Plant Growth and Development

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Plant Growth and Development
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Types of Growth

• Apical meristem plant tissue made of actively
  dividing cells. Primary growth and located at
  the tip of roots and shoots.
• All growth at the apical meristem is called
  primary growth which increases the height, not
  the width.
• Secondary growth arises from lateral meristems
  and that increases the width of a plant.

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Shoot Primary Growth

• Begin with mitosis and once cell division occurs,
  each cell grows longer.
• Then elongated cells specialize (differentiate)
  into different cell types parenchyma, epidermal,
  vascular.
• Tissues form stems, leaves, and organs for sexual
  reproduction.

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Root Primary Growth

• Root apical meristems produce the cells of the
  root cap and all other cell types in the root.
• There are several zones in the root
• Root cap protects the meristem as it pushes
  through the soil.
• Zone of elongation most cells stop dividing but
  increase in length. Phloem matures and xylem
  starts to form.
• Zone of cell division cells formed by meristem
  continue to divide.
• Zone of maturation (differentiation) tissues of
  the root begin to take on specialized roles.
  Root hairs begin to form.

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Secondary Growth

• Happens in woody plants after the first season.
• Wood is a product of secondary growth from
  lateral meristem
• The vascular cambium is an example of a lateral
  meristem as it gives rise to secondary phloem and
  secondary xylem
• Cork cambium is another example as it makes the
  cells that form the bark

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Tropisms and Growth Regulators

• Tropisms are growth responses to external
  stimuli.
• Positive tropisms are growths towards the
  stimulus
• Negative tropisms are growths away from the
  stimulus
• Phototropism stems show a positive tropism
  response to sunlight, where roots show a negative
  tropism response.
• Gravitropism stems show negative to gravity,
  where roots show positive.
• Thigmotropism is a change in the direction of
  growth in response to contact. Climbing vines
  are an example.

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Hormones

• There are three groups of plant hormones auxins,
  gibberellins, cytokinins.
• Auxins
• Plants like grasses have a sheath that covers
  young leaves as the seedling emerges from the
  ground. This sheath responds to sunlight by
  bending towards it.
• Auxin is released from the apical bud (tip) when
  exposed to light and travels away from the light
  and downwards, causing the cells to elongate.
• This results in an uneven elongation of cells,
  and the stem bends towards the light.
• Produced in the roots by the apical meristems.
• Helps the roots grow with gravity, and works the
  opposite to stem cells, high amounts of auxin,
  inhibit elongation.
• Help roots grow around rocks.

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• Some herbicides contain auxins that cause plants
  to undergo elongation at an unsustainably rapid
  rate and die.
• Synthetic auxins are used to induce cell
  elongation in fruits.
• If you cut off the apical bud, you can stunt the
  growth of the plant and possibly cause the plant
  to grow more flowers, fruit or leaves.

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• Gibberellins
• Produced in apical meristems AND in leaf
  chloroplasts
• Work with auxins to elongate stems and roots
• Promote leaf growth and flowering
• Seed germination
• Controls fruit development
• We spray fruit with gibberellins, to get larger
  fruit.

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Cytokinins

• Promote cell division and cell differentiation
• Seed germination and flowering
• Prevent plant from aging too quickly
• Florists spray cut flowers to keep them alive
  longer.