Tissue system and types

Introduction:

It is a group of tissues derived from a portion of meristem that performs a similar function in the plant body irrespective of its position. These tissues form a tissue system. They may have no structural or morphological similarity and may also differ in their origin.
In the case of plants, tissue systems are divided into three types based on their location and structure in the plant body, i.e. epidermal tissue system, ground tissue system, vascular tissue system.
Tissue systems can be defined as how various groups of tissues and their types function differently based on their location.
The three tissue systems that perform the major functions are the epidermal tissues system, ground tissue system and vascular tissue system.
The epidermal tissue system deals with the protection and coverage of the entire plant body. It is the first layer of cells in any plant.
The ground tissue system is responsible for the formation of internal regions of the plant organs excluding epidermis and vascular systems.
The vascular tissue system includes the vascular bundles (xylem and phloem).

Detailed explanation:

i) Epidermal tissue system

Epidermal tissue system is found on the surface of every plant and covers the plant surface throughout.
The epidermal tissue system develops from the protoderm layer (primary meristem layer).
The epidermal tissue system consists of the epidermis, stomata, cuticle and the epidermal appendages like root hair and trichomes(stem hair).

ii) Ground tissue system-

The ground tissue system emerges from the ground meristem, plerome, and periblem.
This tissue forms the main part of the plant body and the internal part of the organs.
It consists of simple permanent tissues such as parenchyma, collenchyma, sclerenchyma, glandular and laticiferous tissues (except epidermis and vascular bundles)

iii) Vascular tissue system-

The vascular tissue system forms the vascular strands that contain the xylem and phloem within.
The strands can be in various forms within the gymnosperms and angiosperms.
They are found in radial or conjoint conditions in roots and stems respectively.

Topics covered:

Epidermal Tissue System
Ground Tissue system
Vascular tissue system
Vascular bundles on the basis of arrangement of xylem and phloem elements
Vascular bundles on the basis of the relative position of primary xylem and metaxylem elements
Vascular bundles on the basis of presence or absence of cambium

Epidermal Tissue System

Introduction:

It forms the outermost protective covering of various plant organs which remains in direct contact with the environment.
It originates from the outermost layer of the apical meristem.
It performs various functions including protection, absorption, excretion, gaseous exchange, restriction of transpiration, secretion etc.
It involves the following parts: (1) Epidermis (2 ) Stomata (3) Cuticle & wax (4) Epidermal Appendages.

Detailed explanation:

The epidermal tissue system is composed of different components and each component has a specific function.

1) Epidermis-

It is uniseriate in most of the plant organs but in some plants it may be multilayered. e.g. Ficus, Nerium.
Its cells are parenchymatous and compactly arranged. The outer tangential walls are usually thicker as compared to inner walls.
Each cell has a large central vacuole & a peripheral thin cytoplasm. They may contain chloroplasts, anthocyanin pigments, tannins, oils and crystals etc.
Some upper epidermal cells in some monocot leaf become larger, thin-walled, have vacuoles & are called bulliform cells. They play an important role in the folding of leaves during deficiency of water e.g. Grasses.
In aerial roots of epiphytes, the multiple epidermal cells are modified to velamen which absorbs moisture from the atmosphere.

2) Stomata-

Stomata are minute pores in the epidermis. Each pore is surrounded by two kidney-shaped cells, called guard cells.
Guard cells are dumb-bell shaped in the members of the Gramineae family (Monocots).
Guard cells contain chloroplasts. The inner wall of the guard cell is thickened.
Guard cells in some cases are surrounded by another group of less modified epidermal cells called subsidiary cells.
All these components together are termed as stomatal apparatus.
Usually, there is a large air cavity below each stomata. It is called a substomatal cavity.
Stomata are absent in roots, underground parts and submerged hydrophytes.
In xerophytes stomata are sunken in grooves due to which transpiration is greatly reduced, stomata are embedded in Pinus.
The function of stomata is the exchange of gases and control of transpiration.

3) Cuticle and Wax-

Cutin is a fatty substance deposited over the outer surface of epidermal cells in the form of a separate layer which is called the cuticle.
The cutinised walls are less permeable to water. The impermeability depends upon the thickness of the cutin.
Cuticle is-
- thick in xerophytes,
- thin in mesophytes and
- absent in submerged parts of hydrophytes.
- It is also absent in underground parts.

4) Epidermal appendages

Trichomes and root hairs represent the epidermal appendages.
Root hairs:

i) The epidermis of the roots of plants bears the hair-like structures called root hairs.
ii) The epiblema cells called Trichoblasts found in roots, enlarge and form the root hairs.
iii) Epiblema cells possess a large vacuole and cytoplasm around them, the nucleus is found at the apex of root hair.
iv) Root hair cells feature thin walls and are pectocellulosic.
v) These root hairs assist the roots in the absorption of water from the soil and also tend to increase the surface area for absorption.
Trichomes:

i) They are found on the stems of plants.
ii) They are multicellular structures, unlike root hairs.
iii) They are found in two different types, i.e. scales and hairs.
iv) These could be branched (scales) or unbranched (hairs).
v) These might be stiff or soft in nature.
vi) These can also act as secretory cells.
vii) They help in preventing water loss due to transpiration.

Ground Tissue system

Introduction:

The ground tissue system is responsible for the formation of internal regions of the plant organs excluding epidermis and vascular systems.
The ground tissue system emerges from the ground meristem, partly plerome, and partly periblem.
This system is composed of different types of tissues and performs many functions. Therefore it is a heterogenous zone.
This tissue system mainly originates from ground meristem.

Detailed explanation:

It is divided into two parts.
(a) Extrastelar ground tissue (b) Intrastelar ground tissue

(a) Extrastelar ground tissue:

It involves only the cortical region. Cortex is the region that lies between the epidermis and pericycle.
It involves the following tissues :

1. Hypodermis:

The layer below the epidermis is called hypodermis which is absent in roots.
It provides mechanical strength and rigidity.
Dicot stem has collenchymatous hypodermis while monocot stem has sclerenchymatous hypodermis.

2. General cortex:

Its cells are thin-walled, parenchymatous & may be rounded, polygonal or cylindrical.
The cells have prominent intercellular spaces. Starch grain, oil, tannin, and crystals of various types are found in cortical cells.
ln hydrophytes the general cortex is aerenchymatous.
This region stores food materials & provides mechanical support.

3. Endodermis:

The innermost layer of the cortex is called the endodermis. Its cells are compactly arranged.
The endodermis is not found in woody stems of dicots and gymnosperms.
In the young stem of angiosperms, endodermis is also called a starch sheath due to abundant starch granules present in them.
There are several functions of endodermis
(i) It acts as a watertight jacket between the vascular & nonvascular regions.
(ii) It stores starch.
(iii) It may serve as a protective layer and maintain root pressure.

(b) Intrastelar ground tissue:

It includes a pericycle, medullary rays and pith.

(1) Pericycle:

It lies between the endodermis and vascular tissues. It is single-layered or multilayered.cc
The pericycle is generally parenchymatous but in some cases, it is made up of many layers of sclerenchymatous cells (Cucurbits stem) or in the form of alternating bands of sclerenchymatous & parenchymatous cells (sunflower stem).
In roots, the pericycle is single-layered & made of thin-walled parenchymatous cells which later develop into lateral roots (endogenous origin of roots).
In the dicot root, the vascular cambium originates from the pericycle. The latter also gives rise to a part of the vascular cambium.

(2) Medullary rays:

They lie between the vascular bundles outside the pith.
These are made of parenchymatous cells and extend from the pith towards the periphery.
They serve the function of lateral conduction.

(3) Pith:

The central portion of roots & dicot stem is occupied by pith. It is also called the medulla.
It is generally composed of large parenchymatous cells with intercellular space.c
The main function of the pith is the storage of water and food reserves.

Vascular Tissue system

Introduction:

It is generally composed of large parenchymatous cells with intercellular space.
The central cylinder of the shoot or root surrounded by cortex is called stele.
These tissues originate from the procambium of the apical meristem.
The varying number of vascular bundles formed inside the stele constitute the vascular tissue system.
Vascular bundles are made up of xylem and phloem with or without cambium.
The main function of vascular bundles is conduction of water and minerals, translocation of organic solutes and to give mechanical support to the plant body.

Vascular bundles on the basis of arrangement of xylem and phloem elements

Introduction:

The xylem and phloem can be arranged in various ways and patterns.
And depending on the arrangement type the vascular bundles they can be grouped into 3 types, i.e. Radial, Conjoint, and Concentric vascular bundles.
All these types may exist in different parts of the same plant or in different plant species altogether.

Detailed explanation:

On the basis of the relative position of the xylem & phloem following types of vascular bundles are recognized:

(1) Radial: When xylem & phloem are located on different radii, in an alternate manner the bundles are called radial. e.g. roots.

Radial-vascular-bundles

(2) Conjoint : A vascular bundle having both xylem & phloem together present on the same radii, is called conjoint. Generally, these are found in stems. They are of two types-

collateral-biocollateral

(a) Collateral: In this type, phloem lies towards outside & xylem towards inner side. e.g. sunflower.
(b) Bicollateral: In such vascular bundles, two patches of phloem one on each side of the xylem are present. e.g. Cucurbits.

(3) Concentric: In this type, any one of the two, xylem or phloem is present in the centre and the other surrounds it. They are also of two types-

concentric

(a) Amphivasal (Leptocentric): Here, the xylem completely surrounds the phloem. e.g. Dracaena, Yucca.
(b) Amphicribal (Hadrocentric): In this, the xylem is in the centre surrounded on all sides by phloem. e.g. Ferns.

Vascular bundles on the basis of the relative position of primary xylem and metaxylem elements

Introduction:

Depending upon the position of primary xylem and metaxylem elements the vascular bundles can be divided into two groups.
These groups represent the way protoxylem and metaxylem is arranged within the vascular bundles and in which direction the development takes place.

Detailed explanation:

On the basis of the relative position of protoxylem and metaxylem elements, the following two xylem conditions are differentiated-

endarch-exarch

(a) Exarch: When the protoxylem lies at the periphery and the metaxylem in the centre, the condition is said to be exarch. The direction of development of the xylem here is centripetal. e.g. Root.
(b) Endarch: When the protoxylem lies towards the centre and the metaxylem at the periphery, the condition is called endarch. The direction of development of the xylem here is centrifugal. e.g. Stem and leaf.

Vascular bundles on the basis of presence or absence of cambium

Introduction:

Vascular bundles can also be differentiated and grouped based on the cambium ring.
There can be two types of vascular bundles based on the presence or absence of cambium, i.e. Open ( with cambium ring) and closed (without the cambium ring).
This ring is responsible for secondary growth hence, plants with this ring present show secondary growth and the plants without the ring show no secondary growth.

Detailed explanation:

i) Open vascular bundles

The vascular bundles in which there is a presence of cambium strip in between the xylem and the phloem.
The cambial strip gives rise to secondary xylem and phloem which is the characteristic of secondary growth.
This particular vascular bundle is found in the stems of dicots and the gymnosperms.

ii) Closed vascular bundles

The vascular bundles in which there is the absence of cambium strip in between the xylem and the phloem.
Secondary growth is absent in the case of the closed vascular system.
This type is exhibited by leaves and monocot stems.

Frequently asked questions (FAQ)

Q1. Why is the function of the cuticle layer?

Ans: Cutin is a fatty substance deposited over the outer surface of epidermal cells in the form of a separate layer which is called the cuticle. The cutinised walls are less permeable to water. The impermeability depends upon the thickness of the cutin. This layer is responsible for controlling transpiration and also protecting them against minor mechanical injury.

Q2. State the features of epidermal appendages.

Ans: Trichomes and root hairs constitute the epidermal appendages.
Root hairs-

Root hairs are the epidermal appendages that assist the roots in the absorption of water from the soil by increasing the surface area of absorption.

Trichomes-

Trichomes are hair-like structures that are found on the epidermis of the stem.
Trichomes are multicellular structures, unlike root hairs.
Trichomes are of two different types, i.e. scales and hairs.
Sometimes, they can be secretory.
Trichomes protect the plants against external agents and also prevent excess water loss.

Q3. How does root hair enhance water absorption?

Ans: Root hairs are thin-walled, unicellular structures formed by the elongation of epidermal cells in roots. They increase the surface area of absorption of water from the soil.

Q4. Define ground tissue system.

Ans: This tissue system mainly originates from ground meristems. It constitutes the main body of the plants. It consists of parenchyma, collenchyma, sclerenchyma, glandular and laticiferous tissues (except epidermis and vascular bundles). This system represents a heterogeneous zone as it is composed of different types of tissues and performs many functions.

Q5. State the functions performed by the cortex.

Ans: The cells of the cortex are thin-walled, parenchymatous & may be rounded, polygonal or cylindrical. The cells have prominent intercellular spaces. Starch grain, oil, tannin, and crystals of various types are found in cortical cells. ln hydrophytes the general cortex is aerenchymatous. This region stores food materials & provides mechanical support.

Q6. What are concentric vascular bundles? State types of concentric vascular bundles.

Ans: Concentric bundles can be characterized by the formation of a solid core by either xylem or phloem and the other surrounds that core like a ring.
The two types of concentric vascular bundles are-
a. Amphicribal is the type of concentric bundle where the xylem is at the core and the phloem surrounds it.
b. Amphivasal is that type of concentric bundle where phloem forms the core and xylem surrounds it.

Q7. How many types of vascular bundles are there on the basis of the presence or absence of cambium?

Ans: Open: When cambium lies between xylem & phloem elements, the bundle is said to be open.e.g. dicot stem. This cambium is called intrafascicular cambium. Due to the presence of cambium, they are capable of exhibiting secondary growth.
(b) Closed: When there is no cambium inside, the bundle is described as closed. e.g. monocot stem. Due to the absence of cambium, they do not exhibit secondary growth.

Q8. State the characteristic features of bicollateral vascular bundles.

Ans: A vascular bundle having both xylem & phloem together, is called conjoint. In bicollateral types of vascular bundles, two patches of phloem one on each side of the xylem are present. e.g. Cucurbits.

Q9. What is the epidermal tissue system and mention its components?

Ans: It forms the outermost protective covering of various plant organs which remains in direct contact with the environment. It originates from the outermost layer of apical meristem. It performs various functions including protection, absorption, excretion, gaseous exchange, secretion etc. It involves the following parts. (1) Epidermis (2 ) Stomata (3) Cuticle & wax (4) Epidermal Appendages.

Q10. What is stele?

Ans: The central cylinder of the shoot or root surrounded by the cortex, is called stele.

Q11. What constitutes the vascular tissue system?

Ans: The varying number of vascular bundles formed inside the stele constitute the vascular tissue system. Each vascular bundle is made up of xylem and phloem with or without cambium.

Q12. Define Exarch xylem.

Ans: Exarch xylem is the xylem tissue where the protoxylem is positioned towards the periphery and the metaxylem towards the centre. Exarch xylem is mostly found in roots.