- The meristematic layer that is responsible for cutting off vascular tissues – xylem and pholem – is called vascular cambium.
- In the young stem, it is present in patches as a single layer between the xylem and phloem, which later forms a complete ring.
Formation of cambial ring
- In dicot stems, the cells of cambium present between primary xylem and primary phloem is the intrafascicular cambium.
- The medullary cells adjoining this intrafascicular cambium become meristematic and form the interfascicular cambium and hence a continuous ring of cambium is formed.
Fig. Vascular cambium
Activity of the cambial ring
- The cambial 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 amount of secondary xylem produced is more than secondary phloem and soon forms a compact mass.
- 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.
Spring wood and autumn wood
- In the spring season, cambium is very active and produces a large number of xylary 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 xylary elements that have narrow vessels, and this wood is called autumn wood or late wood.
- The two kinds of woods that appear as alternate concentric rings, constitute an annual ring.
Fig. Autumn wood and spring wood
Heartwood and sapwood
- The region which comprises of dead elements with highly lignified walls and is called heartwood.
- The peripheral region of the secondary xylem, is lighter in colour and is known as the sapwood.
Fig. Heartwood and sapwood