Plant Growth and Development

1. Chapter at a glance

• Growth is an irreversible permanent increase in size of an organ, its parts or an individual cell, accompanied by metabolic processes at the expense of energy.
• Plant growth is indeterminate (open form) due to meristems; root and shoot apical meristems cause primary (elongation) growth while lateral meristems (vascular cambium, cork cambium) cause secondary (girth) growth.
• Growth occurs in three phases: meristematic, elongation and maturation; it shows arithmetic or geometric patterns leading to a sigmoid curve with lag, log/exponential and stationary phases.
• Differentiation is the maturation of cells from meristems; dedifferentiation allows differentiated cells to regain division capacity (e.g., interfascicular and cork cambium); redifferentiation follows.
• Development is the sum of growth and differentiation; plants exhibit plasticity (e.g., heterophylly) due to intrinsic (genetic, PGRs) and extrinsic (light, temperature, water, oxygen, nutrients) factors.
• Five major PGR groups: auxins and gibberellins (promoters), cytokinins (promoters), ethylene and abscisic acid (inhibitors/stress responses); each discovered accidentally and shows multiple overlapping effects.
• PGRs act synergistically or antagonistically; extrinsic factors often act via PGRs (e.g., vernalisation, flowering, dormancy).

2. Definitions and laws

• Growth: “an irreversible permanent increase in size of an organ or its parts or even of an individual cell.”
• Arithmetic growth: Lt = L0 + rt (Lt = length at time t; L0 = length at time zero; r = growth rate/elongation per unit time).
• Geometric/exponential growth: W1 = W0 ert (W1 = final size; W0 = initial size; r = growth rate; t = time of growth; e = base of natural logarithms).
• Differentiation: maturation of cells derived from meristems to perform specific functions, involving structural changes in cell wall and protoplasm.
• Dedifferentiation: living differentiated cells regain capacity to divide under certain conditions (e.g., formation of interfascicular cambium and cork cambium from parenchyma).
• Redifferentiation: cells produced by dedifferentiated meristems lose division capacity again and mature to perform specific functions.
• Development: “the sum of two processes: growth and differentiation”; includes all changes from seed germination to senescence.
• Plasticity: ability of plants to follow different pathways in response to environment or life phases to form different structures (e.g., heterophylly).

3. Important diagrams and activities

• Figure 13.1 (Germination and seedling development in bean): shows orderly sequence from seed to mature plant.
• Figure 13.2 (Locations of root apical meristem, shoot apical meristem and vascular cambium): illustrates sites of primary and secondary growth.
• Figure 13.3 (Parallel line technique on root tip): demonstrates zones of meristematic, elongation and maturation phases.
• Figure 13.4 (Arithmetic vs geometric growth curves + embryo stages): compares the two growth patterns.
• Figure 13.5 (Linear plot of length vs time): shows arithmetic growth as a straight line.
• Figure 13.6 (Idealised sigmoid growth curve): shows lag, exponential and stationary phases typical of cells/tissues/organs.
• Figure 13.7 (Comparison of absolute and relative growth rates on two leaves): distinguishes total vs per-unit-initial-parameter increase.
• Figure 13.8 (Sequence of developmental processes in a plant cell): flow from meristematic cell through division, expansion, differentiation, maturation to senescence/death.
• Figure 13.9 (Heterophylly in larkspur and buttercup): illustrates plasticity due to environment/phase.
• Figure 13.10 (Coleoptile tip experiment): demonstrates that the tip is the source of transmittable influence (auxin) causing phototropism.
• Figure 13.11 (Apical dominance before/after decapitation): shows inhibition of lateral buds by apical bud and its removal.

4. Common misconceptions and exam pitfalls

• Assuming growth is reversible or measurable only by one parameter (text stresses multiple parameters and irreversibility).
• Confusing arithmetic growth (linear, one daughter cell divides) with geometric growth (exponential, both daughters divide) or mislabelling phases of the sigmoid curve.
• Thinking differentiation is irreversible in plants (text explicitly describes dedifferentiation and redifferentiation).
• Believing all PGRs are promoters or have fixed single roles (ethylene and ABA can inhibit; roles overlap and are context-dependent).
• Overlooking that final structure depends on position of cell relative to meristem (open differentiation).
• Forgetting that development = growth + differentiation and is controlled by both intrinsic and extrinsic factors acting via PGRs.

5. Formula sheet