Growth of the embryo is usually arrested after the meristems and cotyledons are formed. Seeds are adaptively important because they permit the plant to postpone development until conditions are best for growth, allow synchrony of development with optimal weather conditions such as temperature and moisture, protects the young plant embryo, and facilitates dispersal. Many seeds may fail to germinate unless they are exposed to specific stresses, such as fire, the intestinal tract of an animal, or the leaching of specific materials from their seed coats. Primary growth occurs in the apical meristems of roots and stems and is seen in herbaceous plants, shrubs, and trees. The change produced is elongation of the plant body.
Secondary growth occurs in the lateral meristems.
This tissue is found in the vascular cambium and cork cambium. The change produced by secondary growth is thickening of the plant body. Deciduous trees and shrubs lose their leaves on a cyclical basis, evergreens do not. Deciduous trees are encountered in many climates. Loss of leaves may be for conservation of water in times of drought in tropical regions.