Secondary Growth in Dicot Root: Explained

Secondary Growth in Dicot Root: It is a magical experience to stand against an enormous tall tree, which makes us wonder how it grew so large. We must have come across some tall trees with sturdy trunks! These types of trees have strong splayed roots as well. Are you aware that secondary growth is responsible for the production of wood in tall trees?

Dicotyledonous stems and roots show the most secondary increase in thickness. In dicot root showing secondary growth, cork is found. The presence of parenchyma cells beneath the phloem group causes the thickness of a dicot root to expand, forming cambium strips. Two secondary vascular cells, cambium and periderm, are responsible for this expansion. Read through the article that describes various stages of secondary growth found in dicot root!

Secondary Growth in Dicot Root: Definition

Secondary growth increases the thickness or girth of the plant, which is the result of cell division in the cambia or lateral meristem. The secondary growth in the root takes place due to the formation of secondary tissues by lateral meristems. Most of the dicotyledonous roots show secondary growth in thickness, like that of dicotyledonous stems. It occurs by the reappearance of two types of secondary vascular tissues called the cambium and periderm. These tissues formed by meristem are vascular cambium and cork cambium, respectively.

Secondary Growth in Roots: Example

The roots of most dicotyledonous and gymnosperms show secondary growth. The roots of extant vascular cryptogams and most monocotyledons do not show any secondary growth; they remain entirely primary throughout their life.
As in the stem of dicotyledonous plants, secondary growth occurs after the primary growth is over. This increases the girth of the stem and roots and not the length. The roots of a few monocots like Dracaena, Yucca, Agave, Aloe and Sansevieria, show anomalous secondary growth.

Fig: Primary and Secondary Growth in Dicot roots

Secondary Growth in Dicot Root: Process

Secondary growth occurs in all dicotyledonous woody plants—the root increases in girth by stelar and extrastelar cambium activity.

(A) Origin and Activity of Vascular Cambium

1. The vascular bundles that are present in dicot roots are radial.
2. The xylem is exarch. That means the protoxylems are directed towards the periphery and the metaxylem towards the centre.
3. The cambium is absent initially but develops later at the time of secondary growth.
4. The pericycle cells lying outside the protoxylem and the conjunctive parenchyma cells on the lateral side of the phloem bundle become meristematic to form many cambial strips.
5. During development, these strips become continuous laterally. This is due to the tangential divisions of pericycle cells opposite each protoxylem layer. Thus, a wavy continuous cambium ring is produced.
6. This ring is present below the phloem but above the protoxylem and is a secondary meristem.
7. The cambial cells divide tangentially again and again and produce secondary tissues, joined with the first formed cambium strips to form a complete but wavy continuous ring of the vascular cambium.
8. The cambial cells produce more xylem elements than phloem elements and produce secondary xylem and secondary phloem like the secondary growth in stems.
9. First, the strips of cambia below the phloem cut off the new secondary xylem inward.
10. The vascular cambium developed from the pericycle gives rise to only ray initial cells. The development of these ray initials is slower than the formation of secondary vascular tissues.
11. Due to this, the depressed parts of the vascular cambium move outward, and ultimately the cambium becomes circular.
12. Thus, after stellar secondary growth, a solid central cylinder of wood is developed, which is surrounded by a bast.

(B) Origin and Activity of Cork Cambium

1. The cork cambium or phellogen, or secondary meristem, develops because of the tangential division of the outer cells of the pericycle.
2. The activity of cork cambium is like that found in the dicot stem, so it produces cork cells or phellem towards the outside and phelloderm or secondary cortex towards the inside.
3. Suberin is a lipid-like substance secreted from the protoplast of cork cells deposited in the walls.
4. As the cork cambium in the dicot root develops from the pericycle cells, all tissues lying outside the cork (endodermis, cortex) are completely sloughed off.
5. These cells die due to further deposition of suberin. The epiblema dies out earlier, and the lenticels may also be formed.

Fig: Different Stages of Secondary Growth in Roots

Fig: Secondary Growth in Roots

Difference Between Secondary Growth in Dicot Stem and Dicot Root

The difference between the secondary growth in dicot stem and dicot root are mentioned below:

Summary

Most roots of dicotyledonous undergo secondary growth. During this period of growth, the thickness or girth increases by the activity of the vascular cambium and extrastelar cambium. Secondary growth is common in stems of dicots. Dicot roots also show secondary growth, and the mechanism is much similar to that of the stem. Secondary growth in dicot roots is essential to provide strength to the growing aerial parts of the plants. The formation of secondary tissue in the dicot roots and dicot stems are almost similar, but the initiated process in both is different.

Attempt Mock Tests

FAQs About Secondary Growth in Dicot Root

Let’s look at some of the commonly asked questions about Secondary Growth in Dicot Root:

Q Where does secondary growth take place in dicots?
Ans: In secondary growth, the diameter or circumference of the root increases. These secondary merisms are responsible for the formation of vascular cambium and cork cambium. Dicots are the only plants to possess vascular cambium and cork cambium.

Q What is the difference between Secondary Growth in the Dicot stem and Dicot root?
Ans: The difference between secondary growth in dicot stem and dicot root are:
In dicot stem:
1. The formation of the cambial ring is circular in cross-sections from the beginning.
2. The xylem is an endarch in the dicot stem.
In dicot root:
1. In the beginning, the formation of the cambial ring is wavy and later becomes circular in the dicot root.
2. The xylem is exarch in the dicot root.

Q Do gymnosperms show secondary growth?
Ans: Yes. Like dicotyledonous plants, gymnosperms also show profound secondary growth in the stem.

Q Why do monocots lack secondary growth?
Ans: Monocots do not have vascular cambium. So, monocots lack secondary growth. Sometimes plants like Dracaena, Aloe, Agave, Yucca, etc., plants show an increase in girth without secondary growth is referred to as anomalous thickening.

Q Can secondary growth take place in dicot roots?
Ans: Many dicotyledonous roots show secondary growth in thickness. The roots of extant vascular cryptogams and most monocotyledons do not show any secondary growth because they remain entirely primary throughout their life.

Q When does secondary growth begin in the dicot root? What happens first?
Ans: When secondary growth begins in the dicot root first, the cells of the pericycle lying outside the protoxylem also become meristematic to form part of cambium strips.

Study Ground tissue system

We hope you find this article on Secondary Growth in Dicot Root helpful. In case of any queries, you can reach back to us in the comments section, and we will try to solve them.