Role of Vascular cambium in secondary growth in Dicot stem

• Secondary growth signifies the formation of secondary tissue with help of lateral cambium. 

• Secondary tissue  which are formed during the secondary growth named vascular tissue and ground tissue. 

• Vascular tissue and secondary tissue are formed by the vascular cambium and cork cambium respectively.

• Vascular cambium is lateral meristem and form secondary xylem and secondary phloem. The activities of vascular cambium are as follows -

Formation of cambial ring : 

• Vascular cambium of Dicot stem and Gymnosperm partly secondary in origin. 

• Intra fascicular cambium is first component that appears first between the xylem and phloem inside the vascular tissue. 

• Interfascicular is secondary component appears at the level of intrafascicular cambium. 

• The parenchymatous cells of medullary ray become meristmatic through the process of dedifferentiation. 

• The strip of both interfascicular and interfascicular are joined and make complete ring of vascular cambium.

Cells of vascular cambium: 

• Vascular cambium has two types of cells fusiform initials and ray initials. 

• Fusiform initials and ray initials form secondary vascular tissue and vascular ray respectively. 

• The cells of vascular cambium undergo periclinal divison to form  the component of secondary tissue. 

• These divison occur on both the outer side and inner side of cambium ring.  

• At some places, the cambial ring  also divides by anticlinal divison and increase the diameter of vascular cambium. 

Activation of vascular cambium: 

• There are various factor that are involved in activation of vascular cambium. 

• These factors are - Production of hormone by young leaves and bud, increased availability of nutrients, favourable conditions for temperature, Photoperiod, growth stimulator.

Formation of vascular ray : 

• These Rays are formed from ray initials of vascular cambium. 

• These Rays may be uniseriate or multiseriate. These may be homocellular or hetero cellular. 

• These rays has two components - Phloem ray and xylem ray. 

• Xylem and phloem ray are present in secondary xylem and secondary phloem respectively. 

• These vascular rays are also called as secondary medullary  rays. They take part in radial conduction of food, water and gases. 

Formation of secondary phloem or bast fiber : 

• Secondary phloem is formed vascular cambium prominently from fusiform initials. 

• Secondary phloem has same constituent like primary phloem like sieve tube, companion cells  phloem parenchyma and phloem fiber. 

• Out of these, sueve tubes are more numerous. This phloem is also called soft bast or hard bast. 

• The primary phloem and older secondary phloem are degenerated as the newly formed secondary phloem become functional.

Formation of secondary xylem: 

• Secondary xylem is formed vascular cambium prominently from fusiform initials like secondary phloem. 

• It is the woody part of stem. Secondary xylem has same constituent like primary xylem like vessels tracheids ,  xylem parenchyma and xylem fiber. 

• Out of these, vessels  are more numerous. These vessels with large more pore is called porous wood where as large sized vessel  formed during favourable condition called ring porous. 

• Xylem fibers provides hardness to the wood and vascular ray increase softness of the wood. 

• Xylem parenchyma store tannins,  Crystals, and other material.