Question

With the help of diagram, explain secondary growth in dicot roots.

Answer 1

Hint: Width of the secondary tissue increases with the age of plant. Pith initially is narrow but with time it vanished. And growth of the secondary xylem is also affected by the environmental conditions like seasons, temperature, etc.

Complete answer:
To answer this question let us understand the term secondary growth.
Secondary growth: It is defined as the increase in stem diameter due to the activity of the lateral meristem tissues.
Let us now try to understand secondary growth in dicot plants:
Secondary tissues in dicots are formed by lateral meristems which are of two types vascular cambium and cork cambium or phellogen.
Given diagram represents the tissue arrangement of the root starting form the primary root to the secondary root.

1.Vascular rays: In secondary growth first of all, these rays start emerging from the radially arranged cells and they are involved in giving height to the roots. These rays are present in the xylem tissues. They may be made up of one type of cells or may have different types of the cells.
2.Secondary phloem: This is the covering layer outside the vascular cambium. It is made up of the same cells which are found in primary phloem tissue. It is a regular arrangement of the tissues and has no fibre in it.
3.Secondary xylem: This is the woody part of the roots and made up of the vessels, fibres, parenchyma tissues and tracheids.

Note: Origin of secondary vascular tissues: Vascular cambium is involved in formation of secondary vascular tissues. It is formed by two types of meristem tissues intra fascicular cambium and inter fascicular cambium.
Intra fascicular cambium: It is present in between phloem and xylem vascular bundles.
Inter fascicular cambium: It is present in between two consecutive vascular bundles.