What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

Define growth, differentiation and development.

Growth is an irreversible increase in size, differentiation is the maturation of cells into specialised forms, and development is the sum of growth and differentiation through the life cycle.

Why is plant growth said to be open and indeterminate?

Because meristems retain the capacity to divide and add new cells throughout the life of the plant, allowing unlimited growth.

Describe arithmetic and geometric growth.

In arithmetic growth only one daughter cell continues to divide, giving a linear curve, while in geometric growth both daughter cells continue to divide, giving an exponential or sigmoid curve.

What is a sigmoid growth curve?

It is an S-shaped curve typical of growth in a natural environment, with a lag phase, a log or exponential phase and a stationary phase.

Distinguish between dedifferentiation and redifferentiation.

Dedifferentiation is the regaining of dividing capacity by mature cells, while redifferentiation is the loss of that capacity again as the new cells mature to perform specific functions.

Name the five main groups of plant growth regulators.

Auxins, gibberellins, cytokinins, ethylene and abscisic acid.

Why is abscisic acid called the stress hormone?

Because it stimulates stomatal closure and increases the tolerance of plants to various stresses, and it induces dormancy that helps seeds withstand desiccation.

Which PGR would you use to induce rooting in a stem cutting and which to ripen a fruit?

Auxins are used to induce rooting in stem cuttings, and ethylene (e.g. ethephon) is used to ripen fruits.

What is apical dominance and how can it be removed?

It is the inhibition of lateral bud growth by the apical bud due to auxin; removing the shoot tip (decapitation) allows the lateral buds to grow.

What roles do gibberellins and cytokinins play?

Gibberellins promote stem and fruit elongation, bolting and delay senescence, while cytokinins promote cell division, new leaf and shoot formation and delay leaf senescence.

Plant Growth and Development

Class 11 Biology · Biology · Chapter 13

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Summary

This chapter deals with growth, differentiation and development in plants and the regulators that control them. Growth is an irreversible, permanent increase in size, measurable as increase in length, area, volume, weight or cell number. Plant growth is generally indeterminate and open because meristems retain the capacity to divide, giving primary growth from apical meristems and secondary growth from lateral meristems. Growth passes through meristematic, elongation and maturation phases, and its rate may be arithmetic (a linear curve) or geometric (an S-shaped sigmoid curve with lag, log and stationary phases). Differentiation is the maturation of meristematic cells into specialised cells; differentiated cells can dedifferentiate to regain division and then redifferentiate. Because differentiation and growth are open, development is flexible, and plants show plasticity such as heterophylly. Development, the sum of growth and differentiation, is controlled by intrinsic factors including plant growth regulators (PGRs) and extrinsic factors such as light and temperature. The five main PGRs are auxins (apical dominance, rooting, herbicides), gibberellins (stem elongation, bolting), cytokinins (cell division, delaying senescence), ethylene (fruit ripening, abscission) and abscisic acid (the stress hormone, inducing dormancy and stomatal closure). PGRs may act synergistically or antagonistically in events such as dormancy, senescence and abscission.

Growth, its measurement and phasesArithmetic and geometric growth ratesDifferentiation, dedifferentiation, redifferentiationDevelopment and plasticityPlant growth regulators and their discoveryPhysiological effects of the five PGRs

Key terms

Growth

An irreversible, permanent increase in the size of an organ, its parts or a cell.

Differentiation

The maturation of cells derived from meristems into cells that perform specific functions.

Dedifferentiation

The regaining of the capacity to divide by living differentiated cells, as in cork cambium formation.

Plasticity

The ability of plants to follow different developmental pathways in response to environment or age, as in heterophylly.

Plant growth regulator

A small simple molecule (auxin, gibberellin, cytokinin, ethylene or abscisic acid) that controls growth and development.

Apical dominance

The inhibition of growth of lateral buds by the growing apical bud, controlled by auxin.

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Growth is best defined as a(n):

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