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9.1 Essential ideas

9.1.3 Growth

  • The areas of growth in a plant are called meristems. Upward and downward growth occurs at the apical meristems, located in the buds of growing roots and stems. These are sometimes called primary meristems.
  • Many plants also have lateral meristems, which are rings of mitotic cells embedded near the vascular bundles. These are responsible for thickening of stems and are sometimes called secondary meristems.
  • The cells in the meristem regions are undifferentiated (stem cells), allowing for indeterminate growth by mitosis.
  • Auxins are a class of plant hormones that influence gene expression and influence growth through tropisms. The best studied auxin is IAA (indole 3-acetic acid) which plays a role in cell elongation and apical dominance.

Plant hormones and tropisms

Plants adapt their growth in response to the biotic and abiotic environment. When there is a change in the direction of growth or orientation of a plant, this is known as tropism. Tropisms occur in response to a number of different stimuli, as shown in the table below:

Tropism

Growth response

Example

Phototropism

Towards light source (positive)

Away from light source (negative)

Sunflowers orient themselves towards the sun (Figure 9.1.4c)

Gravitropism (geotropism)

Shoot grows upwards (positive)

Roots grow downwards (negative)

This occurs even if the pot or plant is inverted (Figure 9.1.4d)

Thigmotropism

Physical manipulation or touch

Vines on a fence (Figure 9.1.4e)

 

Cell elongation and phototropism

  • Auxin, a hormone that stimulates the growth of cells, is synthesised primarily in the apical meristem of the shoot.
  • When a plant is illuminated only on one side, auxin accumulates in the cells on the shady side of the plant, causing them to elongate.
  • As a result, the growing shoot bends towards the light.

Cell elongation in response to auxinFigure 9.1.3a – Cell elongation in response to auxin

Apical dominance

When the shoot meristem is pruned, a plant tends to become bushier. This is a result of apical dominance. Apical dominance refers to the tendency of the apical meristem to inhibit the axial buds from developing into branches.

  • Auxins produced in the shoot meristem travel down the length of the plant and promote elongation of cells.
  • Auxin inhibits the action of another hormone, called cytokinin, whose role is stimulation of the axial buds to branch and develop.
  • The result is mostly upward growth and very little branching (apical dominance).
  • When the apical meristem is cut or damaged, the source of auxin production is removed, and the plant will branch further because of the antagonistic action of cytokinin.
apical dominanceFigure 9.1.3b – Apical dominance
The shoot meristem is intact on the plant on the left but has been removed from the plant on the right.
sunflowerFigure 9.1.4c – PhototropismgravitropismFigure 9.1.4d – Gravitropsim (geotropism)thigmotropismFigure 9.1.4e – Thigmotropism

Key questions

  • Where are meristems located?
  • How do auxins affect plant growth and differentiation?

Extended essay 

IAA (indole 3-acetic acid, an auxin) is commonly available from scientific suppliers and it can be used in a number of experiments on tropisms. Why not try to improve Darwin’s original experiments?

Did you know?

Did you know that one ripe banana can induce ripening in a bunch of green bananas? The chemical responsible is ethylene.