Question

Explain the process of secondary growth in the stelar region of a dicot stem.

Answer 1

• The growth that results from cell division in the cambia or lateral meristems which causes the stems and roots to thicken is called secondary growth
  
• This type of growth occurs due to the activity of the two lateral meristems- the cork cambium and vascular cambium.
• Secondary growth in dicots:-

1) Formation of cambium ring:

• The cambium of vascualr bundles becomes meristematic.
  
• Some of the medullary ray cells lying at the level of cambium also become meristematic.
  
• It forms a strip of interfasciular cambium together with intrafasciular cambium to form a circular ring called cambium ring.
  
• This cambium ring forms secondary tissues in the stellar region.
  
• The vascular cambium consists of two types of cells- fusiform initials and ray initials.
  
• The fusiform initials are vertically oriented which divide to form the elements of xylem and phloem.

2) Formation of secondary vascular tissues:

• The cambium ring cuts off new cells, both on outer and inner sides,
  
• The new cells formed on the outer side modify into the elements of secondary phloem.
• The cells formed on the inner side modify into the secondary xylem.

3) Vascular rays:

• Ray initials of the cambium ring form narrow bands of parenchymatous cells.
  
• These cells extend radially from the pith to the phloem.
  
• These are called secondary medullary rays or vascular rays.