This document covers plant growth regulators and secondary growth. It discusses how plants grow through cell division via mitosis in meristematic tissues located at root and shoot tips. Auxin influences plant growth through phototropism, the growth response of plants toward light, and cell elongation. When woody dicots undergo secondary growth, concentric rings of secondary xylem form, alternating bands of early and late wood. Photoperiodism refers to the flowering response in plants triggered by day length.

1. RHS Level 2 Certificate
 Week 10 – Plant Growth Regulators and
Secondary Growth.

2. Learning objectives
 Explain how plants increase is size by mitosis
 Identify where this takes place
 State the meristematic tissues within a plant.
 Describe two examples of how auxin influences plant
growth.
 Describe what is meant by the term phototropism.
 State the basic changes that occur when woody
dicotyledons undergo secondary growth/thickening.
 State how strengthening tissue can be established in
monocotyledons allowing perennial growth to occur.
 Define ‘photoperiodism’

3. Cell components
 Cell wall – cellulose mesh, reinforced with lignin in
cells that do not remain active.
 Cell membrane – holds the living contents of the cell.
 Nucleus – contains chromosomes (DNA) and RNA.
 Cytoplasm – jelly like substance that supports and
sustains cell components.
 Plasmodesmata – strands of cytoplasm that
biochemically link cells together.
 Vacuole – sac containing dilute sugars, waste
products and nutrients. Maintains cell shape.

4. Plant cell - structure

5. How do plants grow?
 Growth is produced by cell division in the
meristems of plants
 Mitosis produces identical daughter cells that
expand and differentiate to form the different
tissues of the plant.
 Meristems occur at the tips of roots and
shoots (apical meristems) and around the
sides of the plant (lateral meristems).
 Lateral meristems include the vascular
cambium and cork cambium.

6. Plant Growth Regulators 1
 Control the way in which plants grow.
For example that roots grow downwards
and shoots upwards.
 Chemicals produced by the plant which
control the speed and location of the
division of cells and the differentiation of
those cells once produced

7. Plant Growth Regulators 2
 Auxin – elongation of cells and fruit formation
 Cytokinins - promotes cell division (mitosis), delays the
senescence of leaves.
 Gibberellins - cell division and elongation of stems.
Promote flowering and bolting in ‘long day’ plants and seed
germination.
 Ethylene - ripening of fruit and senescence of leaves and
flowers. Induces abscission of leaves in deciduous plants
(the process of leaf fall).
 Abscisic acid maintains dormancy in seeds and possibly in
buds. It also is responsible for causing the closing of
stomata – produced in response to ethylene(?).

8. Auxin and tropisms
 Tropisms are growth responses to external
stimuli
 Phototropism – plants grow toward the light
 Geotropism – may be positive (growth towards
gravitational pull) or negative (growth away)
 Thigmatropism – growth towards touch.
 Auxin migrates in the stem to cause cell
elongation that causes the above growth
responses

9. Photoperiodism
 The term for the trigger of the flowering
response in plants by day length (or, more
accurately, night length).
 Some plants do not respond to this trigger –
they flower a more or less set number of
weeks after germination, when they reach
mature size.
 Others will only flower if the day length is
correct – short day or long day plants.

10. Annual Rings
 Concentric rings of secondary xylem
 Alternating bands of early and late wood
 Early wood
– Xylem cells with large diameter, thin walls
 Late wood
– Xylem cells with smaller diameter, thicker walls

11. Woody Stem
periderm (consists of
cork, cork cambium,
and secondary cortex)
secondary
phloem
BARK
HEARTWOOD SAPWOOD
vascular cambium

12. Learning outcomes
 Explain how plants increase is size by mitosis
 Identify where this takes place
 State the meristematic tissues within a plant.
 Describe two examples of how auxin influences plant
growth.
 Describe what is meant by the term phototropism.
 State the basic changes that occur when woody
dicotyledons undergo secondary growth/thickening.
 State how strengthening tissue can be established in
monocotyledons allowing perennial growth to occur.
 Define ‘photoperiodism’