Class 11 Biology Anatomy of Flowering Plants Vascular cambium

Vascular cambium

  • 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.

Class_11_Biology_Anatomy_Of_Flowering_Vascular_Cambium

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.

Class_11_Biology_Anatomy_Of_Flowering_Autumn_&_Spring_Wood

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.

Class_11_Biology_Anatomy_Of_Flowering_HeartWood_&_SapWood

Fig. Heartwood and sapwood

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