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Epidermal Tissue System: Epidermis, Stomata, Epidermal Appendages, Practice problems and FAQs

Epidermal Tissue System: Epidermis, Stomata, Epidermal Appendages, Practice problems and FAQs

What is the first line of defence against environmental factors in your body? It is the skin, correct? What is the outer layer of the skin known as? It is known as the epidermis. Did you know that plants also have an epidermis? So does that mean that plants also have a skin similar to ours? Well, the answer is no. The epidermis in plants and animals vary a lot, even though they perform somewhat similar functions.

The epidermis in plants is a part of the epidermal tissue system. You must be wondering what a tissue system is? Let us try and understand that. 

Every job becomes easy if many individuals work together for its completion. A single person could do a simple job. But that person will need some help from others to perform a difficult task. A cell, too, can perform all basic life functions but for multicellular organisms, different cells with a common origin and a common function, come together to get all the necessary jobs done for the organism at a larger scale. These groups of cells form tissues and many such tissues come together to form tissue systems.

In plants, there are different tissue systems to perform different functions. On the basis of the structure and location there are three types of tissue systems in plants - epidermal tissue system, ground tissue system and vascular or conducting tissue system. This classification was proposed by Sachs in 1975. 

Let's discuss more about the epidermal tissue system.

image

Fig: Types of tissue systems

Table of contents:

Epidermal tissue system

The outermost layer or covering of the plant body is formed of the epidermal tissue system. It is derived from the protoderm. The epidermal tissue system consists of the following components-

  • Epidermis
  • Stomata
  • Epidermal appendages

image

Fig: Epidermal tissue system

image

Fig: Components of epidermal tissue system

Epidermis

Epidermis is the outermost layer made up of parenchyma cells. It possesses elongated and compactly arranged cells and is single layered in most cases. A large vacuole and a thin layer of cytoplasm lining the cell wall is present in the epidermal cells. 

image

Fig: Epidermis

In some plants, a few epidermal cells are modified to perform special functions. For example, epidermal cells present in the aerial roots of orchids are modified to a hygroscopic velamen tissue that absorbs water from the atmosphere. Similarly, in some monocot plants like grasses, the epidermal cells are modified into large, thin walled cells with big water-filled central vacuoles. Such cells are known as bulliform cells and help the leaves of these plants to roll up and reduce the rate of transpiration by decreasing the exposed surface area.

Cuticle

Have you ever wondered why leaves of some plants do not get wet even after a heavy downpour? It is because of the presence of a waxy layer named cuticle which coats the epidermis. Cuticle is composed of the deposition of water-resistant substances such as cutin or suberin and protects the epidermal cells from desiccation and mechanical injuries.

The epidermal cells present in roots are devoid of cuticle and are known as epiblema.

image

Fig: Cuticle

Stomata

The epidermis is interrupted at places by the presence of tiny pores known as the stomata. Stomata are seen in the epidermis of leaves and young shoots. They can regulate the process of transpiration and exchange of gases.

image

Fig: Stomata

Structure of stomata

Each stoma consists of a complex called stomatal apparatus. It consists of the following:

  • Stomatal aperture
  • Guard cells
  • Subsidiary cells

image

Fig: Stomatal apparatus

Stomatal aperture or stoma

It is the pore present in between the guard cells. Movement of gases takes place through the pore.

Guard cells

Cells surrounding the stomatal aperture or pore are called guard cells. The guard cells possess chloroplasts and are capable of photosynthesising. The outer wall of a guard cell is thin and flexible whereas the inner wall is thick and rigid. 

image

Fig: Thick inner wall and thin outer wall of guard cells

Guard cells regulate the closing and opening of the stomata. When solute concentration in the guard cells is high, the water from neighbouring cells enters the guard cells by endosmosis and the guard cells become turgid and expand. Due to the presence of a rigid inner wall and a flexible outer wall, expansion causes the guard cells to bulge towards the outer side and results in opening of the stomatal aperture. This mostly happens during the day when the guard cells produce sugars by photosynthesis.

When the solute concentration in the guard cells becomes less, the water from the guard cells leak out by osmosis. This causes the guard cells to become flaccid and collapse which is why the stomatal aperture closes.

Guard cells are bean shaped in dicots. They are dumb-bell shaped in monocots. 

image

Fig: Guard cells in dicots and monocots.

Subsidiary cells

Subsidiary cells are specialised epidermal cells seen around guard cells. When subsidiary cells lie above the guard cells, it is called sunken stomata.

image

Fig: Subsidiary cells

Epidermal Appendages

The outgrowths present on the epidermis are called epidermal outgrowths. On the basis of the location of epidermal appendages, there are of different types as follows: 

  • Root hairs
  • Trichomes
  • Prickles

Root hairs

Root hairs are the unicellular elongations from root epidermis or epiblema.They help in absorbing water and minerals from soil and also help in anchorage. 

image

Fig: Root hairs

Trichomes

Trichomes are unicellular or multicellular outgrowths from the shoot epidermis. Trichomes can be soft or stiff. Trichomes occur in various forms -

  • Nonglandular hair - These can be unicellular or multicellular, branched or unbranched, stellate or T shaped. These trap air on the surface of the leaves and help to reduce loss of water by transpiration and also protect from very high or low temperatures.
  • Glandular hairs are usually multicellular in nature and secrete different substances such as mucilage, salt, nectar, etc. Glandular hairs can also act as digestive glands, aromatic glands or stinging glands.
  • Scales are multicellular, flattened, scale like trichomes that occur in pitcher plants and also form the ramenta (hair-like structures present on the stem) of some ferns.
  • Colletors are unicellular or multicellular glandular trichomes that secrete a sticky substance.
  • Some trichomes are modified to become bladders for water storage.

image

Fig: Trichomes

Prickles

Prickles are multicellular and stiff epidermal outgrowths. They are devoid of vascular tissue and can be easily plucked out. They protect the plant from herbivores and also prevent excess loss of water. Prickles can be found in roses.

Practice Problems

Q1. Which of the following is not a part of the epidermal tissue system?

A. Trichomes
B. Companion cells
C. Guard cells
D. Subsidiary cells

Solution: The companion cells are present in the phloem and support the function of the sieve tubes. Thus, these cells are not a part of the epidermal tissue system but are a part of the vascular tissue system.

Hence the correct option is b.

Q2. Trichomes take part in 

A. Transpiration and exchange of gases 
B. Protection and reduction of transpiration 
C. Exudation of water drops 
D. Desiccation

Solution: Trichomes are unicellular or multicellular appendages arising from the epidermis. Some forms of non-glandular hair like trichomes help to reduce transpiration by trapping air on the surface of the leaves. The stinging glandular hair like trichomes in plants like Stinging Nettle help to protect against grazing animals. Trichomes also protect the plants from invasion by insect pests.

Hence the correct option is b.

Q3. The sugarcane plant has

A. dumb-bell shaped guard cells
B. tap-root system
C. reticulate venation
D. open vascular bundle

Solution: Sugarcane plants are monocots and like all monocots, they have dumb-bell shaped guard cells. 

Hence the correct option is a.

Q4. Choose the odd one out.

A. Cortex, pericycle, pith
B. Trichomes, stomata, cuticle
C. Root hair, guard cells, subsidiary cells
D. Sieve tubes, vessel elements, root hairs

Solution: Here, the pattern that is seen in each set is that it consists of the members of a single tissue system.

Cortex, pericycle and pith are parts of the ground tissue system. Trichomes, stomata and cuticle are parts of the epidermal tissue system. Root hair, guard cells and subsidiary cells are also parts of the epidermal tissue system.

However, in option d, root hairs are extensions of epidermal cells of the roots whereas sieve tubes and vessel elements are parts of the xylem tissue of the vascular tissue system.

Hence the correct option is d.

FAQs

Question 1. Why is the cuticle absent in the roots?
Answer: The cuticle is a waxy layer that is generally present outside the epidermis of leaves and stems. It is a waterproof substance made up of a polymer named cutin and hence it is absent in roots which are actively involved in absorption of water from the soil.

Question 2. Name some plants which have more than one layer of epidermal cells.
Answer: Epidermis is usually single layered in most plants. However, leaves of Nerium have a multilayered epidermis and leaves of Ficus elastica have a double layered epidermis.

Question 3. What is the significance of the epidermal tissue system in plants?
Answer: The epidermal tissue system protects the plants against mechanical injury, parasitic infection, environmental stress, unnecessary loss of water, etc. 

In Stinging Nettle, the epidermal hair protects from herbivorous animals. Stomata on the epidermis help in the exchange of gases and transpiration. Root hairs help in absorption of water and minerals. Trichomes aid in reducing transpiration and prickles provide protection against grazing animals.

Question 4. What is the difference between the arrangement of stomata in monocots and dicots?
Answer: In dicot plants the leaves are dorsiventral in nature, i.e, the dorsal and ventral surfaces are different and have more number of stomata in the lower epidermis than on the upper epidermis. In monocot plants, the leaves are isobilateral in nature with identical upper and lower surfaces. The stomata are distributed equally in both upper and lower epidermis. 

Moreover, the stomata are scattered in the leaves of dicot plants whereas in monocots they are arranged in straight lines.

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