COORDINATION AND MOVEMENTS IN PLANTS IN PLANTS
Types of movements
a.
Nastic movement: is the movement of plant organ
in response to an external no-directional stimuli, ie. Movement which is
independent of the direction of the stimuli. Such movement may be the result of
changes in turgor pressure in certain cells or growth curvature. E.g. the
folding up and drooping of leaves in mimosa pudica when touched.
b.
Tropic movement: Is the directional growth
movement of a plant organ in response to an external stimulus.
-It is Positive
tropism when it is towards the stimulus.
-It is Negative
tropism when it is away from the stimulus.
c.
Tactic Movement: Is a movement of organism or
freely motile part of it, in response to an external directional stimuli.
Towards the stimuli is positive taxis while away from the stimuli is negative
taxis. Examples:- Chlamydomonas swims towards the light to photosynthesise, or,
A male gamete moves towards an egg for fertilisation.
Photoperiodism. Is a flowering
response in plants relative to the length of the day and night.
Groups of movement
1. Growth Movement: Due to unequal
permanent growth of two sides of plant organ (roots and stems).
a.
Autonomic growth. Movements which are self
controlled ie. Growth meristematic regions (tips of stems and roots).
b.
Paratonic growth. Movements induced by external
stimuli. This includes tropic and nastic moventent.
2. Turgor movement: Plant movements
which are revisable and involve changes in the turgor pressure of specific
cell, e.g. Mimosa Pudica and Oxalis. Such movements orient plant leaves and
flowers to protect themselves from drought, cold and other unfavourable
environment.
Plant Hormones (Phytohormones)
Hormones are essential for
normal development of plants.
1. Gibberellins.
Are mixture of chemical
compounds like Gibberellic acid that causes elongation in plants.
-Stimulate rapid growth of
dwarf variety by increasing the length of internode.
-Break seed dormancy.
-Induces parthenocapy.
2. Cytokinins.
They promote growth in the
presence of auxin.
-They promote cell division by
inducing growth of roots, leaves, callus tissues and repair of wounds.
3.
Ethylene
-It speeds up ripening of
fruits such as citrus
4.
Abscisic Acid
-Regulate fruit drop at the
end of the season.
5.
Phytochromes
-
Are pale blue-green compounds consisting pigments
which absorb light energy.
a.
Pr., which absorb red light at a wavelength of 665 mm.
b.
Pfr., which absorb far-red light at a wavelength of
725 mm..
When Pr absorbs red light it
is rapidly converted into Pfr and when Pfr absorbs far-red light it is rapidly
converted into Pr.
The effects of the two
Phytochromes (Pr and Pfr.)
-Elongation of stem is
stimulated by Pfr but inhibited by Pr.
-Leaf expansion is stimulated
by Pr but inhibited by Pfr.
-Lateral roots growth is
stimulated by Pfr but inhibited by Pr.
-Seed germination is
stimulated by Pr but inhibited by Pfr.
6. Florigen
When plant is exposed to
light, phytochrome absorbs light energy and P725 accumulates and initiates
flowering hormone called Florigen. Florigen is transported to
the stem apices to promote flowering.
7. Auxin
Are group of plant hormnoe
originaly isolated from human urine.
IAA=Indole acetic Acid is an
auxin from amino acid tryptophan in root, shoot tips, young leaves and seeds.
Functions of Auxin
-Increase cell wall plasticity
by loosing bond between the cellulose fibres, hence decreases the wall pressure
and allowing more water to enter.
-Important for normal growth
and tropism.
-Accelerate cell division,
formation of callous tissue over the wound.
-Accelerate development of
fruits, stimulate root initiation, inhibit lateral bud development and affect
flowering
Role of Auxin to plant life,
1.
Root Formation especially in species grown
by cuttings. Examples of rooting Auxin: NAA=Naphthalene acetic acid
IBA=Indole butyric acid
2.
Storage: NAC: Increase the period of
dormancy in tubers and bulbs so that they can be stored for prolonged period.
3.
Flower formation. Spray of NAA and 2,4D causes
flowering in pineapple.
4.
Ovary development: Development of ovary into
fruit without fertilisation.
5.
Parthenocarpy: Seed less fruits. Naturally is present in
grapes, bananas and some oranges. IAA and NAA application induces
parthenocarpy.
6.
Apical dominance: The presence of apical bud
inhibits growth of lateral buds. Auxin play an important role in apical
dominance.
7.
Prevention of fruit drop: The pre-harvest
fruit drop can be prevented by application of the auxin 2,4D.
8.
Weed killer: In higher proportion auxin can
interfere with normal plant metabolism and can cause their death. So auxin are used
as herbicides or selective weed killer, e.g. 2,4D is used to destroy broad
leaves weeds.
Effect of auxin concentration
on growth
-Amount of auxin present in
plant usually is tiny. Comparatively higher concentrations of auxin stimulate
growth in shoot while relatively lower concentration stimulate growth in roots,
e.g. concentration of IAA which stimulate shoot growth, usually inhibit root
growth.
Tropic Movement
Are caused by stimuli like
light, gravity, water, touch, water current, air, chemicals and
temperature. The bending of plant organ
away or towards stimuli is caused by unequal distribution of auxin in the
elongation zone.
Types of Tropism
1.
Gravitropism = Geotropism: is the growth
movement of plant parts in response to the direction of force of gravity.
-Roots = Moves towards
gravity, hence positive gravitropic movement.
-shoots = moves away from
gravity, hence negative gravitropic.
2.
Phototropism = Is the growth movement of
plant parts in response to unilateral source of light.
-Shoots grow towards light,
hence positive phototropism.
-Roots grow away from light,
hence negative phototropism. However, most roots are insensitive to light
(neutral).
The percentage of auxin
increases in the side away from light, hence it grows more and the shoot bends
towards the light.
3.
Hydrotropism = Is the growth movement of
plant parts in response to unilateral source of water or moisture.
-Roots grow towards water,
hence positive hydrotropism.
-Shoots grow, either away
(negative) or insensitive (neutral)
In roots the hydrotropic
response overrides the gravitropic response.
4.
Thigmotropism-Haptotropism: Thigma is a Greek word meaning touch.
Thigmotropism is the response of
plant to touch or the stimuli of contact or function with some objects. Is the
result of rapid growth response to touch, e.g.
-Tendrils which curl around
and cling to stem or other objects, ie. Gloriosa, Passiflora.
-Twining plants that coil
around other objects.
Auxin and ethylene can
induce coiling in the Tendril in the absence of any contact stimulus.
5.
Chemotropism = is a growth movement of
plant parts in response to a unilateral source of chemicals, e.g. pollen tubes
grow towards the stigma.
6.
Thermotropism = Is a growth movement of
plant parts in response to heat, e.g. sunflower.
7.
Rheotropism = Growth movements of plant
parts in response to unilateral source of air current.
Usefulness of Tropic movement
-It help plant to survive,
e.g. a seed planted upside down; the radicle will still grow down ward (+ve
gravitropic) and the plumule will grow upward (-ve gravitropic and +ve
phototropic).
-Phototropism help plant to
maximise light absorption hence increase output of photosynthesis process.
-Thigmotropism help plants
without woody stem to obtain mechanical support from hard objects or from woody
stems.
-Thigmotropism help the
climbing plants to search for sun light.
Klinostat:
Is an instrument used for
experimental investigation of geotropism and phototropism in seedlings and cut
shoots.
Nastic movements
Is the movement of plant organ
or parts in response to an external non-directional stimuli, e.g. folding of
leaves due to heat or touch.
1.
Chemonasty = Movement in response to
chemical stimuli, e.g. bending of long tentacles on sundew leaves in response
to presence of organic nitrogenous compound, ie. Insect to be digested.
2.
Haptonasty = movement in response to
contact, e.g. leaf hair of insectivorous plant, Venus's fly-trap when touched
they close as a result of rapid loss of water from the cell along the mid rib.
3.
Hydronasty = movement in response to
changes in atmospheric humidity, e.g. flowers of dandelion open and close in
response to such changes.
4.
Nyctinasty (Thermonasty) =movement in
response to changes in temperature. It
is a thermostatic movement, therefore is also known as thermonasty, e.g. Tulip
flowers open and close as temperature increase and decrease respectively.
5.
Photonasty =Movement of plant parts in
response to light intensity, e.g. some flowers open during daylight and close
at night.
6. Seismonasty = Is a plant
movement in response to the stimulus of shock or vibration, e.g. Mimosa Pudica;
fold its leaflets and lowers its leaves when shaken.
Tactic Movement
Taxis is a movement of
organism or freely motile part of it, in response to an external directional
stimuli. Towards the stimuli is positive taxis while away from the stimuli is
negative taxis. Examples:-
-Chlamydomonas swims towards
the light to photosynthesize.
-A male gamete moves towards
an egg for fertilisation.
Types of Tactic Movement.
1.
Phototaxis = In response to light.
2.
Chemotaxis = In response to chemicals.
3.
Aerotaxis = In response to air or oxygen.
4.
Osmotaxis = In response to osmotic pressure
5.
Rheotaxis = In response to direction of flow of water
e.g. some fish.
Photoperiodism.
Is a flowering response in
plants relative to the length of the day and night. When plant is exposed to
light, phytochrome absorbs light energy and P725 accumulates and initiates
flowering hormone called florigen. Florigen is transported to
the stem apices to promote flowering.
Groups of plants in relation
to photoperiodism.
a.
Short-day plants, they require short
illumination but longer night periods to flower, e.g. chrysanthemum and
poinsettias.
b.
Long-day plants, they require longer
day-length illumination but shorter night periods to flower, e.g. wheat and
lettuce.
c.
Day-neutral plants, are plants that flower
irrespective of day-length or night periods, e.g. cotton and tomatoes
Supportive cells and tissues in plant
For plant to maintain its
erect posture and to hold delicate organs like leaves and flowers to position
where they can efficiently fulfil their functions, plants need supportive
tissues. Such tissues also enable plants to withstand external forces like
strong winds and animals that climb on them.
- Parenchyma: Consists of unspecialized cells, spherical in shape and have thin cellulose wall. They fill spaces between more specialized cells in many parts of the plant. They are major component of the cortex and pith in the stem.
When
turgid they provide main support in herbaceous plants like tomatoes and Irish
potatoes. But when the plants lose water they become flaccid hence wilting.
- Collenchyma: Consist of longitudinally elongated cell whose cell wall are unevenly thickened by deposition f additional cellulose and pectic compounds. They retain their protoplasm at maturity. They are found just beneath the epidermis in young parts of the stem and in the midrib of leaf veins. They are the first mechanical tissues to be formed at the tips of young stem.
- Sclerenchyama fibres: Found just outside the phloem of the vascular bundle. They are long and slender cells with tapering ends. Their cell walls are very thick and heavily lignified. They lose their protoplasm at maturity and die. Function is support and protection of the inner delicate tissues.
- Tracheids: Made of long tapering cells that lose their protoplasm at maturity. Mostly found in gymnosperm like pine. Function is support and water conduction. Their cell walls are highly lignified (for support) and have pits (for water to pass from cell to cell) on their sides.
- Xylem vessels: Found in all angiosperm. Located in vascular bundles. Walls are heavily lignified and thickened. Like tracheids they serve a dual purpose of support and water conduction. Their cells are longitudinally elongated and the end walls are perforated. At maturity they lose their protoplasm and die. Cells are arranged end to end forming a continuous tube from the roots to the leaves.
In
wood plants xylem occupies most of the stem beneath the bark. This forms wood
which confers great strength to stems. As perennials grow year to year, more
wood is added increasing the diameter of the stem. This serves to support the
extra weight of the new tissues.
- Wood fibres: These are long tapering cells whose walls are greatly thickened and heavily lignified. Many fibres overlap thus enhancing their supportive function. They are found together with xylem vessels in vascular bundles forming the wood of perennial plants.
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