The type of dormant response depends on the plant's pattern of growth and death. Perennials are plants that live year after year, undergoing a period of dormancy during the cold season. In herbaceous species, the aboveground portions die, but the plants survive as specialized underground stems. Woody shrubs and trees remain alive above-ground. Deciduous species shed their leaves in winter, while many nondeciduous species, often called evergreens, keep their leaves year-round but dramatically reduce their metabolic rates.
Biennial species live for two years. The first year is devoted to vegetative growth and the formation of underground storage tissues. After the plant lies dormant through the winter months, its second year of growth results in the stored food supply being used to produce flowers and seeds. Annuals are plants that complete an entire life cycle in one growing season. The plants die, producing seeds, which normally remain dormant until the following growing season.
Most perennial trees and shrubs in temperate re gions produce buds in the summer. These buds, which can eventually develop into leaves, stems, or flowers, exhibit reduced metabolic activity even before leaves begin to senesce (age). As temperatures decrease in the fall, complete dormancy sets in. Specialized leaves called bud scales cover the dormant tissue. These scales block the diffusion of oxygen into the bud; they also prevent the loss of water from the tissue.
Almost all flowering plants produce seeds. The seeds develop as ovules within a structural component of the flower called the ovary. As the ovary ripens to form the fruit, the ovules mature into seeds. Each seed is composed of a reserved food supply and a new plant with embryonic root, leaf, and stem tissue. The embryonic plant and reserve food supply are surrounded by a tough seed coat. The seeds of many species, especially trees in the temperate zones, do not germinate immediately after maturing even under ideal moisture, temperature, and nutritional conditions because there is a built-in period of dormancy.
Although seasonal dormancy is most often correlated with temperature changes, variation in precipitation is the primary factor in regions where pronounced wet and dry seasons alternate. Some deciduous trees and shrubs drop their leaves and remain dormant during the dry season and grow new leaves when the rains return. Herbaceous perennials die back and go dormant at the beginning of the dry season, then regrow their aboveground biomass. Many desert annuals have seeds that will only break dormancy when sufficient rains come, which in some regions may be only every few years. Some particularly specialized seeds germinate only after they have been tumbled in the waters of a flash flood, which scrapes their seed coats.
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