Plant Growth and Development notes

Plant growth and development explains how a tiny seed becomes a mature plant through coordinated cell division, enlargement, differentiation and maturation. Growth is an irreversible increase in size, mass, volume or cell number, while development includes all changes from germination to senescence. Plants show open growth because meristems remain active throughout life. Differentiation produces specialised tissues, but plant cells can also dedifferentiate and redifferentiate, showing totipotency. Plant growth regulators such as auxins, gibberellins, cytokinins, ethylene and ABA control germination, elongation, flowering, fruiting, dormancy and senescence. For NEET, focus on growth curves, arithmetic versus geometric growth, plasticity, heterophylly, hormone functions and seed germination types.

Growth in plants is a permanent and irreversible increase in size, dry weight, volume, area or cell number. Unlike animals, plants show open growth because meristematic tissues remain active at root and shoot apices, cambium and other regions. Growth occurs through cell division, cell enlargement and cell differentiation. NCERT describes three phases in a root tip: meristematic phase, elongation phase and maturation phase. Growth may be arithmetic, where one daughter cell continues division while the other differentiates, or geometric, where both daughter cells continue dividing for some time. Growth rate can be absolute or relative. A typical plant organ shows a sigmoid growth curve with lag, log, stationary and senescence phases.

Development is the sum of all changes that a plant undergoes during its life cycle, beginning with seed germination and ending in senescence and death. It includes growth, differentiation, maturation, flowering, fruiting, seed formation and ageing. Plant development is controlled by both intrinsic factors such as genetic programme and plant growth regulators, and extrinsic factors such as light, temperature, water, oxygen and nutrients. A special NCERT concept is plasticity: plants can follow different developmental pathways depending on environment or life phase. Heterophylly is a strong example, where leaves of the same plant differ in shape, such as juvenile and mature leaves in cotton or submerged and aerial leaves in buttercup.

Differentiation is the process by which cells derived from root and shoot apical meristems and cambium mature to perform specific functions. During differentiation, cells undergo major structural changes, such as loss of protoplasm in tracheary elements or wall thickening in xylem. Dedifferentiation is the ability of mature living cells to regain division capacity, as seen when parenchyma forms cork cambium or interfascicular cambium. Redifferentiation occurs when these newly divided cells again mature into specialised tissues. Cellular totipotency is the ability of a living plant cell to generate a complete plant under suitable conditions, forming the basis of tissue culture. NEET often tests the sequence and distinction among these three terms.

Plant growth regulators are small, simple organic molecules that influence growth and development even in very low concentrations. NCERT groups them into five major types: auxins, gibberellins, cytokinins, ethylene and abscisic acid. Auxins promote cell elongation, apical dominance, rooting and parthenocarpy. Gibberellins promote stem elongation, bolting, seed germination and breaking dormancy. Cytokinins promote cell division, shoot formation and delay leaf senescence. Ethylene is a gaseous hormone involved in fruit ripening, senescence, abscission and the triple response. ABA is mainly a stress hormone that induces dormancy, closes stomata and inhibits growth. NEET questions usually ask hormone functions, commercial applications and opposite effects.

Seed germination is the resumption of growth by the embryo after a period of dormancy, leading to formation of a seedling. Dormancy helps seeds survive unfavourable conditions and may be due to hard seed coat, immature embryo, chemical inhibitors such as ABA or absence of suitable environmental conditions. Germination requires a viable seed, water for imbibition and enzyme activation, oxygen for respiration and a suitable temperature for metabolism. The stages include imbibition, enzyme activation, mobilisation of stored food, radicle emergence and plumule growth. In epigeal germination, cotyledons come above the soil due to hypocotyl elongation. In hypogeal germination, cotyledons remain below the soil due to epicotyl elongation.