The Arizona desert is harsh, hot, and extremely dry, yet row upon row of giant saguaro cacti, up to 18 m (60 ft) tall, grow there, all well supplied with water. On a large cactus like the saguaro, the roots cover a vast area, radiating outwards 50 to 100 m (165 to 330 ft) from the stem.
As the desert's most efficient reservoirs of water, cacti must protect themselves from the predations of thirsty animals. The greatest threat comes from rodents, but the spines keep them away from most of the plant's moist pulp. Cactus flowers would also make a juicy meal, so many small cacti open them only in the hottest part of the day - when rodents are hiding in the shade and the only visitors are insect pollinators.
Other species employ a different strategy, producing particularly fragrant flowers that they open at night to be pollinated by moths or nectar-feeding bats. Once they have served their purpose, most cactus flowers wilt on the very same day that they open.