- All living organisms whether unicellular or multi-cellular are made up of cells.
- Multi-cellular organisms are in a more advantageous position as compared to unicellular organisms as they exhibit division of labour. Different groups of cell perform different specialised functions. The specialisation is achieved due to differentiation, where cells come to have a definite shape, size and function.
- Group of cells having a common origin, similar structure and which work together to perform a common function is known as Tissue.A microscopic view of a Scots pine tree (Pinus sylvestris) showing cells of the xylem tissue
- The branch of biology, that deals with the detailed study of tissues, is known as Histology.
- Both plants and animals have similar life processes. However, they do not have similar types of tissues because of differences in their organisation, growth patterns, mode of living, lifestyle, movement and locomotion.
Plant tissues are of two types:
(i) Meristematic tissues
(ii) Permanent tissues
- Meristematic tissues are a thin-walled, compactly arranged group of cells that are preparing to divide or are in the process of multiplication or have the potential to divide.
- They are small, spherical or polygonal cells with dense cytoplasm. The nucleus is large and prominent. Vacuoles are absent or very small in number.
Meristematic tissues can be classified on the basis of their position into:
(i) Apical Meristem (root and shoot apex) occurs at the growing tips of stems and roots and increases the length of the stem and the root.Longitudinal section of shoot apex showing location of meristem and young leaves(ii) Lateral Meristem (cambium, cork cambium) occurs on the sides both in stem and root. It is exclusively present in dicot plants where it increases the girth of stem and root.
It is of two main types - Vascular and Cork cambium.
(iii) Intercalary Meristem (internodes) is present at the base of leaves and internodes. It increases the length between the two nodes. It is usually found in monocot plants.
Try yourself:Cardiac muscleis one of the three major types of muscles, the others being skeletal and smooth muscles. It is found in the walls and histological foundation of the heart. Which one of the following statements is not related to the cardiac muscles?
- Permanent tissues are derived from meristematic cells which have lost the ability to divide. They undergo differentiation to perform a particular function.
- Their cells may be living or dead, thin or thick-walled. The thickenings may be regular or irregular.
- There are two main groups of permanent tissues:
(i) Simple Permanent Tissue
(ii) Complex Permanent Tissue
1. Simple Permanent Tissues
A simple tissue is a group of one type of cells which perform the same function.
Simple tissue is classified into three main types:
- These cells are thin-walled, living, isodiametric cells which form the basic packing tissue of all the plant organs.
- The cells are oval, spherical or polygonal in the outline, that are loosely packed with small and large intercellular spaces in between them. Their primary function is the storage of food.Parenchyma Tissues in Transverse and Longitudinal section
- Parenchyma containing chlorophyll is referred to as chlorenchyma which is the seat of photosynthesis.
- In aquatic plants, parenchyma cells are associated walk large air spaces. These air spaces store gases and provide buoyancy to the plants. Such a specialised parenchyma tissue is known as aerenchyma.
- These cells are living cells having thickened corners. The uneven thickenings of pectocellulose provide mechanical support and flexibility to plant. Because of collenchyma plant can bend without breaking.
- These cells are usually dead, thick and lignified cells with the narrow lumen.Sclerenchyma Tissue
- Sclerenchyma cells are of two types:
(i) Fibres are elongated spindle-shaped cells pointed at both ends
(ii) Sclereids are short and broad cells which occurs singly or in small groups.
- They are also known as stone cells. This tissue provides mechanical support and enables plants to bear various stresses.
Protective tissues constitute an outermost layer of stem, roots, leaves, flowers and fruits. It provides protection against environmental factors and pathogens.
It is of two types namely:
- Cork is the outer protective tissue of older stem and roots. It is formed by a secondary lateral meristem called cork cambium.
- Cork cambium produces secondary cortex or phelloderm on the inner side and cork or phellem on the outer side.
- Cork cells are rectangular in outline which are compactly arranged and are multilayered. Their walls become impermeable due to the deposition of suberin.
- At certain places they possess small aerating pores called lenticels through which exchange of gases takes place.
- Older cells become dead and are filled with tannins, resins and air.
- Since cork is light, impervious, compressible and non-reactive it is commercially used in the manufacture of sport goods, stoppers for bottles, linoleum, insulation boards and shock absorbers.
- The epidermis is the outer protective layer of the plant parts that prevent the entry of pathogens and pests.
- Cells of the epidermis are elongated and closely packed. The cells are generally thick on their outer and radial sides and thin-walled on the inner side.
- The outer thick walls are impregnated with an impermeable fatty substance called cutin.
- Generally, the epidermis is single-layered and is discontinuous at certain places as it is interrupted by small pore-like structures called stomata. In xerophytes, the epidermis is multilayered to prevent transpiration.
Transpiration is the process of loss of excess water in the form of vapours through stomata.
- Epidermal cells of root bear long hair-like projections called root hair. Root hairs increase the absorptive surface area thereby facilitates increased absorption of water and minerals from the soil.
- In many plants, the aerial surface also bears cutinised hair over their epidermis. They are called trichomes. Trichomes may be glandular or non-glandular in nature. It produces an insulating stationary layer of air on the surface.
Try yourself:While doing work and running, you move your organs like hands, legs, etc. Which among the following is correct?
2. Complex Permanent Tissues
Complex permanent tissues are made up of more than one type of cells. All these cells coordinate to perform a common function.
Complex tissue is classified into two main types:
- Xylem is a complex permanent plant tissue which plays an important role in the conduction of water and minerals from the soil to the various plant parts via roots.
- Xylem consists of four types of elements:
(iii) Xylem fibres
(iv) Xylem parenchyma.
- Tracheids, vessels and xylem fibres are dead cells whereas xylem parenchyma is living.
- Vessels and tracheids are the tracheary elements of xylem which occur in flowering plants. Tracheids are abundant in lower plants (Pteridophytes and Gymnosperms) while vessels are found in angiosperms.
- Tracheids are long tubular dead cells with lignified walls, wide lumen and tapering end. There are certain unthickened areas called pits on the thick lignified walls which help in the movement of water from one tracheid to another.
- Vessels are long tubes which are formed by the end to end union of a large number of dead cells. The walls are lignified and possess pits.
- Xylem parenchyma stores food while xylem fibres provide mechanical support.Working of Xylem and Phloem in plant
- Phloem is the conducting tissue which translocates food manufactured in leaves to other parts of the plant.
- Cells of phloem are living and consists of four tracheary elements namely — phloem sieve tubes and sieve cells, companion cells for short-distance transport, parenchyma for storage and phloem fibres for mechanical support.
Animal tissues are of four different types depending upon their structure and function.
- It covers the entire body externally as well as internally.
- The cells are tightly held together with various types of junctions and a small amount of cementing materials. Intercellular spaces are nearly absent.
- Epithelium rests over an extracellular layer of collagen fibres and dense matrix called basement membrane which connects epithelium with underlying connective tissue.
- Epithelia are classified on the basis of the arrangement of layers, cells shapes and functions.
- In simple epithelium, cells are arranged in a single layer and in the stratified epithelium, cells are arranged in more than one layer.
- In squamous epithelium cells are made up of flat cells, cuboidal epithelium comprises of cuboidal or cubical cells whereas columnar epithelium is made up of oblongated cells.
- A contractile tissue which possesses contractile proteins inside cells held together by connective tissue. Cells of muscular tissue are elongated and are called as muscle fibres.
Muscle fibres are of three types:
(i) Striated Muscles
- These muscles are attached to the bone and work according to our conscious will, hence also known as skeletal or voluntary muscles.
- The cells of these muscles are elongated, cylindrical, unbranched, non-tapering, showing dark and light bands and contain many nuclei situated towards the periphery of muscle fibre.
(ii) Unstriated or Smooth Muscles
- They are also referred to as non-striated or involuntary or visceral muscles.
- They are found in various visceral organs of the body e.g., stomach, intestine, ureter, bronchi. These muscles do not work according to our will hence also known as involuntary.
- Since they do not have dark and light bands they are also called smooth muscles.
- The muscle cells are tapering at both ends with a single nucleus at the centre.
(iii) Cardiac Muscle
- They are found exclusively in the heart.
- The cells are cylindrical, branched and have light and dark bands. They do not work according to our will. So they are involuntary in action.
- The cells are uninucleate and are rich in glycogen and mitochondria. They develop small lateral branches to form a highly interconnected network that helps in the quick spread of impulses.
- Intercalated discs are present between adjacent cells. These muscle fibres do not get fatigued and work throughout life.
- A fundamental animal tissue that binds, supports and packs different organs of animal body.
- It helps in the transport of gases, hormones, nutrients and excretory products.
- This tissue comprises the abundant matrix, secreted by living cells of connective tissue.
- It can be jelly like, fluid or solid.
- On the basis of the nature of the matrix, the connective tissue is of three types:
(i) Skeletal tissue (solid matrix)
(ii) Connective tissue proper (jelly-like matrix)
(iii) Vascular tissue (fluid matrix)
(i) Skeletal Tissues
- Our skeleton is made up of bone (non-flexible) and cartilage (flexible).
- Bone becomes hard due to the deposition of calcium and phosphorus in the matrix. Bone cells are known as osteocytes. They have a Haversian canal which contains blood vessels and nerve fibres.
- On the contrary, cells of cartilage are known as chondrocytes and the matrix is not solid but flexible. Cartilage occurs in the nasal septum, pinna, the tip of the nose, epiglottis, larynx, trachea, bronchi, sternal ends of ribs intervertebral discs and tips of several bones.
(ii) Connective Tissue Proper
- Tendons are strong and connect muscle to the bone. Ligaments are elastic and connect bone to bone.Illustration of Tendons and Ligaments
- Areolar Tissue forms a packaging tissue between organs lying in the body cavity.
- Adipose Tissue stores fats.Areolar and Adipose Tissue
(iii) Vascular Tissue
- Blood is a fluid connective tissue due to the presence of a fibreless fluid matrix. Blood comprises plasma and blood cells or corpuscles.
- Blood cells are of three types namely red blood cells (RBC), white blood cells (WBC) and blood platelets. RBC contains haemoglobin and help in transport of O2 and CO2. WBC provides immunity to the body and aids in defence of body against pathogens whereas platelets help in clotting of blood.
- Plasma is straw-coloured fluid which comprises of inorganic and organic substances. Albumin, globulin and fibrinogen are major soluble proteins present in the plasma. Na+, Cl-, glucose, amino acids, vitamins, fatty acid are the constituents of plasma.
- Lymph is similar to plasma but lacks RBC and blood platelets. It contains a large number of lymphocytes. Lymph helps in maintaining blood volume by removing or adding plasma. Lymph nodes and lymphoid organs function as traps of microbes where they can be destroyed.
- Nervous tissue is specialised for receiving and transmitting impulses.
- It comprises the brain, spinal cord and nerves.
- The neuron is the basic structural and functional unit of the nervous system. It is made up of expanded cell body and processes like dendrites and axon.
- Interneuron junctions where nerve endings of one neuron are in close contact with the dendrites of a successive neuron is referred to as a synapse.
Try yourself:Which of the following statements is incorrect?
i. Parenchyma tissues have intercellular spaces.
ii. Collenchymatous tissues are irregularly thickened at corners.
iii. Apical and intercalary meristems are permanent tissues.
iv. Meristematic tissues, in its early stage, lack vacuoles.