This document discusses structural adaptations, tropisms, and hormonal control in plants. It covers how plants photosynthesize and use water and nutrients. It describes how plants adapt structurally to factors like light, temperature, water availability, and fire. It discusses tropisms like phototropism, geotropism, and thigmotropism which allow plants to grow toward beneficial stimuli. Finally, it outlines the roles of major plant hormones like auxins, gibberellins, cytokinins, abscisic acid, and ethylene in controlling growth, development, and responses to environmental conditions.

1. Structural Adaptations, Tropisms and Hormonal Control

2.  Plants are photosynthetic organisms that
make their own carbohydrates for energy.
 They need carbon dioxide, light and water for
photosynthesis.
6CO2 + 6H2O → C6H12O6 + 6O2
+ +
6x Carbon Dioxide 6x Water Glucose 6x Oxygen

3.  They also need oxygen for respiration.
Glucose + Oxygen → Carbon Dioxide + Water + Energy
Or as a balanced chemical equation:
+ + +
Energy
Glucose 6x Oxygen 6x Carbon Dioxide 6x Water

4.  They use different ions as nutrients (equivalent to
vitamins and minerals in humans).
 Plants have leaves that contain chloroplasts that
absorb light energy for photosynthesis.
 Stomata on the under-side of the leaves control gas
exchange and water loss. (carbon dioxide moves in,
water and oxygen move out of the leaf)

5.  Temperature is important to plants as it affects
metabolic rate (the rate of chemical reactions in
the plant essential to life processes).
 Metabolic rate controls growth and development.
 The higher the temperature the higher the
metabolic rate- up to a limit.

6.  We will cover plant adaptations to:
 Light
 Temperature
 Water
 Gaseous Exchange
 Support
 Fire
 We will also look at the special case of epiphytes

7. Adaptations to water availability

8.  Plants like other organisms also need to
maintain a constant water balance.
 For plants this is especially important as
water constitutes 90-95% of the living tissue
of plants.
 Plants therefore have specialized
mechanisms to both conserve water and
minimize loss.

9. Most leaves are covered by a
water proof layer called the
cuticle.
Stomata: Stomata’s(mostly on the underside of
the leaf) allow gas exchange; since a lot of water
vapour can be lost through the stomata they
only open for photosynthesis in daylight; at
night they close to reduce loss of water vapour.

10. Open Stomata Closed Stomata
 By opening and closing the stomata regulate the amount
of water loss. Unfortunately 98% of water is lost here.

11.  Transpiration explains how water
moves up the plant against gravity
in tubes made of dead xylem cells
without the use of a pump.
 Water on the surface of spongy and
palisade cells (inside the leaf)
evaporates and then diffuses out of
the leaf. This is called transpiration.
 As more water is lost more is
drawn up through the plant to
replace it.
 This creates a continuous tube
from the leaf, down the stem to
the roots, and acts like a drinking
straw, producing a flow of water
and dissolved minerals from roots
to leaves.

12.  For plants that are exposed to sufficient amounts of
water the opening and closing of stomata is
sufficient to control water balance.
 Many plants however live where water exposure is
low and the challenge is to conserve water and
reduce water loss

13. A rolled leaf  Some plants have hard, thick cuticle
which reduces evaporation of water.
 Some plants have a reduced number of
stomata or hairs on the surface of their
leaves which trap water and increase the
humidity at the surface of the leaf.
 Some plants leaves roll inwards and
therefore the stomata are covered. When
water does evaporate it increases the
humidity around the leaves reducing
future water loss.
 Some plants such as cacti and succulents
store water in their leaves and stems.
Cacti

14.  Mesophytes - Plants in areas with adequate water
 Hydrophytes - Aquatic plants
 Halophytes - Salt-tolerant plants
 Xerophytes - Plants in areas where water is scarce

15.  Mesophytes require an environment that
is neither too wet nor too dry.
 Water lost from stomata is matched by
water gain from the environment
 Under stress (Like winter) these plants
shed their leaves
 Perennials survive unfavourable
conditions by dying down and surviving
underground. Annuals survive as
dormant seeds.
Most plants fall into this category

16.  Hydrophytes are plants
that require a large supply
of water.
 They can grow wholly or
partly submerged in water.
 The stems and leaves have
little to no cuticle (outer
waxy layer of leaf) as they
do not need to conserve
water

17. Mangrove Succulent Plant
 Salt tolerant
 Store water in special tissue
 Tissue has lots of air spaces
 Some can excrete salt though special glands or by
dropping yellowish leaves where salt has been
accumulated.
 Many are succulents (Water retaining plants)

18. All Cacti are xerophytes

19.  Grow in hot, dry environments therefore have
adapted to conserve water and to prevent leaf
temperature from rising too much
 Often these adaptations are of the leaves
Cacti Marram Grass

20. Adaptation How it works Example
thick cuticle stops uncontrolled evaporation
through leaf cells
small leaf surface less surface area for conifer needles, cactus
area evaporation spines
low stomata density smaller surface area for
diffusion
sunken stomata maintains humid air around marram grass, cacti
stomata
stomatal hairs maintains humid air around marram grass, couch
(trichores) stomata grass
rolled leaves maintains humid air around marram grass,
stomata
extensive roots maximise water uptake cacti

22.  Temperature can affect the growth
potential of a plant and plants have
several adaptations designed to
control heat gain.
 Leaves with a smaller surface area
do not absorb as much heat.
4.44 C 35.56 C
Plants with leaves that dangle reduce
shiny leaves reflect light and heat. their exposure to the sun.

23. Effect
Photosynthesis: Increases with temperature to a point.
Respiration: Rapidly increases with temperature.
Transpiration: Increases with temperature.
Flowering: May be partially triggered by temperature.
Sugar storage: Low temperatures reduce energy use and increase
sugar storage.
Dormancy: Warmth, after a period of low temperature, will
break dormancy and the plant will resume active
growth.

24.  Water plants have more difficulty than land plants in obtaining
the light they require for photosynthesis.
About 30% of light striking the surface of
water is reflected.
by 1 m about 60% of the light is absorbed.
by 10 m about 85% of the light is absorbed.
by 150 m about 99% of light has been absorbed

25. Surface Depth Algae  As seen on the previous slide water
does not absorb all lengths of light
equally. Blue and green light is better
able to penetrate water and reach
deeper.
 Algae at surface depths (0-10m) will be
predominately green as it can absorb
Deep water Algae the red and orange light that
penetrates this region.
 As we move deeper however the algae
will turn brown and then red. The
brown and red algae are better able to
absorb blue light.

26.  Water plants have more difficulty than
land plants in exchanging the required
gases.
 These plants may have stomata on
surfaces other than their leaves.
 Mangroves have special aerial roots called
pneumatophores (peg roots) that extend
out of the water. These roots obtain
oxygen for respiration through special
pores located on the root.

27.  The role of a plant’s roots is to anchor the A kelp holdfast
plant to the ground and also absorb water
and nutrients from the soil.
 Water plants may have weaker roots
systems as they rely on the water for
buoyancy and support.
 Water plants in fast moving waters have
holdfasts.

29. Before
After

30.  Wild fires started most often by
lightening are a natural occurrence
and plants have adapted strategies
to survive fires.
 Plants have developed two strategies
which they can either use separately
or in combination.
 Producing a large volumes of seeds.
 Structures and mechanisms for
regeneration Some native plants actually rely on
frequent fires to flower and cause
seeds to sprout. Banksias require
frequent fires to produce seeds.

31. Banksia seed pods  The first strategy is to produce a
large volume of seeds that only
germinate after a fire.
 Advantage: Seeds have access to
increased minerals from the ash in
the soil.
 Disadvantage: If the time between
fires is too long the seeds may not
mature and the next generation may
be lost.

32.  Many trees have thick bark that
protects the internal structure of the
tree. Under this bark are epicormic
buds that sprout quickly after fire.
 Many plants have shouts or roots
called lignotubers underground that
are protected by soil or dead plant
matter during a fire.
 Some plants combine both
epicormic buds and lignotubers to
completely regenerate plants after a
fire.

34.  Epiphytes are unique given that they grow on other
plants and have no contact with the soil.
 The advantage of growing on other trees is that they
have better access to light than they would if they were
located on the ground.

35. Tank Bromeliads  So how to epiphytes obtain
water and nutrients?
 Epiphytes such as mosses absorb
and store water releasing it when
water is scarce.
 Bromeliads have leaves that are
rolled and form funnel like
structures that capture rain
water and plant debris- a source
of nutrients.
 The tank bromeliad above can
hold up to 8 Litres of water!
Bromeliads

36.  Epiphytes are plants which, like a parasite,
grows on a host, but unlike a parasite, takes
no nutrients from the tree itself and relies on
nutrients from the air, falling rain, and the
compost that lies on tree branches.
 Epiphytes do not directly cause damage to
the host plant they are on.

38.  Plants need to respond to stimuli in the
environment. They do so through the use of
plant hormones.
 There are several types of responses that plants
may display in response to certain stimuli.
 These responses may be negative (away from
the stimuli) or positive (towards the stimuli).

39.  We can group plant responses into four
broad groups:
 Taxis
 Tropism
 Nastics
 Nutation

40.  movement of a whole organism in response
to a stimuli; e.g. algae moving towards a light
source (positive phototaxis) or the movement
of algae away from chemicals (negative
chemotaxis)

41.  growth movement in response to an external
stimulus; the direction of the stimulus
determines the direction of plant growth

42.  When a shoot is
illuminated from one side,
an auxin is transported
across to the shaded side.
 Cells on the shaded side
elongate.
 The shoot then is able to
bend towards the light.

43.  In geotropism:
 Roots show positive geotropism
 Stem/shoot show negative geotropism
 There are two different theories for
geotropism:
 redistribution of auxins to the lower side of
root. Causing growth downward.
 the pull of gravity is detected by cells near
the stem or root tip (apex). These cells
contain starch grains that change their
location in the cell if the plant is moved
from a vertical to a horizontal position.

44.  hydrotropisms is defined
as movement towards
water.
 In this case roots show a
positive tropism towards
water sources.

45.  Thigmotropism is a
plant's response and
movement to physical
contact.
 This phenomenon is
clearly illustrated by the
climbing tendrils of some
plants, such as the sweet
Thigmotropism: the hop vine responding pea. The tendrils actually
to contact with the support string.
"feel" the solid object,
which results in the coiling
response

47. Venus fly trap closing to capture an Thigmotropism in response to touch in
insect Mimosa Pudica
 Nastic movements of a plant are rapid movements of
plant organs.

48.  Nutation describes movements
of plant structures that are in
response to internal rather than
external stimuli.
 Slow, upward, helical growth
movements of seedlings have
been caught by time-lapse
photography.
 Seemingly random movements
of climbing plant stems increase
the chance of making contact
with a supporting structure.

49.  Auxins play an importance role in
phototrophism (plants bending towards the
light).
 They cause the shaded side of a stem or
shoot to grow more (elongate) causing the
whole stem or shoot to bend toward a light
source. The higher the concentration of
auxins the greater the elongation and
curvature of the stem.

50.  Auxins are also thought to play a role in geotropism.
Greater amounts of auxins have been found in the
lower side of horizontal organs.
 The evidence is not convincing however and a more
likely explanation is the statolith hypothesis which
states that cells near the stem and root tip detect
gravity. They detect gravity using starch molecules
within the cell that change location when the plant is
moved from a vertical to a horizontal position. This
position shift is thought to active enzymes.

51. The role of hormones

52.  Plants cannot move (they are sessile) when they
are exposed to adverse conditions. For this
reason they need to take advantage of
favourable conditions and often events in their
life cycle are controlled to coincide with
favourable external conditions.
 Events such as germination, growth, flowering,
seed setting and budding are often signalled by
changes in the environment around them.

53.  Therefore there exists in plants just as other
organisms a system that responds to the external
environment.
 Plants have hormones, just as animals do though
they are not as complex and numerous. These
hormones are known collectively as
phytohormones (phyto = plant).
 Unlike in animals where hormones are produced by
glands, any plant tissue is capable of producing
hormones.

54.  There are five groups of plant hormones that
together control the growth and
development of the plant. These hormones
are produced in response to the environment
external to the plant.

55.  The effect of auxins on a plant are widespread and
they often work with other hormones.
 Auxins influences the length of a plant cell, ripening
of fruit, falling of leaves and growth of shoot tips.
They inhibit the growth of lateral buds and promote
root growth from cut stems.
 Auxins increase the circumference of a stem or
trunk.

56.  Gibberellins promote
cell division and
elongation in plant
shoots.
 They also extend
internodes and can
raise flower heads.

57.  Cytokinins stimulate cell division/replication.
 They tend to be concentrated in the starchy
material in seeds (endosperm) and in young fruit.

58.  Abscisic acids
promote the closure of
stomata during times
of water stress.
 They also stimulate
dormancy in seeds and
buds during
unfavourable
conditions.

59.  Ethylene ripens fruit by stimulating the conversion of starch
to sugar.
 It also stimulates colour change and softening of fruit tissue.
Before After

60.  Phytochrome is a light receptor sensitive to red
light found in a plants leaves.
 It is involved in seed germination, stem
elongation, expansion of leaves, growth of
lateral roots and leaf fall.
 When exposed to light, phytochrome causes the
above events to occur.

61.  Photoperiodism is the reaction of plants to the
length of daylight.
 Phytochrome plays a role in regulating the cycles
of flowering plants in response to the length of
sunlight in a day.
 The length of day light and darkness controls
flowering. Different plants will flower in response
to long days or short days.

62.  Different plants react differently to the photoperiod; some
plants are described as ‘short-day’ plants and others as
‘long-day’ plants.
The example to the left is
of a short-day plant
flowering. If a dark period
is interrupted by a light
flash, no flowering occurs.

63.  The length of dark is a trigger to flowering