When it comes to surviving in hot, dry environments, no plant does it better than the cactus.

A suite of evolutionary adaptations, both anatomical and physiological, has allowed them to live where water is scarce. Cacti have shallow roots that quickly soak up infrequent desert rainfalls, and most species do not have leaves, which are easily desiccated. Instead, photosynthesis has moved indoors, taking place in their thick, succulent stems. How did these special features arise? A new study of cactus evolution suggests that the plant's water-saving strategies might have come first, followed only later by dramatic changes in the plant's anatomy.