Basic types of tissues

 Meristematic Tissue in Plants: A meristem may be defined as a region of localized mitosis.

    Plant cells are formed at meristems, and then develop into cell types which are grouped into tissues.

 Plant cell types rise by mitosis from a meristem.

    Meristematic tissue or meristems, as they are also called are tissues that have the ability to enlarge, stretch and differentiate into other types of cells as they mature. The cells of this tissue are generally young and immature, with the power of continuous division.

 Meristematic cells are all living cells. The meristematic cells can be oval or rounded or polygonal in shape. They have a large nucleus with no vacuoles. Intercellular space between cells is absent. The cells are also small in size but have a high capacity of cell division.

Types of Meristematic Tissue 

1. Depending on the time of formation of the meristematic tissue on the plant body, we can classify the meristems into two types: Primary meristem & Secondary meristem.

i   .Primary Meristem:- 

                                         Primary meristem is the initial form of meristem of a plant and it is derived from the direct descendants of the embryonic cells. Since it occurs in the apical regions of the plant, this type of meristem is known as apical meristem.

  Two types of apical meristems occur for a plant. They are the shoot apical meristem and the root apical meristem.

 Shoot apical meristem is the source for the above-ground plant organs including leaves, flowers, etc. On the other hand, the root apical meristem gives rise to the cells of an adult root. Most importantly, primary meristem is responsible for the longitudinal growth of the plant, increasing the length of the plant. Furthermore, the cells of the apical meristem give rise to three types of primary meristems known as protoderm, procambium, and ground meristem.

              

                            Note                                    

 Protoderm lies outside the stem and it differentiates into the epidermis. 

 Procambium lies just inside the protoderm and it differentiates into the primary xylem and primary phloem.

 During secondary growth, procambium gives rise to the vascular cambium and cork cambium, which belong to the secondary meristem.

 Ground meristem occurs in the center and it differentiates into the pith and cortex.

ii. Secondary Meristem:-

                                              Secondary meristem is a type of meristem which arises during the secondary growth of the plant. Secondary meristem arises from the permanent tissues of the plant. Generally, this meristem occurs in the lateral regions of the plant; therefore, we call it the lateral meristem.

 The main function of the secondary meristem is to increase the width of the plant, which is the lateral growth.

 Two types of secondary meristems in a plant are vascular cambium and cork cambium.

II. Depending on the occurrence of the meristematic tissue on the plant body, we can classify the meristems into three types. They are:

1. Apical Meristems – 

                                     These meristems are located on the tip of the root, stem etc. They help in the growth of the root system as well as the shoot system. The various cell divisions along with the cellular enlargement help in the growth of the stem above the ground and the growth of the root below the ground.

2. Intercalary Meristem –

                                                The intercalary meristems are located at the internodes or the base of the leaves. The intercalary meristems help in increasing the length of the internode. This is usually seen in monocotyledonous plants.

3. Lateral Meristems – 

                                           The lateral meristems are present on the lateral side of the stem and root of a plant. These meristems help in increasing the thickness of the plants. The vascular cambium and the cork cambium are good examples of a lateral meristematic tissue.

 Plants have only three tissue types:

 1) Dermal;                                       2) Ground; and                             3) Vascular.

1. Dermal tissue:-

                                  Dermal tissue covers and protects the plant, and controls gas exchange and water absorption (in roots). Dermal tissue of the stems and leaves is covered by a waxy cuticle that prevents evaporative water loss.

Stomata are specialized pores that allow gas exchange through holes in the cuticle.Unlike the stem and leaves, the root epidermis is not covered by a waxy cuticle whichwould prevent absorption of water. Root hairs, which are extensions of root epidermalcells, increase the surface area of the root, greatly contributing to the absorption of water and minerals.

Trichomes, or small hairlike or spikey outgrowths of epidermal tissue, may be present onthe stem and leaves, and aid in defense against herbivores.

2. Ground tissue comprises the bulk of the primary plant body. Parenchyma,collenchyma, and sclerenchyma cells are common in the ground tissue. Ground tissuecarries out different functions based on the cell type and location in the plant, andincludes:

i. Parenchyma (photosynthesis in the leaves, and storage in the roots),

ii. Collenchyma (shoot support in areas of active growth), and

iii. Sclerenchyma (shoot support in areas where growth has ceased) is the site ofphotosynthesis, provides a supporting matrix for the vascular tissue, providesstructural support for the stem, and helps to store water and sugars

.3. Vascular tissue transports water, minerals, and sugars to different parts of the plant.Vascular tissue is made of two specialized conducting tissues: xylem and phloem.                                                  

  i . Xylem tissue transports water and nutrients from the roots to different parts ofthe plant, and also plays a role in structural support in the stem.  

ii. Phloem tissue transports organic compounds from the site of photosynthesisto other parts of the plant.

The xylem and phloem always lie adjacent to each other in a vascular bundle.

Ground tissue

Parenchyma

These are alive, polygonal cells with a big central vacuole, and have intercellular spaces amidstthem. Parenchymatous cells create ground tissue and pith.

1. Parenchyma consisting of chloroplasts are termed as chlorenchyma. The chlorenchymahelps in photosynthesis.

2. Parenchyma which consists of big air voids is called aerenchyma. Buoyancy is the mainpurpose the aerenchyma.

3. Some parenchymatous cells perform as storage chambers for starch in vegetable andfruits.

Collenchyma

These are stretched out, living cells with minute intercellular gaps. Their cell walls are made upof pectin and cellulose. Collenchyma is found in the marginal regions of leaves and stems andoffers flexibility with the structural framework and mechanical support to plants.

Sclerenchyma 

These are elongated, dead cells with lignin deposits in their cell wall. They have no intercellulargaps. Sclerenchyma is found in the covering of seeds and nuts, around the vascular tissues in stems and the veins of leaves. Sclerenchyma provides strength to the plant.

Protective tissues

These provide fortification to the plant. They include the cork and epidermis.

1. Epidermis – It is a layer of cell that makes up an outer casing of all the structures in theplant. The stomata perforates the epidermis at certain places. The stomata help in loss ofwater and gaseous exchange.

2. Cork – This is the external protective tissue, which substitutes the epidermal cells inmature stems and roots. Cork cells are lifeless and lack intercellular gaps. Their cellwalls are coagulated by suberin, which makes them impervious to gas and Water Molecules.

 vascular tissues

Vascular tissue transports water, minerals, and sugars to different parts of the plant. Vasculartissue is made of two specialized conducting tissues: xylem and phloem.XylemIt helps in the transport of dissolved substances and water all through the plant. The diversecomponents of the xylem include vessels, tracheids, xylem fibres and xylem parenchyma. Xylemfibres and Tracheids are made up of lignin, which provides structural support to the plant.PhloemThis tissue helps in the transportation of food all through the plant. The diverse elements of phloem include phloem fibres, sieve tubes, phloem parenchyma and companion cells. Difference Between Xylem And PhloemPlants are classified based on many criteria and one such classification is the presence or absence of a vascular system. Essentially, a vascular plant has specialized features that help it to absorb water and minerals from the soil. These include special tissues such as xylem and phloem.Xylem and PhloemXylem and Phloem are two different types of vascular tissues, which are mainly involved in the transportation process. These tissues form a vascular bundle and these work together as a unit. The movement of xylem is unidirectional, while the movement of phloem is bidirectional.

Vascular bundle and their types

Vascular Bundle may be defines as a strand of conducting vessels in the stem or leaves of a plant, typically with phloem on the outside and xylem on the inside.

There are four Main Types of Vascular Bundle 1. Collateral Bundle 2. Bicollateral Bundle 3. Concentric Bundle 4. Radial Vascular Bundle. Type # 1. Collateral Bundle: A vascular bundle in which a strand of phloem is present external to the strand of xylem on the same radius side by side is known as collateral bundle.

Cambium may be present or absent in between xylem and phloem, and so there are the following two types of collateral bundle: (a) Closed collateral bundle:

 In this type cambium is absent in between xylem and phloem. Therefore stems having this type of bundle do not have normal secondary growth. Ex. Monocotyledonous stem.

 Usually these bundles are enclosed within bundle sheath made up of sclerenchyma and those that lack the sheath are considered as anomalous (e.g. Asparagus stem).

(b) Open collateral bundle:

An open collateral vascular bundle has cambium called fascicular cambium between xylem and phloem. The bundles can increase in diameter by normal secondary growth with the help of fascicular cambium. Ex. Dicotyledonous stem.

Type # 2. Bicollateral Bundle: A vascular bundle with phloem situated on the peripheral and inner side of xylem is known as bicollateral bundle. A strip of cambium termed outer cambium is present between the peripheral phloem and xylem; another strip of cambium, termed inner cambium, is also present between inner phloem and xylem. The peripheral or external phloem is termed as outer phloem whereas the inner or internal phloem is called inner phloem. The sequence of vascular tissues in the bicollateral bundles from periphery toward centre is outer phloem, outer cambium, xylem, inner cambium and inner phloem. These bundles are open type as strips of cambia are present but the secondary thickening occurs only by the outer cambium, i.e. cambium present between the outer phloem and xylem. Ex. Cucurbita stem.

Type # 3. Concentric Bundle: A vascular bundle in which one type of vascular tissue surrounds the other is known as concentric bundle. In this bundle xylem either encircles or is encircled by phloem and accordingly the following two types are recognized:(a) Amphivasal bundle:A vascular bundle in which xylem encircles the central strand of phloem is known as amphivasal bundle, also called leptocentric bundle. Ex. Dracaena, Yucca.(b) Amphicribral bundle: A vascular bundle in which phloem encircles the central strand of xylem is called as amphicribral bundle, also known as hadrocentric bundle. Ex. Selaginella (Pteridophyte). The concentric bundles, either amphivasal or amphicribral, are closed as there is no cambium in between xylem and phloem.

Type # 4. Radial Vascular Bundle:

 A vascular bundle, in which the primary xylem and primary phloem strands are separated from each other by non-vascular tissues and they are situated on alternate radii of an axis, is known as radial vascular bundle or radial bundle.

 These bundles are the characteristic of roots. There is no primary cambium in this bundle and the secondary thickening occurs by the secondary cambium that originates at the time of secondary growth in dicotyledonous root only.

 The dicot roots usually have four to six number of protoxylem poles in contrast to monocot root where many poles of xylem (more than six) are present. The number of protoxylem poles in a root may be 1, 2, 3, 4, 5, 6 or more. Accordingly they are called monarch, diarch, triarch, tetrarch and so on. The term polyarch is used when the number of protoxylem poles are more than six .