Key Points

Growth is an irreversible and permanent increase in the size of an organ, its parts, or an individual cell, which is generally accompanied by metabolic processes.

Plant growth is unique as it is indeterminate, meaning plants can grow throughout their life due to meristems at specific locations. This is known as the open form of growth.

The period of growth is divided into three phases: meristematic (active cell division), elongation (cell enlargement and vacuolation), and maturation (cells attain their maximal size and differentiate).

In arithmetic growth, one daughter cell continues to divide while the other differentiates, resulting in a linear increase. The formula is $L_t = L_0 + rt$, where $L_t$ is length at time 't'.

In geometric growth, both progeny cells divide, leading to an initial lag phase, an exponential log phase, and a stationary phase, forming a typical S-shaped or sigmoid curve. The formula is $W_1 = W_0 e^{rt}$.

Absolute growth rate is the measurement of total growth per unit time. Relative growth rate is the growth per unit time expressed on a common basis, such as per unit initial parameter.

Differentiation is when cells mature to perform specific functions. Dedifferentiation is when differentiated cells regain the capacity to divide. Redifferentiation is when these cells mature again to perform new functions.

Development is the sum of all changes an organism goes through, including growth and differentiation. Plasticity is the ability of plants to form different structures in response to environment, such as heterophylly in buttercup.

PGRs (phytohormones) are small molecules controlling growth. They are divided into growth promoters (Auxins, Gibberellins, Cytokinins) and growth inhibitors (Abscisic Acid, Ethylene).

Auxins like IAA promote rooting in stem cuttings and cause apical dominance. Synthetic auxins like 2,4-D are widely used as herbicides to kill dicot weeds.

Gibberellins like $GA_3$ promote stem and fruit elongation, delay senescence, and promote bolting (internode elongation) in rosette plants. They are used to increase the yield of sugarcane.

Cytokinins promote cell division (cytokinesis), help overcome apical dominance by promoting lateral shoot growth, and delay leaf senescence by promoting nutrient mobilization.

Ethylene ($C_2H_4$) is a gaseous PGR that hastens fruit ripening, promotes senescence and abscission of leaves and flowers. The rise in respiration rate during ripening is called the respiratory climactic.

Abscisic acid (ABA) is a growth inhibitor that acts as a stress hormone by stimulating the closure of stomata during water stress. It also promotes seed dormancy and abscission.

Apical dominance is a phenomenon where the apical bud inhibits the growth of lateral buds, an effect mediated by auxins. Decapitation (removal of the shoot tip) promotes lateral branching.