The Secondary growth in plants causes the thickening of stem and root due to cell division in the lateral meristems. In contrast, primary growth occurs due to the cell division of apical meristems and increases the length of stems and roots.

Plants, for the most part, keep growing throughout their lives. Plants, like other multicellular creatures, grow by a combination of cell division and growth. Cell division (mitosis) increases the number of cells, whereas cell expansion increases cell size. Through cellular differentiation, plant cells get specialised into multiple cell types as they grow. Let us learn about Secondary Growth in Plants through this article.

Secondary Growth in Plants Definition

The formation of secondary tissues which lead to an increase in girth is called secondary growth. Secondary tissues are formed by two types of lateral meristems– vascular cambium and cork cambium. (Source: Arihant Biology Handbook). Or in other words, Secondary growth in plants is the growth that results from cell division in the cambia (or the lateral meristems) which causes the stems and roots to thicken

Types of Growth in Plants

Growth is an irreversible, permanent increase in plants. Growth is mainly two types in plants: Primary growth and Secondary growth.

(a) Primary growth– Primary growth increases the length of the shoot and the root of the plant body, which is the result of cell division in the shoot apical meristem. 
(b)Secondary growth-Secondary growth increases the thickness or girth of the plant, which is the result of cell division in the cambia or lateral meristem. 

Secondary Growth Occurrence

Secondary growth takes place in dicots (angiosperms), and gymnosperms. It is rarely found in monocots. Secondary growth occurs in the stem and roots of a plant and causes the thickening of the stem and roots due to cell division.

In Pteridophytes and most monocotyledonous plants, the primary plant body is nearly completed in itself and does not produce any secondary tissue. In most of the dicotyledonous plants, there are distinct phases of secondary growth that increases the girth.

Types of Secondary Growth in Plants

Secondary growth in dicots occur in two ways:

1. Secondary tissue formed by the vascular cambium or true cambium or intrastelar cambium results in the formation of secondary xylem and secondary phloem.
2. Secondary tissue formed by the cork cambium or phellogen or extrastelar cambium results in the formation of the secondary cortex (or phelloderm) and cork cells (or phellem).

Tissue Responsible for Secondary Growth in Plants

The most important tissue responsible for secondary growth in plants is termed Cambium (plural cambia). This is a layer of tissue that provides partially undifferentiated cells needed for plant growth, found mainly in between the xylem and the phloem inside the stele. There are several types of cambia:

1. Vascular cambium is the lateral meristem present inside the stele (vascular tissue). Dicots have open vascular bundles where a stripe of cambium is present between the xylem and phloem.
2. Cork cambium is also called Phellogen which is basically a permanent tissue regaining the meristematic activity. This is a part of the periderm. This attributes to the extrastelar secondary growth.
3. Unifacial cambium produces cells to the interior of its cylinder. This only forms xylem and not the phloem to the exterior.

Significance of Secondary Growth

Most Gymnosperms and most perennial dicot trees show marked secondary growth. The following are the major significance of such growth:

1. Due to an increase in the girth (diameter), the aerial parts of the plant get better support to grow further.
2. The formation of cork, bark, etc., around the trunk protects the tall tree from abrasion, infection, etc.
3. The formation of new conducting tissues efficiently conducts sap and food for a longer distance. The non-functional, old conducting tissues are replaced.
4. Complex organic deposits in the heartwood (like tannins, gums, etc.) makes them resistant to microbial attack. Thus, secondary growth also contributes to the longevity of plants.

Secondary Growth in Gymnosperms

Certain Gymnosperms show typical secondary growth. They are tall, perennial, woody genera, like Pinus. Typically, the secondary wood in most Gymnosperms are manoxylic (non-compact wood with a large amount of parenchyma, large pith and the cortex mixed with less tracheids) or pycnoxylic (wood with a large amount of xylem tracheids or wood and a small amount of cortex and pith with little parenchyma).

1. The vascular cambium contributes more secondary xylem than secondary phloem. Xylem is typically devoid of vessels. There are distinct late wood and early wood formations contributing to the development of prominent annual rings. An annual growth ring is composed of the early wood and late wood formed in a single year.
2. The periderm layer is also significantly prominent. The cork cells may be empty and highly suberised. The cork cambium, which may be one or two cells thick, with prominent nuclei, cut off phelloderm or secondary cortex below them.

Fig: Secondary Growth in Gymnosperms.

Secondary Growth in Dicot Stem

The secondary growth in the dicot stem takes place by the activity of cambium at the stellar region and cork cambium at the extra stellar region. 

(i) Vascular Cambial Activity

1. Some of the parenchyma cells of the medullary rays adjoining the vascular cambium become meristematic and constitute interfascicular cambium.
2. A cambium ring is formed by the joining of the interfascicular and intrafascicular cambiums.
3. Secondary medullary rays provide radial conduction of food from the phloem and water and mineral salts from the xylem. 
4. The wood formed in the spring is known as springwood, and that formed in the dry summer or cold winter, autumn wood or latewood.
5. The formation of latewood and early wood forms the annual growth rings, which is a characteristic feature of dicot stems.
6. The numbers of annual rings determine the age of a tree. This branch of science is known as dendrochronology.

(ii) Cork Cambial Activity

1. Phellem, phellogen and phelloderm are collectively called periderm.
2. Cork cambium contributes to the formation of the secondary cortex inward, which is termed as phelloderm.
3. The cork cells are also formed outward of the cork cambium.
4. All dead tissues lying outside the active cork-cambium are collectively known as bark.

Secondary Growth in Dicot Stem Diagram

Fig: Secondary Growth in Dicot Stem

Secondary Growth in Dicot Roots

Most of the dicotyledonous roots show secondary growth in thickness, similar to that of dicotyledonous stems. However, there are certain differences.

1. The secondary vascular tissues originate as a result of the cambial activity. The phellogen gives rise to the periderm.
2. The cells of the pericycle lying outside the protoxylem also become meristematic to form part of cambium strips.
3. These cambial strips join the first formed cambium strips to form a complete but wavy ring of the vascular cambium.
4. From the outer layers of the pericycle arises the phellogen, which cuts phellem (cork) on the outer side and secondary cortex or phelloderm toward the inner side. However, lenticels may not be formed.

Secondary Growth in Dicot Root Diagram

Fig: Secondary Growth in Dicot Root

What is Anomalous Secondary Growth in Plants?

Anomalous secondary growth is an abnormal type of secondary growth that is not commonly seen and is present in a few limited families or genera.
a) In certain arborescent monocots like Dracaena, Yucca, Agave, Aloe and Sansevieria, the secondary growth occurs from the abnormal cambium.
b) The storage roots like carrot and beetroot form accessory cambial rings outside the endodermis.
c) Bougainvillea is an example of a dicotyledonous stem that shows anomalous secondary growth in which a series of cambia arise outside the oldest phloem.
d) Boerhaavia stem show anomalous secondary growth takes place by accessory cambium formation and its activity.
e) Nymphaea, Piper, Papaver, etc., dicotyledons show scattered vascular bundles.

Summary

Secondary Growth in plants is an increase in the diameter of the stem and root of that plant. It takes place due to cell division in lateral meristems. Primary and secondary growth takes place in two different parts of the woody plant. Secondary growth increases the width of the stem and roots, whereas primary growth is responsible for the elongated growth of the plant.

Stems have comparably more secondary growth than roots. Most of the dicots and gymnosperms show secondary growth in their roots. Secondary growth takes place by the formation of two types of tissue: secondary vascular tissue and Periderm.

Frequently Asked Questions

We have provided some frequently asked questions about secondary growth in plants here:

Q.1. Why is secondary growth important to plants?
Ans: The secondary growth increases the thickness or girth of the plant. It produces a corky bark around the tree trunk, which protects the interior of the plant.

Q.2. Which is the best example of anomalous secondary growth?
Ans: Bougainvillea is the best example of the dicotyledonous stem, which shows anomalous secondary growth, a series of cambia that arise outside the oldest phloem.

Q.3. Where does secondary growth occur in plants?
Ans: Secondary growth occurs in dicots, angiosperms, and gymnosperms. It is rarely found in monocots. Secondary growth occurs in the stem and roots of a plant.

Q.4: What are secondary tissues in plants?
Ans: The secondary tissues in plants are vascular cambium and cork cambium.

Q.5. What is the basic difference between primary and secondary growth?
Ans: Primary growth increases the length of the plant, whereas secondary growth increases the thickness of the plant. The apical meristem causes the primary growth, whereas the lateral meristem causes the secondary growth of the plant.   

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