Transcript Plant Growth and Development

Plant Growth and
Development
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.
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.
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.
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
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.
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.
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.
 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.
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.