About one third of the earth is covered by deserts, areas that typically receive less than 10 inches of rain per year.
The most diverse deserts receive rainfall in two seasons – winter and summer. The harshest deserts get moisture in only one season. In some extreme cases, water arrives in the desert mostly in the form of dew.
Deserts are marked by extremes in temperature and dryness, or aridity. Plants have evolved a variety of adaptations to survive in this harsh climate.
Many unrelated plants have evolved in desert environments around the world and arrived at very similar adaptations. This process is called convergent evolution. Some adaptations include hairs or spines, waxy surfaces, fleshy leaves or stems, and deep stem grooves.
The features of agave and aloe plants are a good example of convergent evolution. Agave is native to the Americas, and aloe is native to Africa and Madagascar. They are unrelated but look similar, having developed the same adaptations to very dry climates.