secondary growth the growth of and stems in length with the help of apical meristem is called the primary growth apart from primary growth most dicotyledons plants exhibit an increasing girth this increase is called the secondary growth the tissues involved in secondary growth are the two lateral meristems a vascular cambium and B cork cambium a vascular cambium the meristematic layer that is responsible for cutting off vascular tissues xylem and phloem is called vascular cambium in the young stem it is present in patches as a single layer between the xylem and phloem later it forms a complete ring formation of cambia ring in die-cut stems the cells of cambium present between primary xylem and primary phloem is the interface ocular cambium the cells of medullary cells adjoining these intravascular cambium become meristematic and form the interface ocular cambium thus a continuous ring of cambium is formed activity of the cambia ring the cambium ring becomes active and begins to cut off new cells both towards the inner and the outer sides the cells cut off towards pith mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem the cambium is generally more active on the inner side than on the outer as a result the amount of secondary xylem produced is more than secondary phloem and soon forms a compact mass the primary and secondary phloem get gradually crushed due to the continued formation and accumulation of secondary xylem the primary xylem however remains more or less intact in or around the center at some places the cambium forms a narrow band of parenchyma which passes through the secondary xylem and the secondary phloem in the radial directions these are the secondary medullary rays Springwood and atom would the activity of cambium is under the control of many physiological and environmental factors in temperate regions the climatic conditions are not uniform through the year in the spring season cambium is very active and produces a large number of Zeiler elements having vessels with wider cavities the wood formed during this season is called spring wood or early wood in winter the cambium is less active and forms fewer salary elements that have narrow vessels and this wood is called Adam wood or late wood the spring wood is lighter in color and has a lower density whereas the adam wood is darker and has higher density the two kinds of woods that appear as alternate concentric rings constitute an annual ring annual rings seen in a cut stem give an estimate of the age of the tree heartwood and sapwood in all trees the greater part of secondary xylem is dark brown due to deposition of organic compounds like tannins raisins oils gums aromatic substances and essential oils in the central or in the most layers of the stem these substances make it hard durable and resistant to the attacks of microorganisms and insects this region comprises dead elements with highly lignified walls and is called heartwood the heartwood does not conduct water but it gives mechanical support to the stem the peripheral region of the secondary xylem is lighter in color and is known as the support it is involved in the conduction of water and minerals from root to leaf