A tap root system has one main primary root growing vertically downward from which lateral roots (secondary and tertiary) arise; it develops from the radicle and is characteristic of dicots (e.g. mustard, mango). A fibrous root system has a cluster of thin, thread-like roots of similar size arising from the base of the stem; the primary root is short-lived and it is characteristic of monocots (e.g. wheat, grass).

Modifications of the stem include: (1) underground stems for storage such as the tuber of potato and the rhizome of ginger; (2) stem tendrils for climbing, as in gourds and grapevine; (3) thorns for protection, as in Citrus and Bougainvillea; and (4) runners or stolons for vegetative propagation, as in grass and strawberry. These modifications help in storage, support, protection or propagation.

A typical flower has four whorls: (1) Calyx (sepals) - the outermost green whorl that protects the flower in the bud stage; (2) Corolla (petals) - usually coloured, it attracts pollinators; (3) Androecium (stamens) - the male reproductive whorl producing pollen; and (4) Gynoecium (carpels/pistil) - the female reproductive whorl containing ovules that develop into seeds after fertilisation.

Venation is the arrangement of veins and veinlets in the lamina of a leaf. In reticulate venation the veinlets form a network, typical of dicot leaves (e.g. mango); in parallel venation the veins run parallel to one another, typical of monocot leaves (e.g. grass). Phyllotaxy is the pattern of arrangement of leaves on the stem or branch; it may be alternate, opposite or whorled.

Meristematic tissue consists of actively dividing cells that are responsible for the growth of the plant. The cells are small, thin-walled, with dense cytoplasm and prominent nuclei, and little or no vacuole. Based on position the three types are: apical meristem (at the tips of root and shoot, causing increase in length), intercalary meristem (at the base of internodes or leaves) and lateral meristem (along the sides, e.g. cambium, causing increase in girth).

(1) In a dicot root the number of xylem bundles (vascular bundles) is few, usually two to four (tetrarch or less), whereas in a monocot root they are many (polyarch, often more than six). (2) A dicot root has secondary growth due to the presence of cambium, whereas a monocot root usually lacks secondary growth as cambium is absent. Also, the pith is small or absent in dicot roots but large and well developed in monocot roots.

Stomata are tiny pores present mainly in the epidermis of leaves. Each stoma is bounded by two bean-shaped (kidney-shaped) guard cells that contain chloroplasts; in monocots the guard cells are dumb-bell shaped. Functions: stomata allow the exchange of gases (carbon dioxide and oxygen) between the plant and the atmosphere and control transpiration (loss of water vapour). The opening and closing of the stoma is regulated by the turgidity of the guard cells.

The three types of muscular tissue are: (1) striated (skeletal) muscle - cylindrical, unbranched, multinucleate and voluntary, attached to bones for movement; (2) smooth (unstriated) muscle - spindle-shaped, uninucleate and involuntary, found in the walls of internal organs such as the stomach, intestine and blood vessels; and (3) cardiac muscle - branched, striated and involuntary, found only in the wall of the heart.

A neuron is the structural and functional unit of nervous tissue. It has three main parts: (1) the cell body (cyton), which contains the nucleus and cytoplasm with Nissl granules; (2) dendrites, which are short branched processes arising from the cell body that receive impulses and carry them towards the cell body; and (3) the axon, a single long process that carries impulses away from the cell body to the next neuron or effector. The axon may be covered by an insulating myelin sheath.

The cell theory was proposed by M. Schleiden (a botanist) and T. Schwann (a zoologist) in 1838-39. Its postulates are: (1) all living organisms are composed of cells and products of cells; (2) the cell is the basic structural and functional unit of life. Later Rudolf Virchow (1855) added that all cells arise from pre-existing cells (omnis cellula-e-cellula).

The endoplasmic reticulum (ER) is a network of membranous tubules and sacs in the cytoplasm. Rough ER (RER) has ribosomes attached to its surface and is mainly involved in the synthesis and transport of proteins. Smooth ER (SER) lacks ribosomes and is involved in the synthesis of lipids and steroids and in detoxification. Both help in intracellular transport of materials.

The Golgi apparatus consists of a stack of flattened membrane-bound sacs called cisternae arranged parallel to one another, along with associated vesicles. Functions: it packages, modifies and sorts materials (such as proteins and lipids) received from the endoplasmic reticulum and secretes them in vesicles. It is also the site of formation of lysosomes and the synthesis of certain complex carbohydrates and cell wall materials.

Ribosomes are non-membrane-bound organelles made of RNA and protein; they are the site of protein synthesis. Lysosomes are single-membrane-bound vesicles containing powerful digestive (hydrolytic) enzymes that digest worn-out organelles, food and foreign material. They are called suicidal bags because on rupture their enzymes can digest the cell's own contents, causing the cell to die (autolysis).

Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that yield them on hydrolysis) and act as the main source of energy. They are classified as: (1) monosaccharides - simple single-unit sugars such as glucose and fructose; (2) oligosaccharides (mainly disaccharides) - such as sucrose, maltose and lactose, which yield 2 to 10 monosaccharides on hydrolysis; and (3) polysaccharides - large polymers such as starch, glycogen and cellulose.

Lipids are organic compounds that are insoluble in water but soluble in organic solvents; they include fats, oils, phospholipids and steroids and are generally esters of fatty acids and glycerol. Functions: (1) they are a concentrated store of energy (fats give about twice the energy of carbohydrates); (2) phospholipids form the structural basis of cell membranes; and they also act as thermal insulation and protect internal organs.

Nucleic acids are polymers of nucleotides and are of two types, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid); they store and transmit genetic information. Each nucleotide has three components: (1) a nitrogenous base (adenine, guanine, cytosine, and thymine in DNA or uracil in RNA); (2) a pentose sugar (deoxyribose in DNA, ribose in RNA); and (3) a phosphate group.

Essential amino acids are those that the body cannot synthesise and must be obtained from the diet (for example lysine and valine). Non-essential amino acids are those that the body can synthesise itself (for example glycine and alanine). A peptide bond is the covalent amide bond formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of the next, with the removal of a water molecule.

The cell cycle is the sequence of events by which a cell duplicates its contents and divides into two daughter cells. It has two main phases: interphase (the preparatory, non-dividing phase) and the M phase (mitosis or division). Interphase is divided into three sub-phases: G1 phase (cell growth and normal metabolism), S phase (synthesis, in which DNA is replicated) and G2 phase (further growth and preparation for division).

Mitosis produces two daughter cells that are genetically identical to the parent cell, each with the same (diploid) number of chromosomes. Its significance: (1) it is responsible for growth of the body by increasing the number of cells; (2) it repairs and replaces old, worn-out or damaged cells and tissues; and (3) it maintains the constant chromosome number and enables asexual reproduction in some organisms.

(1) Mitosis involves a single division producing two daughter cells, whereas meiosis involves two successive divisions producing four daughter cells. (2) In mitosis the daughter cells have the same (diploid) chromosome number as the parent and are genetically identical, whereas in meiosis the chromosome number is halved (haploid daughter cells) and genetic variation is introduced through crossing over. Mitosis occurs in body (somatic) cells while meiosis occurs in reproductive cells.