PLANT AND ANIMAL RESPONSES
(a) (i) The types of plant responses.
● To include the response to abiotic stress and herbivory e.g. chemical defences (such as
tannins, alkaloids and pheromones), folding in response to touch (Mimosa pudica)
● the range of tropisms in plants.
Plants have evolved physical defences to prevent herbivory, such as barbs, spikes, spiny leaves,
and inedible tissue. However, they also have a number of chemical defences. Tannins are a
group of phenols produced by plants that are toxic to microorganisms and larger herbivores. In
leaves, they are found in the upper epidermis and make the leaves taste bitter and in the roots,
they prevent infiltration by pathogenic microorganisms. Alkaloids are a group of bitter-tasting,
nitrogenous compounds found in growing tips and flowers, and peripheral cell layers of stems
and roots, acting as a feeding deterrent. Terpenoids are a group of compound that are toxic to
insects and fungi.
Many plants produce pheromones—chemicals that when released by one plant can affect the
physiology of another. A response to water stress or herbivory can cause the release of
pheromones which warn other plants. Maple leaves when attacked by insects induce callose
deposition in the sieve tube plates of others. Other plants release chemicals that alter the
behaviour of other organisms, usually insects. When cabbages are attacked by caterpillars they
release a chemical signal which attracts the parasitic wasp Cotesia glomerata.
Tropisms are directional growth responses to external stimuli in plants. The leaves of plants are
positively phototropic and grow towards the light, whereas roots are negatively phototropic. The
main reason roots grow downward, however, is because of geotropism—they grow toward
gravity, allowing them to grow into the soil to obtain water to keep plants turgid, as well as
nitrates to synthesise amino acids. Chemotropism causes pollen tubes to grow down the style,
attracted by chemicals, towards the ovary where fertilisation occurs. The buds of shooting plants
are thigmotropic, growing toward a particular direction in response to touch, allowing them to
wind around a solid structure.
Non-directional responses to external stimuli are nastic responses. For example, Mimosa pudica
folds its leaves in response to touch. This is a thigomonasty, but can be found in response to
other external stimuli, such as light.
(ii) Practical investigations into phototropism and geotropism.
In order to test phototropism two seeds should be germinated, one under the exposure of
unilateral light, and one under the and one with directional light, and compare the horizontal
distance between two marked points on the stem to measure the distance the plant has grown
toward the light. The exact behaviour of auxin can be measured using impermeable and
permeable membranes inserted to the stems, opaque caps on the tip of the plant, or cutting off
the tip.
Geotropism can be measured using a clinostat, which slowly rotates. As it does so the
directional response of the plant to geotropism will change the relative direction of the growth of
the shoot, causing it to spiral. This should be conducted in the dark to prevent phototropism
affecting growth.
(b) The roles of plant hormones.
(a) (i) The types of plant responses.
● To include the response to abiotic stress and herbivory e.g. chemical defences (such as
tannins, alkaloids and pheromones), folding in response to touch (Mimosa pudica)
● the range of tropisms in plants.
Plants have evolved physical defences to prevent herbivory, such as barbs, spikes, spiny leaves,
and inedible tissue. However, they also have a number of chemical defences. Tannins are a
group of phenols produced by plants that are toxic to microorganisms and larger herbivores. In
leaves, they are found in the upper epidermis and make the leaves taste bitter and in the roots,
they prevent infiltration by pathogenic microorganisms. Alkaloids are a group of bitter-tasting,
nitrogenous compounds found in growing tips and flowers, and peripheral cell layers of stems
and roots, acting as a feeding deterrent. Terpenoids are a group of compound that are toxic to
insects and fungi.
Many plants produce pheromones—chemicals that when released by one plant can affect the
physiology of another. A response to water stress or herbivory can cause the release of
pheromones which warn other plants. Maple leaves when attacked by insects induce callose
deposition in the sieve tube plates of others. Other plants release chemicals that alter the
behaviour of other organisms, usually insects. When cabbages are attacked by caterpillars they
release a chemical signal which attracts the parasitic wasp Cotesia glomerata.
Tropisms are directional growth responses to external stimuli in plants. The leaves of plants are
positively phototropic and grow towards the light, whereas roots are negatively phototropic. The
main reason roots grow downward, however, is because of geotropism—they grow toward
gravity, allowing them to grow into the soil to obtain water to keep plants turgid, as well as
nitrates to synthesise amino acids. Chemotropism causes pollen tubes to grow down the style,
attracted by chemicals, towards the ovary where fertilisation occurs. The buds of shooting plants
are thigmotropic, growing toward a particular direction in response to touch, allowing them to
wind around a solid structure.
Non-directional responses to external stimuli are nastic responses. For example, Mimosa pudica
folds its leaves in response to touch. This is a thigomonasty, but can be found in response to
other external stimuli, such as light.
(ii) Practical investigations into phototropism and geotropism.
In order to test phototropism two seeds should be germinated, one under the exposure of
unilateral light, and one under the and one with directional light, and compare the horizontal
distance between two marked points on the stem to measure the distance the plant has grown
toward the light. The exact behaviour of auxin can be measured using impermeable and
permeable membranes inserted to the stems, opaque caps on the tip of the plant, or cutting off
the tip.
Geotropism can be measured using a clinostat, which slowly rotates. As it does so the
directional response of the plant to geotropism will change the relative direction of the growth of
the shoot, causing it to spiral. This should be conducted in the dark to prevent phototropism
affecting growth.
(b) The roles of plant hormones.
