronounced ribs may be straight or spiral, and bands of small white woolly scales cross the green surface. Closely spaced areoles carry yellow to brown spines. Large yellow flowers develop at the crown, opening during the day and closing at night for several successive days. It is cultivated as an ornamental cactus for its striking ribbed form. Wild populations are classified as Vulnerable. Photographed in Colombia.