The picture above depicts the inside of a root of an oak tree. Inside of oak (and other) trees, there is a layer called the cambium. The cambium lies between the xylem and phloem, and is a layer of continuously dividing cells, which helps the secondary growth of stems and roots. The cambiums active division increases the width of the plants parts instead of length. The cambium is made up of meristematic tissue, which is extremely thin, and produces a new phloem on the outside.