Tissues in Action

• Hierarchy in multicellular organisms: cells → tissues → organs (more than one type of tissue) → organ systems → organism.
• Unicellular (eg: amoeba): one cell performs all life functions. Multicellular: different groups of cells perform different functions, giving division of labour, which makes the body efficient.
• Examples: muscle tissue → movement; nervous tissue → carries messages; xylem → water and minerals; phloem → food.

• Plants grow in length (stem height, root depth), in girth (stem thickness) and by regrowth after cutting or grazing.
• Growth needs actively dividing cells, which together form meristematic tissue.

• Node: point on a stem where branches or leaves arise. Internode: part of the stem between two nodes.
• Annual growth rings: ring-like patterns on a cut tree trunk (T.S.), made by lateral meristem. Wide ring = favourable year, narrow ring = unfavourable year. Counting rings gives the age of the tree and the climate it grew in. Eg: a 40-year-old teak has 40 rings.

• Differentiation: some new cells lose the ability to divide and change in structure and function, becoming permanent tissue specialised for support, transport or storage.

Functionally, permanent tissues can be grouped as protective, supporting and conducting tissue.

(i) Protective tissue: Epidermis

• Outermost layer of the plant body: a tightly packed single layer of flat, rectangular cells. Protects against mechanical injury, water loss, microorganisms and extreme conditions.
• Cuticle: waxy layer of cutin over epidermal cells; thick in plants of very dry habitats to reduce water loss; also protects against injury and parasites.
• Root hair: hair-like projections of root epidermis that increase surface area for absorbing water and minerals.
• Stomata: pores in leaf epidermis for gaseous exchange and transpiration (evaporation of water vapour). Transpiration creates a transpiration pull in xylem (helps water transport) and removes wastes.
• Cork (bark): in older plants, some cells below the stem epidermis start dividing, forming the cork cambium, which gives cork cells: dead, compactly arranged, impermeable to water and gases. This forms the bark.

(ii) Supporting tissue: Simple permanent tissues

(iii) Conducting tissue: Complex permanent tissues

Xylem and phloem are complex because they are made of different cell types working together; together they form the vascular tissue.

• Sieve tubes: long tubular cells joined end to end by perforated walls (sieve pores). Their functions are regulated by companion cells (specialised parenchyma) which monitor loading and unloading of sugars.
• Permanent tissue is simple (one type of cell) or complex (more than one type). So "a tissue is a group of similar cells with similar function" is fully true only for simple tissues.

• T.S. of a sunflower stem (from outside in): cuticle and epidermis (with epidermal hair), then collenchyma, parenchyma and sclerenchyma (ground tissue), phloem, lateral meristem, xylem, and ground tissue at the centre.

Animal tissues are mainly of four types: epithelial, connective, muscular and nervous.

• Epithelial tissue forms the outer covering (skin) and lines internal organs (mouth, lungs, blood vessels, intestine).
• Closely packed cells with very little space between them: prevents entry of germs, reduces water loss, helps absorption, secretion and movement of substances.

• Why single thin layers for exchange/absorption: short distance allows quick passage of materials across them.

• During exercise, muscles need more oxygen, so breathing and blood flow increase (face looks red).
• Stem cells in bone marrow divide and make new cells; in a bone marrow transplant they are given to patients with blood cancers (leukaemia) or disorders such as thalassaemia.

• Voluntary movements are under conscious control (running, writing). Involuntary movements occur automatically (food movement in intestine, heartbeat).
• Muscles cannot work independently; they receive instructions from nervous tissue.

• Body's control and coordination network: sensing, communicating, responding (eg: pulling a hand away from heat, memory). The brain is the control centre.
• Cells of nervous tissue are neurons (nerve cells), specialised to receive, process and transmit messages.

• Eg: during exercise the brain signals the heart to beat faster to meet the higher oxygen demand.

• Components: bones, muscles, joints, cartilage, tendons and ligaments. Helps us stand upright, move, maintain posture and protect delicate organs; works under control of the nervous system.
• Movement: muscles pull on bones. Muscles attach to bones through tendons; when a muscle contracts, the tendon transmits the force to the bone, causing movement at a joint.
• Joint: junction between two or more bones. Joints allow movement but cannot move bones on their own.
• The adult human skeleton is about 12–15% of body weight (adult males about 40–50% muscle, females about 30–40%). Estimate bone or muscle weight $=$ body weight $\times$ percentage.

• The shoulder (with the collarbone) forms the shoulder girdle, connecting the arm to the skeleton.

• Framework of bones giving strength and protection: skull, vertebral column and rib cage.
• Vertebral column (backbone/spine): series of small bones called vertebrae; supports the body and keeps it upright. A cartilage disc between vertebrae cushions and allows bending and twisting without injuring the spinal cord.
• Rib cage: 12 pairs of ribs, attached to the spine at the back and to the breast bone (sternum) in front by flexible cartilage. Protects the heart and lungs. Its flexibility lets it expand and contract during breathing, changing chest space so air moves in and out.
• Yoga (21 June, International Yoga Day), correct posture, nutrition and exercise keep bones strong, muscles fit and joints flexible.

• Table 3.6 results: with light and air in a liquid nutrient medium, fresh weight increased by 20%; with light in a solid medium (no air) or without light in liquid medium (with air), weight reduced. So growth needed light, air and a liquid medium with nutrients.
• Sipra Guha Mukherjee & S. C. Maheshwari: grew a complete plant from anther culture in an artificial medium; important for crop improvement.
• Crown gall disease: tumour-like swellings from uncontrolled cell division, caused by the bacterium Agrobacterium tumefaciens. Studying how it transfers genetic material into plant cells led to its use as a tool in genetic engineering and tissue culture (useful genes for improved and disease-resistant crops).
• B. G. L. Swamy: Indian botanist (plant morphology and anatomy); his Kannada book Hasuru Honnu won the Kendra Sahitya Akademi Award (1978).