Question

explain the secondary growth in the vascular tissue of a dicot stem

Answer 1

Secondary growth in dicot stem involves two major steps:
A. Formation of Secondary Vascular Tissues and
B. Formation of periderm.

A. ​Formation of Secondary Vascular Tissues:
Intrafascicular and interfascicular cambia join together to form vascular cambium.
On inner side, they give rise to secondary xylem and outerside they give rise to secondary phloem.
In between the secondary vascular tissues, secondary vascular rays are formed.
The primary and older secondary phloem present on outer side gets crushed with the development of new phloem.
Width of the secondary xylem grows with age of the plant.
The yearly growth of secondary xylem is distinct in areasmwhich experience two seasons, Favourable (Spring) and unfavourable (Autumn).
The wood of the older stems gets differentiated into two zones, light coloured sapwood and dark coloured heartwood.

B. Formation of Periderm:
Outer cortical cells dedifferentiate to form cork cambium.
It divides to form cells on its inner and outer side sides called Secondary cortex and Cork respectively.

Phelloderm, Phellogen and Phellem together constitute the Periderm.