Chapter 13: Plant Growth and Development | Summary

Growth is generally divided into three phases: meristematic, elongation, and maturation. In the meristematic phase, cells at the root and shoot apex continuously divide; these cells are rich in protoplasm and have thin, cellulosic primary cell walls with abundant plasmodesmatal connections. In the elongation phase, cells show increased vacuolation, cell enlargement, and new cell wall deposition.

In the maturation phase, cells attain their maximal size through wall thickening and protoplasmic modifications. Growth rates can be arithmetic (one daughter cell continues to divide, producing a linear curve) or geometric (both daughter cells divide, producing an S-shaped sigmoid curve). The sigmoid curve, with lag, exponential, and stationary phases, is characteristic of living organisms growing in a natural environment.

Essential conditions for plant growth include water, oxygen, and nutrients. Water is required for cell enlargement and turgidity, which supports extension growth; it also serves as the medium for enzymatic activities. Oxygen is necessary for releasing metabolic energy for growth activities. Nutrients — both macro and micro essential elements — are required for the synthesis of protoplasm and serve as sources of energy.

Temperature also plays a critical role, as every plant has an optimum temperature range for growth; deviation from this range can be detrimental to survival. Environmental signals such as light and gravity further affect specific phases and stages of plant growth.

Differentiation is the process by which cells derived from meristems undergo structural changes in their cell walls and protoplasm to mature and perform specific functions. For example, to form a tracheary element, cells lose their protoplasm and develop strong, elastic, lignocellulosic secondary cell walls to conduct water under extreme tension. Differentiation in plants is considered 'open' because the final structure of a cell at maturity is also determined by its position within the organ.

Dedifferentiation is the phenomenon where living differentiated cells that had lost the capacity to divide regain that capacity under certain conditions, as seen in the formation of interfascicular cambium and cork cambium from differentiated parenchyma cells. The meristems produced by dedifferentiation can divide and produce cells that once again lose the capacity to divide and mature to perform specific functions — a process called redifferentiation.

Development includes all changes an organism undergoes during its life cycle, from seed germination to senescence. It is broadly considered the sum of growth and differentiation, both of which are under the control of intrinsic (genetic, chemical/PGR) and extrinsic (light, temperature, water, oxygen, nutrition) factors. The sequence of developmental events in a plant cell proceeds from a meristematic cell through plasmatic growth, expansion, differentiation, and maturation to a mature cell, with senescence and death representing the final stages.

Plants exhibit plasticity, meaning they follow different developmental pathways in response to the environment or phases of life to form different structures. Heterophylly — the production of differently shaped leaves in juvenile versus adult plants, or in response to aquatic versus terrestrial habitats (as seen in buttercup, coriander, larkspur) — is a classic example of developmental plasticity.

Plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition, including indole compounds (IAA), adenine derivatives (kinetin), carotenoid derivatives (ABA), terpenes (GA3), and gases (ethylene). They are broadly classified into growth promoters (auxins, gibberellins, cytokinins) and growth inhibitors (ABA), with ethylene being largely inhibitory but fitting either group. The discovery of all five major PGR groups was accidental: auxin was isolated by F.W. Went from oat coleoptile tips; gibberellin was linked to Gibberella fujikuroi by Kurosawa (1926); kinetin was identified by Miller et al. (1955) from autoclaved herring sperm DNA; ABA was independently characterised in the mid-1960s; and ethylene was confirmed by H.H. Cousins (1910).

PGRs are variously referred to as plant growth substances, plant hormones, or phytohormones. Their roles in a plant can be complementary, antagonistic, individualistic, or synergistic. In every phase of growth, differentiation, and development, one or more PGRs play a regulatory role, often interacting with extrinsic factors like light and temperature.

Auxins (e.g., IAA, IBA, NAA, 2,4-D) are produced at growing apices and promote rooting in stem cuttings, flowering in pineapples, and prevent premature fruit and leaf drop, though they promote abscission of older organs. Auxins cause apical dominance by inhibiting lateral bud growth; removal of the apical bud (decapitation) releases this inhibition. They also induce parthenocarpy in tomatoes, control xylem differentiation, and are used as herbicides (2,4-D kills dicotyledonous weeds without affecting mature monocots). Gibberellins (more than 100 types; most studied is GA3) promote elongation of internodes, delay senescence, speed malting in brewing, cause bolting in rosette plants, and increase sugarcane yield. Cytokinins (e.g., zeatin from corn kernels) promote cell division, new leaf and chloroplast formation, lateral shoot growth, help overcome apical dominance, and delay leaf senescence by promoting nutrient mobilisation.

Ethylene (gaseous PGR) is produced by senescing tissues and ripening fruits; it promotes fruit ripening, abscission, root growth, root hair formation, breaks dormancy, and induces female flower formation in cucumbers. The compound ethephon is widely used as a commercial source of ethylene. Abscisic acid (ABA), the stress hormone, acts as a general growth inhibitor: it stimulates stomatal closure, inhibits seed germination, promotes dormancy and maturation of seeds, and helps seeds withstand desiccation. ABA acts as an antagonist to gibberellins in most situations.

Plant Growth and Development is a chapter that integrates the concepts of growth (an irreversible increase in size), differentiation (structural and functional specialisation), and development (the sum of both processes across the entire life cycle). The chapter establishes that plant growth is indeterminate due to persistent meristematic activity, follows measurable mathematical patterns (arithmetic and geometric/sigmoid), and is governed by both internal factors — particularly the five classes of plant growth regulators (auxins, gibberellins, cytokinins, ABA, and ethylene) — and external environmental signals.

From a board examination perspective, this chapter carries high weightage in both short-answer and long-answer sections. Students must be able to clearly define and distinguish terms such as differentiation, dedifferentiation, and redifferentiation; compare arithmetic versus geometric growth with their mathematical expressions; and explain the physiological effects and agricultural applications of all five PGRs. Questions on apical dominance, the stress hormone ABA, the sigmoid growth curve, and conditions for growth are frequently asked in CBSE exams and require conceptual clarity rather than rote memorisation.