Red Top Cactus

The Gymnocalycium cultivar -- sometimes called ruby ball cactus or moon cactus -- is actually two cacti in one. A pure red cactus seedling lacks the ability to produce chlorophyll and will die unless it is grafted onto a green one. The green feeds its mutant mate sugar molecules produced from water and carbon dioxide. Once established, the two parts grow together so you can't even see the seam. These cacti typically live only a few years and do not grow appreciably, making them ideal for apartment dwellers.

Source: homeguides.sfgate.com