Secondary Growth in Dicot Stem: What to Know

Secondary Growth in Dicot Stem: Some trees like Eucalyptus, Bombax, Neem, Mahogany, etc., are exceptionally taller than others. They will also have thicker and woody stems. But do you know how these trees develop such thick stems? This thick stem is an example of Secondary growth in the Dicot Stem. 

Secondary growth is described as the formation of secondary tissues from lateral meristems. As a result, the diameter of the stem is increased. Secondary growth usually occurs in the stems and roots of perennial Gymnosperms and Dicots such as trees and shrubs due to cell division in the cambium. The thickness of a dicot stem grows due to the addition of secondary tissues, separated by the cambium and the cork cambium in the stelar and the extra stelar regions. Scroll down to learn more about the Secondary growth in Dicot stems.

Secondary Growth in Dicot Stem Definition

The formation of secondary tissues which lead to an increase in girth is called Secondary Growth in Dicot Stem. Secondary tissues are formed by two types of lateral meristems– vascular cambium and cork cambium.

Arihant Biology Handbook

Vascular Cambiums and Cork Cambium produce the Secondary Vascular Tissue and Periderm, respectively. Secondary growth in the stem increases the diameter of the stem. It takes place in perennial Gymnosperms and Dicot trees, and many shrubs. Secondary growth is the formation of secondary tissues from lateral meristems. In woody plants, secondary tissues constitute the bulk of the plant. 

Fig: Increase in Girth of Eucalyptus Wood

Secondary Growth in Dicot Stem Examples

Now the students know the answer to the question ‘describe the process of secondary growth in dicot stem’. Secondary Growth usually occurs in most of the stems of woody trees like Cassia, Dalbergia sissoo, Acacia, Teak, Olender, mango, neem, Eucalyptus, etc. Secondary Growth is also exhibited in tomatoes, potatoes, etc. though they are not woody.

As we all know, secondary Growth is absent in monocots due to the lack of cambium, but Palm, Yucca, Dracaena, etc., are few exceptions of secondary Growth in monocots.

Secondary Growth in Dicot Stem Process

Normal secondary growth in the dicot stem increases the diameter of the plant stem due to the activity of the vascular cambium.

A. Formation of Cambium Ring

1. The vascular bundles in the dicot stems are conjoint, collateral, and open (cambium is present) and are arranged in a ring.
2. The cambium present between the xylem and phloem in vascular bundles is called Fascicular or Intrafascicular cambium.
3. In secondary growth, some cells of Medullary rays that are present between vascular bundles also become meristematic, and it is known as Interfascicular cambium.
4. Both intrafascicular cambium and interfascicular cambium collectively constitute a complete ring which is known as the Cambium ring.
5. The cambium ring acts to give rise to secondary growth.

Fig: Cambium Ring

B. Formation of the secondary tissues

1. The cambium ring cuts off cells on both sides. On the outer sides, it produces secondary phloem and towards the inside, secondary xylem.
2. The amount of secondary xylem cut off is more than the secondary phloem.
3. The cambium ring moves towards the periphery because more secondary xylem cells are formed towards the centre.
4. The primary xylem and primary phloem, which were initially closed, move apart due to the formation of the Secondary xylem and Secondary phloem.
5. The layers of secondary tissues are gradually added continuously to both the inner and outer sides of the cambium throughout the life of the plant.

C. Formation of secondary medullary rays

1. The cambium cells are of two types: Fusiform initials and Ray initials. 
2. The fusiform initials are vertically elongated cells that give rise to vertical elements, for example, vessels, tracheids, etc.
3. The ray initials are horizontal in position, smaller. They give rise to rays in secondary tissue and produce Parenchymatous cells.
4. The primary and the secondary phloem gets gradually crushed due to the continued formation and accumulation of secondary xylem.
5. The primary xylem, however, remains intact near the centre.
6. At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and secondary phloem in the radial direction. These are called Secondary Medullary rays.
7. The secondary medullary rays provide the radial conduction of food from the phloem and water and mineral salts from the xylem.

Fig: Secondary Medullary Rays

D. Formation of Annual Rings

1. The activity of cambium is affected by the change in season.
2. During springtime, due to favourable weather and thus, more activity of cambium, Springwood or earlywood is formed. The lumens are wider in springwood.
3. In winter or autumn, the activity of cambium is less, and it forms narrower and smaller vessels. The wood formed in winter and autumn is called Winter and Autumn wood or Latewood.
4. Alternate Springwood and Autumn wood of a year constitutes Annual ring or Growth ring. This is a reliable method to find the age of a tree (by counting the number of annual rings). The science dealing with the age-counting of trees is called Dendrochronology.

Fig: Annual Ring

E. Formation of Heartwood and Sapwood

1. The secondary wood of the inner side loses the power of conduction in old trees where enough secondary growth has taken place.
2. The central portion of the stem is dark, hard, and tough due to the presence of tannins, resins, gums, and essential oils. This region is called the Heartwood or Duramen.
3. The heartwood ceases the function of conduction due to the formation of tyloses in vessels. Hence heartwood provides mechanical support to the stem.
4. The outer region of the trunk is lighter and soft, which performs the functions of conduction of water and minerals. This region is called Sapwood or Alburnum.

Fig: Transverse Section of Stem Showing Heart Wood and Sap Wood

F. Secondary growth in extrastelar region due to activity of cork-cambium

1. The increase in diameter or thickness of the stem is due to the activity of the vascular cambium. 
2. This results in the breaking down of outer cortical and epidermis layers and needs to be replaced to provide new protective cell layers.
3. The outer cortical cells or some cells of the hypodermal layer become meristematic and begin to divide, which forms cork cambium or phellogen.
4. Then the division of cork cambium takes place to form secondary tissue on both internal and external sides. 
5. The cells cut off on the outer side are phellem, or cork cells and those on the inner side form a secondary cortex or phelloderm.
6. Thus Phellogen, Phellem and Phelloderm collectively called Periderm.

Fig: Extrastelar Secondary Growth in Dicot Stem

Fig: Cork Cambium

G. Formation of Bark

1. The bark is defined as all the tissues outside the vascular cambium. 
2. The bark that is formed at the beginning of the season is called early or soft bark, and at the end of the season, late or hard bark is formed.
3. This layer is highly suberised and impermeable to water.

H. Formation of Lenticels

1. Lenticels are some loosely arranged areas in the periderm. 
2. Due to more activity of certain portions of phellogen, the phellem cells are cut off very rapidly and hence these cells are loosely arranged with many intracellular spaces. They help in gaseous exchange and transpiration.
3. These loosely arranged cells are called Complementary cells.
4. Lenticels are characteristics of woody stems.

Fig: Lenticel

Secondary Growth in Dicot Stem Diagram

Fig: Secondary Growth in Dicot Stem

Difference Between Springwood and Autumnwood

The difference between Springwood and Autumnwood is given below:

Difference Between Sapwood and Heartwood

The difference between Sapwood and Heartwood is given below:

Summary

Secondary growth occurs in most of the stems of dicots—the thickness or girth of the stem in dicot increases due to secondary vascular tissues and periderm. The vascular bundles in the dicot stem are arranged in the form of a ring. The activity of the cambium ring gives rise to secondary growth in dicot stems. The secondary growth in the dicot stem helps in the conduction of water and nutrients, storage of food, and it also provides mechanical support to the stem.

FAQs on Secondary Growth in Dicot Stem