Science, Tech, Math › Animals & Nature 3 Tree Structures Where Growth Occurs Share Flipboard Email Print Alex Belomlinsky/iStock Vectors/Getty Images Animals & Nature Forestry Tree Structure & Physiology Tree Identification Basics Arboriculture The Science Of Growing Trees Conifer Species Individual Hardwood Species Pests, Diseases, and Wildfires Tree Planting and Reforestation Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Dinosaurs Evolution View More By Steve Nix Forestry Expert B.S., Forest Resource Management, University of Georgia Steve Nix is a natural resources consultant and a former forest resources analyst for the state of Alabama. He is a member of the Society of American Foresters. our editorial process Steve Nix Updated August 16, 2019 Little of a tree's volume is actually "living" tissue. Just 1% of a tree is actually alive and composed of living cells. The major living portion of a growing tree is a thin film of cells just under the bark (called the cambium) and can be only one to several cells thick. Other living cells are in root tips, the apical meristem, leaves, and buds. The overwhelming portion of all trees is made up of non-living tissue created by a cambial hardening into non-living wood cells on the inner cambial layer. Sandwiched between the outer cambial layer and the bark is an ongoing process of creating sieve tubes which transport food from leaves to roots. So, all wood is formed by the inner cambium and all food-conveying cells are formed by the outer cambium. Apical Growth Tree height and branch lengthening begin with a bud. Tree height growth is caused by the apical meristem whose cells divide and elongate at the base of the bud to create upward growth in trees with a dominant crown tip. There can be more than one developing crown if a tree's top is damaged. Certain conifers cannot produce these growth cells and height growth stops at the crown tip. Tree branch growth works in a similar way using buds at the apex of each twig. These twigs become the future branches of trees. Transfer of genetic material in the process will cause these buds to grow at determined rates, creating a tree species' height and form. Tree trunk growth is coordinated with the increase of tree height and width. When buds begin opening in the early Spring, cells in the trunk and limbs get the signal to increase in girth by dividing and in height by elongating. Root Cap Growth Early root growth is a function of meristematic root tissue located near the tip of the root. The specialized meristem cells divide, producing more meristem called root cap cells which protect the meristem and "undifferentiated" root cells while pushing through the soil. The undifferentiated cells become the primary tissues of the developing root during elongation and the process that pushes the root tip forward in the growing medium. Gradually these cells differentiate and mature into specialized cells of the root tissues.