Secondary Growth in Stem: Formation of Secondary Vascular Tissues, Cork and Secondary Cortex, Practice problems and FAQs

Cinnamon is the first spice that comes to mind while thinking of buying spices for biryani. A chicken biryani without cinnamon and cardamom is just chicken and rice. Cinnamon is a very expensive spice that is used in garam masala. But do you know what exactly cinnamon is? It is a part of the tree. But do you know which part? 

                        Fig: Cinnamon

Cinnamon is obtained from the Cinnamomum tree's inner bark. Do you know what is bark and what is it made up of? Bark includes all the living and non-living tissues that lie outside the vascular cambium, such as the primary and secondary phloem, primary cortex and cork cambium. You must be wondering, I had so far known about primary phloem but what is this secondary phloem? Where does it come from? And why does the plant have different types of cambium? What are their functions? 

Well, the secondary tissues are formed during the secondary growth of the plant which helps the plant to increase in girth. The vascular cambium and cork cambium are largely responsible for secondary growth in plant stems. So now we are going to discuss more about the secondary growth in stem. Do you think secondary growth is present in both dicots and monocots? Let’s find out by understanding the process.of secondary growth in stems.

Table of contents:

• Secondary growth in dicot stem
• Secondary growth in monocot stem
• Practice Problems
• FAQs

Secondary growth in dicot stem

Growth is an irreversible increase in the size of the body or organs. Plants show both primary growth (increase in length due to action of apical meristem) and secondary growth (increase in girth). 

Secondary growth in the stem is seen only in dicot plants and is due to the activity of the vascular cambium and the cork cambium. The vascular cambium helps in the formation of the secondary vascular tissues and the cork cambium helps in the formation of the cork and secondary cortex. Let’s discuss more about this.

Formation of secondary vascular tissues 

If we observe the cross section of a dicot stem, we can see the layer of meristematic cambium present in between the xylem and phloem. This makes the vascular bundle open to secondary growth. 

Two types of cambium are present in dicot stems. They are intrafascicular cambium and interfascicular cambium.

                           Fig: Types of vascular cambium

Intrafascicular cambium is the patchy or non-continuous cambium between primary xylem and primary phloem. 

When the cells of medullary rays next to the intrafascicular cambium become meristematic, the interfascicular cambium is formed.

                         Fig: Intrafascicular and interfascicular cambium

Formation of cambial ring

As the interfascicular cambium and intrafascicular cambium start dividing during secondary growth, they eventually unite together to form the cambial ring or vascular cambial ring. 

                           Fig: Cambial ring formation

Activity of the cambial ring

The cambium ring becomes active and the cells start dividing to cut off cells on both sides. Vascular cambium possesses two types of cells and they are fusiform initials and ray initials.

Fusiform initials are spindle shaped cells and form the vascular tissue mother cells which gives rise to the secondary xylem and secondary phloem. Secondary xylem is formed towards the inside and the secondary phloem is formed towards the outside. Fusiform initial divides to form two daughter cells. One of the two daughter cells continues to divide and remains as cambial cells. The other daughter cell is differentiated into secondary phloem (if on the outer side) or xylem (if on the inner side). Cambium is more active towards the inside, hence more secondary xylem is formed than secondary phloem. 

Secondary xylem consists of thick vessels, tracheids, xylem fibres and xylem parenchyma and the secondary phloem consists of sieve tubes, companion cells, phloem fibres and phloem parenchyma.

The primary phloem gets completely crushed during secondary growth. Primary xylem is pushed into the pith. Hence the pith becomes narrow. Secondary xylem forms a continuous cylinder around the pith and forms the bulk of the plant.

                                                     Fig: Cambial ring

Ray initials are small isodiametric cells. They give rise to the xylem and phloem rays known as the secondary medullary rays. They form the radially elongated parenchymatous cells which pass through the secondary xylem and secondary phloem. They conduct water and food in the radial direction. They help in the storage of food and exchange of gases also. 

                                        Fig: Secondary medullary rays

So, now we know how the cambial ring is formed and how it contributes to the development of secondary vascular tissues. Let us now see how the cork cambium functions to form the secondary cortex and cork.

Formation of cork and secondary cortex

The formation of secondary vascular tissues puts pressure on the peripheral tissues. It results in the rupture of the peripheral cortex. In order to protect the inner tissues, a layer of secondary meristem originates from the outer layer of the cortex. This is known as cork cambium and gives rise to periderm. Periderm consists of phelloderm (secondary cortex), phellem (cork) and phellogen (cork cambium). 

                                                        Fig: Periderm

Activity of cork cambium or phellogen

Cork cambium divides on both sides and forms secondary tissues. The secondary tissue formed inside is called the secondary cortex or phelloderm. Cork cambium produces cork or phellem on the outer side. 

Phelloderm or the secondary cortex is made up of parenchyma or collenchyma cells.

Due to the secondary growth of the vascular cambiums, the epidermis bursts. The epidermis is replaced by a protective layer known as the cork or phellem which is formed by the cork cambium or phellogen on the outer side. It consists of rectangular and thick walled dead cells. Cells are packed compactly. They possess lignified and suberised walls. Cork is impervious to water due to the presence of suberin in the cell walls. Pressure is formed on the layers outside the cork by the activity of the cork itself. The layers which are peripheral to phellogen die and slough off. 

Bark 

Layers beyond vascular cambium are termed as the bark. It is a non-technical term. It includes both living tissues (primary and secondary phloem, primary cortex and cork cambium) and non-living tissues (cork).

                                                     Fig: Bark

Classification of bark

It is classified on the basis of the season in which it is formed. Early bark or soft bark is formed early during the spring season. Late bark or hard barkis formed towards the end of the spring season.

Do you know what is the reason for the texture of the bark? It is due to the cork cambial activity. As a result of the division of cork cambium, pressure builds up on the phellem or cork and the phellem parts die and fall off. This gives the texture to the bark.

Lenticels 

Lenticels are aerating pores in the bark of woody plants. These lens-shaped openings are created when the epidermis ruptures. They exchange gases between the outer atmosphere and the internal tissues. They are found as raised circular, oval, or elongated parts on the surface of the bark. Phellogen or cork cambium in this region gives rise to complementary cells instead of cork or phellem. Complementary cells are closely arranged parenchyma cells which enclose intercellular spaces. They mostly occur in woody trees.

                                                Fig: Lenticel

Secondary growth in monocot stem

Primary tissues make up the majority of monocotyledons. This group lacks the typical vascular cambium, hence there is no regular secondary development. In some monocots, however, the action of primary thickening meristem, diffuse secondary thickening due to the action of ground parenchyma and the procambium cells, and the action of the secondary thickening meristem help in thickening and elongation of the stem.

Significance of secondary growth in stem

Secondary growth helps to increase the girth of the plant and also helps in providing mechanical support to the growing plant. It helps to replace old and non-functioning conducting tissues. Production of cork helps to protect underlying layers from injury, heat, cold, desiccation, etc.

Practice Problems

Q 1. The intrastelar region experiences secondary growth as a result of the action of

a. interfascicular cambium
b. intrafascicular cambium
c. cork cambium
d. both (a) and (b)

Answer: All tissues within the endodermis are included in the intrastelar area. The pericycle, vascular bundles, medullary rays, and pith are all included. Only dicot stems contain stele. Secondary growth refers to the expansion of the plant's circumference. The intrafascicular vascular cambium (found between the xylem and phloem of a vascular bundle) becomes active during secondary growth and joins the interfascicular cambium (present between the vascular bundles). Together they unite to form the vascular cambium ring which divides and adds secondary xylem on the inner side and secondary phloem on the outer side.

Hence the correct option is d.

Q 2. Which of the following statements concerning secondary growth is false?

a. A plant's girth grows as a result of it.
b. It's found in dicotyledonous and gymnosperm plants.
c. Cambium is formed when medullary ray cells redifferentiate.
d. To accommodate the increased demand for food and water, it adds new conducting tissue.

Answer: The procambium's surviving cells create the intrafascicular cambium. To regain meristematic potential, some differentiated (permanent) cells of the medullary rays, which surround the vascular bundles in the dicot stem, dedifferentiate. The interfascicular cambium is generated between the vascular bundles by these meristematic cells.

Hence the correct option is c.

Q 3. How many of the tissues listed below have a secondary origin?
Interfascicular cambium, Phellem, Intrafascicular cambium, Phelloderm, Phellogen

a. 2
b. 3
c. 4
d. 5

Answer: All tissues derived from the secondary meristem are referred to as secondary tissues. Secondary development from medullary ray cells forms the interfascicular cambium. Phellem, often known as cork, is a secondary tissue derived from the cells of the cork cambium. Intrafascicular cambium is a primary tissue meristem present in the vascular bundles of dicot plants between xylem and phloem. The secondary cortex, also known as phelloderm, is a secondary tissue that develops on the inner side of the cork cambium. It is made up of parenchymatous cells with thin walls. Phellogen, also known as cork cambium, is a secondary meristem that arises from the outer layer of the cortex.

Hence the correct option is c.

Q 4. Choose the most accurate statement about medullary rays.

a. The nature of the medullary rays can be parenchymatous or collenchymatous.
b. Within primary vascular bundles, primary medullary rays can be found.
c. The cork cambium produces secondary medullary rays.
d. They aid in the radial transmission of food and water.

Answer: Medullary rays are radially oriented parenchymatous cells between adjacent vascular bundles. Because they are extensions of pith, they are also known as pith rays. They reach all the way to the pericycle and cortex. The vascular cambium forms secondary medullary rays, which travel horizontally across the secondary xylem and phloem as radiating stripes or bands. The radial conduction of food, water, and minerals is aided by both primary and secondary medullary rays.

Hence the correct option is d.

FAQs

Q 1. When does a stem's secondary growth begin?
Answer: Secondary growth develops in the stem of many seed plants in their first year and lasts for several years. These are woody plants. Secondary tissues such as periderm and wood, as well as tertiary structures such as bark, form in them.

Q 2. Which plants do not have a woody stem?
Answer: Herbaceous plants are those that don't have a woody stem that sticks out above ground. They are categorised as annuals, biennials, or perennials based on their life cycle.

Q 3. How do you distinguish between lenticels and stomata?
Answer: Plants have two different types of tiny pores: stomata and lenticels. They are in charge of the gas exchange in general. Stomata and lenticels differ primarily in that stomata are found in the lower epidermis of leaves, whilst lenticels are found in the periderm of woody trunks and stems. 

Q 4. Which tree is largely used for the commercial production of cork?
Answer: Cork is obtained commercially from the Oak tree (Quercus suber) which is a tree found in the Western Mediterranean regions. Spain, Portugal, france, Italy, tunisia and Morocco are some of the largest producers of this tree. Cork is largely used for the purpose of insulation, as bottle stoppers, shock absorbers, etc.

YOUTUBE LINK: https://www.youtube.com/watch?v=tG4Yw6eBotY

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