Secondary growth is plant growth by width, such as with trees. Trees continue to grow laterally to provide adequate support for the tree's height and leaf mass. At the same time that a tree's stem, or trunk, increases its girth, the root system undergoes a similar change. The roots of a seedling may be thin and hair-like, but thin roots could not support a full-sized tree. A good example is a mangrove that has both trunk and root structures above ground level. The trunk of a mangrove is thin compared to oaks or elms. Its root system sends out numerous "knees" above the water line where mangroves are found. The roots are long, tangled, and thick, looking more like tree branches than a root system.
Plants may have a determinate growth pattern, which limits the size and shape of the plant, after which growth ceases because the plant has reached its maximum proportions. Other plants may have indeterminate growth patterns, with no obvious limit to height or girth, such as redwoods. Plants with indeterminate growth continue to add branches, stems, roots, and leaves throughout their life spans. Indeterminate growth is the result of meristem, tissue that undergoes repeated, continuous cell division. Meristem tissue produces the basic tissues of plant growth: vascular xylem and phloem, roots and root hairs, and external stem tissue, called epidermal tissues.
Meristematic tissues exist in three structural positions on plants. Apical meristems (meristems at the tip of a plant shoot or root) produce root and stem tissue seen in plant growth; the growth of a cornstalk from germinated seed to a 1.5 meter stalk is the result of apical meristematic tissue generation. Lateral meristems, in which secondary growth occurs, account for the increased thickness of a plant, such as the expansion from a pine sapling to a pine tree with a trunk of 60 cm diameter. Intercalary or basal meristems account for the continued growth of grass, for example, despite repeated mowing to decrease grass height.A plant's growth begins with the shoot system, which includes stem nodes, stem internodes, leaves, branches, and reproductive organs (flower, fruit, and cones). The shoot system originates in a segment of the plant called the shoot apical meristem. The apical meristem is located at the apex of a plant stem, so that primary growth extends upward from the top, not the base, of a plant stem. Likewise, roots grow from the tips of roots, not from the base root.