If you were a plant in a desert you would have to wait along time for water. It would not just be a few days or weeks between showers of rain. It might be many months or even a couple of years. It may seem impossible for plants to survive in such conditions but after rain, seemingly dead plants spring into life and leaves and flowers emerge from the desert soil.

There are two ways that a plant can survive in a desert. It can survive by spending most of its time as a seed then completing its life cycle very quickly or it can develop a body which can collect and hold onto as much water as it can and grow for many years.

Plants which spend most of their lives as seeds are annuals. The seeds are made by small plants. They have no means to survive in the dry conditions and die when the seeds have been released. Inside the seed the tiny plant has just enough water to stay alive and the seed coat prevents this water from escaping. When it rains again the seed takes in water, swells up, germinates and quickly produces a root to take in as much water as it can from the desert soil. The plant grows quickly and soon produces flowers to be pollinated by desert insects. The next generation of seeds are produced as the previous generation of desert annuals dry up and die.

Plants which live for many years are called perennials. Desert perennials have a large system of roots. It may grow out from the plant just below the surface of the desert soil like the roots of the saguaro cactus or it may grow deep into the soil like the long tap root of the creosote bush. The roots near the surface collect the water as it enters the soil. The deep roots collects water that the shallow roots have missed. The root systems of desert perennials do not intertwine. In fact they help to keep the plants spaced apart so each one can collect the water it needs.

When the desert perennial has collected its water it has to store it and prevent it from escaping. Two parts used for water storage are the root and the stem. Plants which store water in the stem, like the cactus have to defend it from animals. Cacti have a rows of spines down their stems which stick into the mouths of any animal that tries to bite its way in to the water.

Plants use water to make food. They also use sunlight but with sunlight comes heat which can make water evaporate from the surface of a plant. Many desert perennials are adapted to avoid water loss by having small leaves which present a small surface area for evaporation. The leaves are also covered with wax to stop water passing out through the surface. The cactus goes a stage further and does no use leaves at all. It makes all the food it needs with its green stem and branches.