What are the two main types of Plant Tissues ?

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In multicellular organisms, various functions were assigned to different groups of cells (tissues), organs and organ systems.

what are the two main types of plant tissues

But…

Before knowing more about ‘two main types of plant tissues‘, you should know about ‘what are Tissues ?‘ and ‘what are plant tissues and its types ?‘.

So lets get start…

What are Tissues ?

Tissues are groups of similar cells, having same origin and performing a specific function. The name tissue was given by Bichat. The study of tissues is known as histology, a term introduced by Meyer.

According to Wikipedia, Tissue is a cellular organizational level between cells and a complete organ. A tissue is a group of similar cells and their extracellular matrix from the same origin that together perform a specific function.

The tissues first evolved in coelenterata. Tissues are categorized into the following types:

(A) Simple tissues (B) Compound tissues

If all the cells present in a tissue are similar in structure, functions and origin, it is called simple tissue.

But…

A tissue is said to be compound tissue if it contains more than one type of simple tissues. Tissues are present in all multicellular organisms, whether plants or animals.

Two main types of Plant Tissues

In higher plants, the cells and tissues differ very much in their kind, form and origin. Based upon the capacity to divide, the plant tissues have been classified into the following two types :

Meristematic tissue
Permanent tissue

Meristematic Tissue or Meristem

This type of tissue is composed of a group of similar and immature cells. These cells divide to form new cells and help in increasing the length and girth of plant.

The main characteristics of this tissue are :

The cells are small and thin walled.
The cells are closely packed without intercellular spaces.
The cells are isodiametric, i.e., they have identical length, breadth and height.
The cells may be rounded, oval or polygonal.
The cells have a large conspicuous and dense cytoplasm.
Vacuoles are absent or very small.
The cells keep on dividing and produce new cells.
New cells, thus produced, are converted into mature permanent cells.
The meristematic cells are parent cells from which all other types of cells are formed.

This type of tissue is found at all the growing points of a plant, such as tips of roots, stems and branches. It is also present between the bark and the wood of trees where it leads to increase in the thickness of stem.

Depending upon the location, meristematic tissue can be categorized into three types:

1. Apical (Terminal) Meristem : This meristematic tissue is present at the growing tips of roots, stems and also in the growing young leaves near the tips of young stems.

2. Lateral Meristem : This meristematic tissue is present below the bark and in the vascular bundles in dicot stem. It is responsible for increasing the diameter of the stem e.g. Cambium.

3. Intercalary : This meristematic tissue is present at the internodes of the stems and at the leaf- bases.

Functions of Meristematic tissues

The main function of meristematic tissue is to form a number of new cells by repeated divisions. The following functions are performed by different types of meristematic tissues:

(i) Apical meristem brings about the elongation of the roots and stems, hence, increases length of the plant. It is called primary growth.

(ii) Lateral meristem helps in increasing the diameter and girth of the plants. It is called secondary growth.

(iii) Intercalary meristem causes increase in the length of an organ.

Permanent Tissue

The permanent tissues are composed of differentiated cells which have lost their capability to divide. They have definite shape, size, thickness and functions. The permanent cells may be dead or living.

Depending upon the structure, the permanent tissues are classified into the following two types :

Simple permanent tissues.
Complex permanent tissues.

Simple Permanent Tissues

These tissues are made-up of cells which are structurally and functionally similar. They include protective and supporting tissues.

Protective Tissues

These tissues are primarily protective in function and include :

(i) epidermis, and (ii) cork or phellem.

Epidermis : Epidermis forms one cell thick outermost layer of various body organs of plants, such as, leaves, flowers, stems and roots.

Cork or Phellem : In older dicot roots, stems, tissues at the periphery become cork cells or phellem cells. Cork is made-up of dead cells with thick walls and do not have any intercellular spaces.

Read about Cell Theory and types of Cell Division.

Supporting Tissues

Though these tissues are primarily concerned with providing support to the plants, yet they also perform many other functions. These tissues are categorized into three types :

(a) Parenchyma (b) Collenchyma (c) Sclerenchyma

Parenchyma : (Gr. para-beside ; enchyma- in filling) Typically, parenchyma consists of isodiametric living cells which may be oval, rounded or polygonal in shape.

They have a thin, smooth and elastic cell walls. Small intercellular spaces are generally present for the exchange of gases.

Internally, each cell encloses a large central vacuole and peripheral cytoplasm containing nucleus. Parenchyma is the most abundant tissue in the plants.

Also…

It is found in the non-woody or soft parts of the stems, roots, leaves, flowers, fruits, etc. It is also distributed in pith, medullary rays and packing tissue in xylem and phloem.

Functions of Parenchyma :

It is meant basically for the storage of food e.g. starch present in the parenchyma of cortex of potato tuber.
It helps in the slow transport of various materials through its cells or cell walls.
It also provides turgidity to the plants.
Parenchyma serves as a packing tissue to fill the spaces between other tissues.
It helps to maintain the shape and firmness of the plant due to its turgid cells.
Waste products of plants, such as tannin, gum, crystals, resins of inorganic wastes, etc., are stored in the parenchyma cells.
Chlorenchyma (Chloroplast containing parenchyma tissue) performs photosynthesis, e.g., mesophyll of leaves.
Aerenchyma enclose large air-spaces in aquatic plants for buoyancy and exchange of gases.

Collenchyma : (Gr. kolla-glue) Collenchyma is made-up of living cells which are usually elongated with thick corners. Intercellular spaces are usually absent in this tissue.

Collenchyma cells may be oval, circular or polygonal in shape. Few chloroplasts are usually present in the cells of collenchyma.

Collenchyma is found in the leaf stalks, below the epidermis of leaves and stems of herbaceous dicotyledonous plants. Collenchyma is absent in the roots, leaves and stems of monocot plants.

Functions of Collenchyma :

Collenchyma is a mechanical tissue and provides both mechanical strength and elasticity.
It prevents the tearing of leaves.
It allows the growth of plants because the cells are capable of elongation.
Chloroplast containing collenchyma manufactures sugar and starch.

Complex Permanent Tissues

The complex tissue consists of more than one type of cells which work together as a unit. Complex tissue helps in the transportation of organic materials, water and minerals up and down the plants.

That is why…

It is also known as conducting or vascular tissue. The common types of complex permanent tissues are : (A) Xylem or wood (B) Phloem or bast. Xylem and phloem together form vascular bundles.

Xylem : Xylem is also known as wood. It is a vascular and mechanical tissue. Xylem cells are thick walled. They are found in the form of tubular passages. Xylem consists of four types of cells called xylem elements. These are :

(a) Tracheids : Single-celled elongated angular dead cells .

(b) Vessels or Tracheae : Much elongated multicellular tubes.

(d) Xylem sclerenchyma or wood fibres : More elongated fibres with thick walls and narrow SE cavities.

Except xylem parenchyma, all other xylem elements are dead. Vessels are the most important cells of all the four types of xylem elements.

The transverse walls between the vessel elements are partially or fully dissolved to form continuous channels or water pipes. The rings present in the trunk of an old tree are the xylem rings. By counting the number of rings, we can determine the age of a tree.

Functions of Xylem :

The main function of xylem is the upward movement of water and dissolved minerals (ascent of sap) from the roots to different parts of shoots.
Tracheids, vessels and xylem sclerenchyma provide mechanical strength to the plant body.

Phloem : Phloem is a living, conducting or vascular tissue. It contains tubes, like xylem, but it performs no mechanical function. Phloem consists of four elements or cells.

These are : (a) Sieve tubes, (b) Companion cells, (c) Phloem parenchyma, and (d) Phloem fibres.

(a) Sieve tubes : These are slender, tube-like structures made-up of elongated, thin-walled cells placed end to end. The end walls of sieve tube cells are perforated by numerous pores and are called sieve plates. The nucleus of sieve tubes degenerates on maturity, however, cytoplasm persists.

(b) Companion cells : These are thin walled and elongated cells attached to the lateral sides of the sieve tubes. These cells have very active cytoplasm and large elongated nucleus. These cells are living. They control the activity of sieve-tube elements.

(c) Phloem parenchyma : They are thin walled living cells. They store food and help in the lateral transportation of food.

(d) Phloem fibres : These are scleren- chymatous fibres which provide mechanical strength to the plant.

Functions of Phloem :

Phloem helps in the transportation of organic food (photosynthesized by the leaves) to all the parts of a plant.
Phloem parenchyma helps in the storage of food.

SUMMARY

Title : What are the two main types of Plant Tissues ?

Living organisms appeared on Earth as single celled (acellular) organisms. They were able to perform all the functions necessary for life.

Even today, millions of such acellular organisms are present, e.g., Amoeba, Paramecium, Chlamydomonas, etc. They can perform all the activities of life, but without much efficiency.

Moreover…

Such organisms cannot perform different activities simultaneously. To attain more efficiency and perform such activities, multicellular organisms came into existence.

Thus, the body of a multicellular organism may have three tier organization leading to division of labour. To perform the assigned functions efficiently, tissues, organs and organ systems got modified, specialised and differentiated.