How organisms position themselves
in relation to their surroundings
What are the abiotic stimuli?
Water (or Humidity)
Current (in water)
PhotoGraviThermoHydro- (or Hygro-)
Simple Animal Orientation Responses
• Taxis
• Kinesis
TAXIS (plural Taxes)
– Orientation and movement of whole
animal towards or away from external
stimulus that is coming from one side only
– Described as positive (toward) or negative
(away from) a stimulus
– Moving toward light = positive photo–taxis
Examples of animal taxes
• Earthworms and slaters move away from sunlight
back down into the soil
= Negative phototaxis
• Snails move away when their feelers touch
something ….
= Negative thigmotaxis
• Flatworms and sharks move towards meat
= Positive chemotaxis
Mosquito moves toward warm skin
What’s the advantage?
• Puts the animal in a more favourable position
• Avoids unfavourable conditions (hot, cold etc)
• Increased survival and reproduction
How do animals determine the
direction of the stimulus?
• Two sense organs: By comparing the input
from receptors on each side of the animal at
the same time
• Differences between sides show the direction
of the stimulus (eg. Snail antennae)
• OR One sense organ: Compare the change
from a single sensor over time, using its body
orientation to determine stimulus direction
KINESIS (pl. kineses)
= Non-directional response to a change in
stimulus intensity ….. It is not orientating
– Animal may simply turn randomly or change its
rate of movement
– Slater moves more
when it is hot
= thermokinesis
More terms to know
Orthokinesis = the stimulus intensity
determines the organism’s speed of
Klinokinesis = the stimulus intensity
determines the organism’s
rate of turning
Examples of Kineses
• Slaters move faster in bright light and slower
in dim light
= photo-orthokinesis
• Woodlice: move faster and turn more in high
humidity vs. low humidity (hygro-)
= hygro-orthokinesis, hygro-klinokinesis
• Human body lice turn more at 35°C than at
lower temps
= thermo-klinokinesis
What’s the advantage?
• Puts the animal in a more favourable position
• Eg. Slaters will end up in dark, damp places
which prevents dessication
• Avoids unfavourable conditions (hot, cold etc)
• Increased survival and reproduction
BIOZONE questions
• Taxes and Kineses
• Do all the questions on page
NCEA 2006 question
• Much of animal behaviour is innate, or inborn. Such
behaviour patterns may be quite simple, or are
produced in response to simple stimuli. They include
kineses and taxes.
• (a) Describe an example of kinesis in a named
• (b) Explain why this behaviour would be an
advantage to your named animal in its normal
Plant Orientation Responses
Plants are capable of a number of movements in
response to environmental stimuli.
eg. Temperature, humidity, light, touch, chemicals
1. NASTIC RESPONSES: non-directional responses
to stimuli
2. TROPISMS: Directional growth responses
towards or away from a directional stimulus
Nastic Responses
• The rate or frequency of these
responses increases as intensity of
the stimulus increases.
• They are named with the suffix
"-nasty" and have prefixes that
depend on the stimuli
Eg. Photonasty, thigmonasty
Examples on you tube
• Mimosa pudica: the sensitive plant
• Venus fly traps – jaws of death
Eg. Mimosa plant
• When the sensitive leaves are touched, they
droop down and fold up rapidly
• Thigmonasty
• Advantage: why?
• Reduces the plant’s
Surface area for grazing
And abiotic stress.
Venus Fly Trap Plant
What happened?
What was the stimulus?
Other examples
• Opening/closing of tulip flowers due to
changes in air temperature
– Called?
• Opening of evening-primrose flowers at dusk
– Called?
• Directional growth response that occurs in
response to an external directional stimulus
• May be positive (towards stimulus) or
negative (away from stimulus)
• Tropism comes from a Greek word ‘tropos’
meaning “to turn” or “to change”
• Plants can alter their growth so they can grow
towards more favourable conditions
(eg. More light, more water etc)
• Must detect where the conditions are better
then alter their growth to "move" in the
appropriate direction
= the directional growth response of a plant in
response to a light stimulus.
• Different parts of a plant exhibit different
reactions to light.
• Stems and shoots exhibit positive phototropism
(grow toward light)
• Most roots exhibit negative phototropism (grow
away from light)
= the directional growth response of a plant in
response to gravity.
• Roots exhibit positive gravitropism (towards)
• Stems and leaves exhibit negative gravitropism.
• Thigmotropism is the growth
response of a plant to physical
contact (touch).
• Plants that cling to physical
structures such as sticks exhibit
positive thigmotropism.
• Directional growth in
response to presence
of water in the soil
• Roots = positive
(grow toward water)
Note: stronger than
• Directional growth in response to a chemical
• eg. Roots can grow towards or away from
chemicals in the soil (copper pipe)
• eg. Growth of pollen tube towards ovary in
flowers (ovary releases chemicals)
An alteration in growth patterns caused by touch
(eg. wind, rain)
Type of tropism? Positive or negative?
One Advantage?
Roots of a seedling grow down ……………………..
Stem of vine winds around a branch………………..
Leaves of pot plant turn toward window………..
Roots of willow grow sideways toward water……
Roots grow away from copper pipes in soil….
Shoots of seedling grow upward in dark lab…
Pollen tube in flower grows toward ovary…..
Tree grows sideways on an exposed mountain…
Control of Plant Growth
• Plant growth is controlled by HORMONES
• These are chemicals produced in one part of
the plant and transported to where they
produce a growth response.
Coleoptile (co – le – op – tile)
is the protective sheath
covering the emerging
shoot in plants such as oats
and grasses.
• A group of hormones that
regulate plant growth
• Indole Acetic Acid, IAA
(the first auxin isolated)
• Causes cell elongation in
Phototropism in Coleoptiles….
• Tip of shoot detects light
stimulus, auxin is produced
• Auxin causes cell elongation
in the stem
If light comes from an angle:
• Auxin moves to shaded side
of stem, cells elongate
• Shoot bends towards light
Auxin moves to shaded side and causes cell
elongation - stem bends towards light source
• More auxin on
the shaded side
• Cells elongate
• Stem bends
toward light
over time
TROPISMS….. You tube clip:
Plant Physiology: Phototropic Response
Experiments with Grass/Oat Coleoptiles
Gravitropism in roots
• In roots, perception of gravity appears to
depend on the settling of specialised
organelles called statoliths in root-cap cells.
• When the plant is turned, within minutes the
statoliths sink toward the source of gravity, to
the side that is down.
• Auxin builds up on the lower side of the root
cap, causing cell elongation and downwards
growth of the root
• Growth is inhibited on the side
of stem being touched
• Cells elongate on the nontouching side due to auxin
• Causes the stem to coil around
the object being touched
• Have a go at all
the questions
Terminology time!
COLEOPTILE = the protective sheath covering tip of the first new shoot from a seed
Darwin studied phototropism in canary grass and oat coleoptiles. The coleoptile is a
hollow sheath of tissue which surrounds the apical axis (stem) of these and other
grasses. Darwin demonstrated that these coleoptiles are phototropic in that they
bend toward a light source. When he covered the tips of the coleoptiles, they were
not phototropic but when he covered the lower portions of the coleoptiles, they
were phototropic. Darwin concluded from these and other experiments that (a)
the tip of the coleoptile is the most photosensitive region; (b) the middle of the
coleoptile is responsible for most of the bending; and (c) an influence which
causes bending is transmitted from the top to the middle of the coleoptile.
Read more: Phototropism - History Of Phototropism Research - Coleoptile,
Darwin, Coleoptiles, Plant, Agar, and Light
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