Question

Continuous state of dividing tissue is called meristem. In connection to this, what is the role of lateral meristem?

Answer 1

The secondary growth in dicots and gymnosperms is brought about by two lateral meristems.

• Vascular cambium:
  

  The vascular cambium is the lateral meristem that produces the secondary vascular tissues, i.e.. secondary xylem and secondary phloem.

  Origin and Formation of Vascular Cambium:

1. A strip of vascular cambium originate from the procambium is present between xylem and phloem of the vascular bundle. This cambial strip is known as intrafascicular or fascicular cambium.
2. In between the vascular bundles, a few parenchymatous cells of the medullary rays that are in line with the fascicular cambium become meristematic and form strips of vascular cambium. It is called interfascicular cambium.

Organization of Vascular cambium:

1. The active vascular cambium possesses cells with large central vacuole (or vacuoles) surrounded by a thin, layers of dense cytoplasm.
2. The most important character of the vascular cambium is the presence of two kinds of initials, namely fusiform initials and ray initials.

Fusiform Initials:

1. These are vertically elongated cells. They give rise to the longitudinal or axial system of the secondary xylem (tracheary elements, fibres, and Axia? parenchyma) and pholem (sieve, elements, fibres, and axial parenchyma).
2. Based on the arrangement of the fusiform initials two types of vascular cambium are recognized.

Stoned (Stratified cambium) and Non - storied (Non - stratified cambium)-

1. If the fusiform initials are arranged in horizontal tiers, with the end of the cells of one tier appearing at approximately the same level, as seen in tangential longitudinal section (TLS) it is called storied (stratified) cambium. It is the characteristic of the plants with short fùsiform initials.
2. In plants with long fusiform initials, they strongly overlap at the ends, and this type of cambium is called non - storied (non-stratified) cambium.

Ray Initials:

These are horizontally elongated cells. They give rise to the ray cells and form the elements of the radial system of the secondary xylem and pholem.

Activity of Vascular Cambium:

1. The vascular cambial ring, when active, cuts off new cells both towards the inner and outer side. The cells which are produced outward form secondary phloem and inward secondary xylem.
2. Due to the continued formation of secondary xylem and phloem through vascular cambial activity, both the primary xylem and phloem get gradually crushed.

• Phellogen (Cork Cambium)-

1. It is a secondary lateral meristem. It comprises homogenous meristematic cells, unlike vascular cambium. It arises from the epidermis, cortex, pholem or pericycle (extrastelar in origin). Its cells divide periclinally and produce radially arranged files of cells.
2. The cells towards the outer side differentiate into phellem (cork) and those towards the inside as phelloderm (secondary cortex).