How many protective mechanisms did you think of? Most desert cacti have thick, waterproof skins. That cuts down losses by evaporation. What else? ... Yes, they protect themselves with fierce spines, as many of you will already have discovered!.
Some of them also store poisons in their skins, which makes life difficult for browsing animals. And evaporation can be reduced still further by keeping evaporative surfaces - like branches and leaves to a minimum - hence the saguaro's tall, columnar stems.
