Tree growth is a response to the environment and to the tree's genetic make-up. The environment is made up of factors such as water, light, temperature, air, pests, and cultural practices. These factors impact physiological processes such as photosynthesis and respiration, and the final response is the tree grows vigorously or slowly. A tree's genetic make-up also influences the way a tree grows. These genetic characteristics along with its environment determine how a tree grows.
Growth occurs in meristems.
A meristem is a tissue that contains cells that have the capacity to divide and make new cells. Some of these new cells continue to function as meristematic cells. Other new cells grow and differentiate into structures such as roots and shoots. For example, an apical meristem in a bud produces new meristems called leaf primordia - each single primordium will grow into a new leaf. Root primordia grow inlo new roots.