My Discocactus placentiformis
From central South America (Bolivia, Brazil, Paraguay), Discocactus consists of a dozen species of globular cacti. These cacti grow in cracks of rock outcrops in tropical forests. Because of the specialized climate these cacti are both water and humidity loving and can't take cold. In cultivation they should be watered adequately in summer and kept warm in winter. Most species are solitary but some such as D.zehntneri form offsets.
All are characterized by a wooly cephalium. A feature evolved to protect young flower buds and fruit. Discocactus stop growing once mature and instead put all their energy into the cephalium. In many cases the cephalium can grow to be as large as the plant itself. They bloom intermittently in summer and the flowers are generally white.
Though tropical cacti are used to plentifull rainfall they still grow mainly in gravelly soils with little humus. The potting medium should be mineral based and fast draining to prevent root rot. Some Discocactus are grafted to make them hardier. But this doesn't work well with the larger species.