How have terrestrial organisms met the environmental challenges of living on land? Life began in the oceans, but many life forms have since adapted to terrestrial life in a sea of air. Every single organism living on land has to meet the same environmental challenges: obtaining enough water; preventing excessive water loss; getting enough energy; and in polar regions, tolerating widely varying temperature extremes. How those challenges are met varies from one organism to another, and in large part explains the diversity of life encountered on land today. Some animals avoid colder temperatures by migrating to warmer climates for the winter, whereas others avoid the cold by passing the winter in a dormant state called hibernation. Many plants also spend winter in a dormant state. The aerial parts of some plants die during the winter, but the underground parts remain alive; the following spring they resume metabolic activity and develop new aerial shoots. Many trees are deciduous; that is, they shed their leaves for the duration of their dormancy. Shedding leaves is actually an adaptation to the “dryness” of winter. Roots cannot absorb water from ground that is cold or frozen; by shedding its leaves the plant reduces water loss during the cold winter months when obtaining water from the soil is impossible.