Peruvian apple cactus looks very much like the cacti that most people invariably think about when they hear the word cactus. Tall, spiny, and green, it makes for a great indoor plant -- even in places where it does not actually thrive.
The species is a typical columnar cactus with green to blue-green stems and clusters of brown spines rising from cushiony areoles along well-defined ribs. The variety called curiosity plant, which sports bizarre knoblike protuberances, is frequently sold.
The Peruvian apple cactus is often used as a living sculpture in totally inappropriate conditions, surviving for years with no growth and rarely needing any water -- then it suddenly rots and dies. In full sun it will grow quite rapidly. The sweetly scented, night-blooming flowers are unfortunately rarely produced indoors.