Like roots, trunks and branches grow in length from apical meristems found in buds, which are essentially telescoped shoots, leaves, and/or flowers. Buds containing all of the above are referred to as mixed, while those containing one or the other are referred to as either leaf buds or shoot buds. The terminal bud, located at the apex of the main stem, forms the trunk of the tree over time. Lateral buds, formed at the leaf axils and nodes along the trunk, grow into branched and flowers.
Within the bud, two growth habits are possible, fixed growth and free growth. Fixed growth occurs in species such as pines, hickory, and oaks, where the buds contain a preformed shoot. All of the components of next year's shoot are contained in the bud formed this year; the number of leaves and nodes is predetermined by this year's environmental conditions. The length between leaves and nodes is influenced by the environmental conditions the tree encounters next year. Free growth, in species such as cottonwood, willow, and silver maple, occurs when buds contain shoots with some preformed leaves, but which are also capable of forming additional leaves. These species can continue to grow as long as environmental conditions are favorable. Recurrently flushing growth occurs on many shrubs. These species produce a series of buds at the tip of the same elongating shoot in waves or flushes. Some fixed growers, under favorable growing conditions, are also capable of a second flush of growth in one season.