3.5.2 Responses in the Flowering Plant
Objectives – What you will need to
know from this section
Explain the term: growth regulation.
Define the following terms: tropisms, phototropism,
geotropism, thigmotropism, hydrotropism,
Name examples of phototropism & geotropism.
Define the term: growth regulator.
Outline the transport of regulators via the vascular
Explain the term: Combined Effect
Explain the term: Growth Promoter.
Explain the term: Growth Inhibitor
Name 4 methods of anatomical/chemical adaptations
that protect plants.
State 2 examples of the use of plant regulators.
Mandatory Activity :
Effects of I.A.A. Growth Regulators on Plants
3.5.2 Responses in the Flowering Plant
The ability to detect change and to respond is called
sensitivity. Response is a form of defence that allows
organisms to survive.
Plant adapt to new situations by modifying their growth,
by means of chemicals called growth regulators
Growth is the increase in the number, size and volume
A hormone is a chemical produced in one part of an
organism, transported to other tissues where it has
External factors that regulate the growth of plants are light
intensity, day length, gravity, temperature.
Development means the
changes that lead to specialised
tissues and organs.
Plant growth regulators [hormones] are chemicals that
interact with one another to control a particular
development or response.
Plants only grow at their tips,
in small regions of active cell
division called meristems—
the tips of shoots, roots and
Plant growth regulators are
produced in the meristems
and transported through
the vascular system of the
Plant Growth Regulators
• They are active in very small amounts
• They are produced in the meristems
• They are transported in the xylem and
• Their effects are dependent on
• A small amount of growth regulator can
have great effect on growth
• What is the difference between growth and
• What is a growth regulator?
• List the properties of plant growth
• What is a meristem?
A tropism is a plant’s response to a stimulus coming from
one direction, e.g. sunlight, gravity.
Phototropism is a
growth response of
a stem towards
light, so that it can
of light for
The coleoptile is the sheath
around the leaves and shoot
of grass seedlings, such as
corn or oats.
Geotropism is the
growth of a plant in
response to gravity—it
allows the root to get the
water and nutrients it
the soil, and raises the
leaves up into the light
Thigmotropism is the
growth of a plant in
response to contact,
e.g. vines wrap around
objects which help support
Hydrotropism is the
response of roots to water
– they will grow towards
Chemotropism is the
response to chemicals,
e.g. pollen tubes
growing down the
carpel in response to
by the ovule.
What is meant by the term “tropism”.
What is a coleoptile?
What is a stimulus.
Name 5 different tropisms and the stimulus
involved in each case.
• List the advantage to the plant of each
Plant growth regulators
Plant growth regulators interact with one another to
control a particular development or response.
promote growth e.g.
Some regulators inhibit growth
e.g. abscisic acid and ethene.
promote cell enlargement and growth,
are involved in phototropism and apical dominance
In shoots, light causes auxin to move down the shaded part of
the stem, causing the shoot to grow towards the sunlight.
This is called PHOTOTROPISM.
here due to more
Artificial auxins can also be used to kill weeds,
stimulate root formation in cuttings, …
cause stem lengthening,
Mobilise the stored food in
Break dormancy in buds
and seeds in Spring.
Cytokinins stimulate cell division (mitosis)
and trigger leaf growth in spring.
Abscisic acid triggers bud and seed
dormancy in autumn and inhibits
Ethene promotes ripening of fruit and the
fall of leaves, flowers and fruits.
Uses of plant regulators
Artificial auxins are used in rooting
powders to stimulate root formation in
A synthetic auxin [2,4-D] is used as a
Fruit is transported green and
unripe, and can then be
quickly ripened by spraying it
• What is a plant growth regulator?
• Explain the terms “promote” and “inhibit”.
• Name 5 growth regulators and a function of
• Give 3 examples of how we make use of
plants growth regulators [natural or
The growth and development responses
described so far are a form of defence that
allows a plant to survive difficult conditions
[environmental stress] in its habitat.
Plants can’t move and so need to defend
themselves against being eaten or damaged.
Some adaptations that plants use to protect themselves
1. Spines, thorns or stinging
hairs to deter animals from
eating them, e.g. cacti,
2. Toxins that cause illness or death,
e.g. Cassava is an important food source in tropical
countries, but its leaves and roots produce cyanide
poison to protect it against insects and other herbivores.
The roots must be processed properly if humans are not
to be poisoned.
Cassava root tuber
3. In autumn, trees recycle all the nutrients they can out of
the leaves for storage in phloem tissue.
The red pigments of autumn serve as a kind of
botanical sunscreen, a defence mechanism against sun
damage that could interfere with the storage process
and cause a leaf to drop before the tree was done with
4. Heat-shock proteins [stress proteins] are created when
cells are exposed to higher temperatures or to other
kinds of environmental stress, such as UV light. Their
activities are part of a cell's repair system and allow
the plant to tolerate extra heat, light, etc. for a limited
period, and resume normal cellular activities when the
Stress proteins act like
chaperons and provide
"shelters" in which new protein
chains can be "incubated" until
they have folded properly
Plants need to protect themselves from their
They protect themselves against:
• Loss of water
• Infection from micro organisms
• Being eaten by herbivores
Adaptations for Protection
Plants can adapt themselves for protection in
1. Structural or anatomical adaptations
2. Chemical adaptations
• Bark/epidermis prevents entry of microbes
and reduces loss of water
• Thick cuticle on stem or leaves in plants that
live in dry places
• Thorns prevent plants from being eaten by
• Stinging cell in epidermis prevent plants been
• Guard cells change shape when they lose water
which causes stomata to close and this reduces
• Production of Tannins to make the plant
• Production of toxic chemicals to prevent
growth of insect larvae
• Production of strychnine and nicotine in
legumes that damage nerves and muscles
• Production of heat shock proteins to protect
enzymes when temperatures are high
Most important adaptation
Plants can grow and regenerate throughout
They can produce new roots, stems, leaves.
• Why is defence important to plants?
• List four methods used by plants to defend
• Explain the term “structural adaptation”.
• What is a toxin?
• What advantage is it to a plant that it grows
throughout its life [from meristems]?