TROPISMS

the movement is always a growth movement

are responses by part of a plant towards or away from a stimulus coming from one direction

Tropic responses can be:

Negative –if growth is away from the stimulus

Positive –if growth is towardsthe stimulus

Gravity

Growth

Growth

Phototropism

is a growth response to light

Light

Geotropism

is a growth response to gravity

A shoot is:POSITIVELY

PHOTOTROPIC NEGATIVELY GEOTROPIC

Advantages of a shoot growing: Towards light:

To trap more light for photosynthesis

Away from gravity:

shoot of a germinating seed grows out of the soil

more chances for pollination & seed dispersal

A root is: POSITIVELY GEOTROPIC

NEGATIVELY PHOTOTROPIC

I am growing towards gravity, but where is the

water????

Advantages of a root growing towards gravity:

To anchor the plant

To search for water

To ensure that the root of a germinating seed always grows downwards, whatever, the position of the seed in the soil

Study the pictures below and then complete the table by putting a plus (+) if the shoot or root grows towards the stimulus and a minus (-) if it grows away from it.

StimulusLight Gravity

Shoot

Root

Study the pictures below and then complete the table by putting a plus (+) if the shoot or root grows towards the stimulus and a minus (-) if it grows away from it.

StimulusLight Gravity

Shoot + -Root - +

ANSWER THE FOLLOWING

Study the diagrams. What would you expect to happen and why.

The shoot with the covered tip grows (straight up / towards light) because it (can sense light / cannot sense light).

Study the diagrams. What would you expect to happen and why.

The shoot with the covered tip grows (straight up / towards light) because it (can sense light / cannot sense light).

The shoot with the exposed tip grows (straight up / towards light) because it (can sense light / cannot sense light).

The shoot with the exposed tip grows (straight up / towards light) because it (can sense light / cannot sense light).

Look at the diagrams below. What does this experiment tell you about the tip of the shoot and the part it plays in growth?

Tip produces chemicals for growth.

Look at the diagrams below. How would you explain the results of this experiment?

Bending resulted due to unequal amount of chemical . Light affects distribution of chemical.

AUXINS

Auxins

• are plant hormones, or growth regulating chemicals produced in the root and shoot tips that affect growth

What is the effect of auxins on plant cells?

Cells absorb water and elongate.

Three regions of a root tip

Cell division region

Cell differentiation region

Cell elongation region

[Cells become specialised]

Which region does auxin affect?

Cell division region

(root tip that makes auxin)

Cell differentiation region (cells change their shape; no effect by auxin)

Cell elongation region (auxin makes cells absorb water)

Explain why:

markings have moved away from each other ONLY in the lower region.

Cell elongation region

A shoot growing in even illumination grows straight up. Why?

auxin

Cells on both sides of the stem elongate equally.

A shoot growing in one-sided illumination.

A shoot growing in one-sided illumination grows towards light. How is it possible?

Cells on the two sides of the stem

GROW UNEQUALLY!!

Remember: UNEQUAL growth of stem sides results in BENDING of stem

Lighted side:normal size of cells

Shaded side of stem: cells elongate

Light

Describe how a shoot bends towards one-sided illumination.

Auxins are produced at the ___________ tip and diffuse down the stem.More auxin collects on the ____________ side of the stem.Auxins cause the cells to absorb water and so elongate in the cell elongation region.Thus the ___________ side grows more than the lighted one.This results in bending of the stem towards light.

shoot

shaded

shaded

Auxins sometimes stop growth

Auxin passes down the stem

So the plant grows tall and straight.

1.Apical bud produces auxin.

2.

Lateral buds do not grow into side branches.

3.

If the top is cut off from the plant:

flow of auxin stops

side branches will grow out

Sometimes gardeners cut the tops off plants to make them more bushy

QUESTION: MAY, 2009

Give a biological explanation for each of the following statements:

cutting off the tip of a shoot will stop the shoot from getting taller but it will increase bushiness; (3)Cutting off the tip of a shoot removes the source of auxin.

Lack of auxin stimulates lateral buds to develop and side branches grow.

EXPERIMENTS TO SHOW:

1. Geotropism in a shoot

2. Geotropism in pea radicles

3. Phototropism in oat coleoptiles

4. Phototropism in a shoot

All make use of an apparatus

called:

clinostatA motor that rotates

slowly – 4 times per hour

A clinostat

makes factors uniform (evenly distributed)

for control experiments

contains a cork disc which can be set to rotate in a vertical or horizontal plane by a motor

AIM: To show geotropism in a plant shoot.

APPARATUS:

CONTROL

Give a precaution for this experiment.Even illumination.

The apparatus was left for 3 days.

Why did the shoot attached to the clinostat grow horizontally ?

As shoot rotated, gravity acted on all sides.

No unilateral stimulus to respond to.

AIM: To show geotropism in pea radicles.

APPARATUS:

The apparatus was left for 3 days.

QUESTION: MAY, 2005

A biology student wanted to test whether the position in which seeds are placed in the soil affects the direction of shoot and root growth.

Design a simple experiment which the student can use to carry out this investigation.(5)

Seeds of the same type are placed at different orientations in a single container.

A similar container is set up as a replicate.

Left in darkness for a few days.

Radicles are observed.

QUESTION: MAY, 2005

When growing plants from cuttings, farmers usually dip the cutting into Rooting Hormonepowder before planting it in the soil. Briefly explain how this increases the chances of successful plant growth. (2)

Roots are stimulated to form. Thus plant can absorb more water and ions for growth due to more root hairs present.

AIM: To show phototropism in oat coleoptiles.

APPARATUS:

METHOD:1. Thirty oat grains were soaked in water for 1 day.2. Ten grains were placed in each of three small pots of

compost and left to germinate. 3. When grown to a height of 1cm, they were treated as

follows: One set was covered with a large box, excluding

light; Another set was covered with a box with a slit at

the side, allowing one-sided illumination; The control was left in the light.

4. The pots were left at room temperature for a few days.

RESULT: In the dark: coleoptiles were yellow, straight and

very long. In one-sided illumination: coleoptiles bent towards

the light. In the light: coleoptiles were green, straight and

shorter than those in the dark.

CONCLUSION: The coleoptiles responded to the stimulus of one-

sided illumination by growing towards it. Coleoptiles are positively phototropic.

Light is needed for chlorophyll to form.

AIM: To show phototropism in a shoot.

APPARATUS:

unilateral light

unilateral light

light-proof box

clinostat

A B

unilateral light

unilateral light

light-proof box

clinostat

The shoots in pot A respond by growing towards the light source. The shoot in pot B do not show any

curvature but grow vertically upwards...

What has happened to the shoots of plants A and B ?

A B

unilateral light

unilateral light

light-proof box

clinostat

A B

What is the effect of the clinostat on the shoot of plant B ?

The revolving clinostat cancel out the effect of unilateral light on the shoot of plant B.

VIEW THE ACTUAL EXPERIMENT IN THE NEXT

3 SLIDES

Question: MAY, 20065. The experiment shows the effect of a stimulus on the growth of a root and a shoot. The seedlings were kept in the dark throughout the experiment.

a) Continue the diagram to show the direction of growth of the shoot and root in:

i) the stationaryii) rotating clinostat. (4)

b) What type of stimulus response is exhibited by the seedlings? (1)

Geotropism.

c) Why were the seedlings kept in the dark? (2)

Any changes observed would be due to gravity.

d) Name the substance produced by the plant which brings about the growth changes. (1)

Auxin

e) This experiment was investigated in an orbiting spacecraft where weightlessness is observed. Predict the results obtained in this situation. (1)

Grow in all directions.

Under which condition (light/dark) was each seedling of bean grown?

Grown in the light

Grown in the dark

What is the advantage of a longer stem in seedlings

grown in the dark?

light

dark

Four differences between a seedling growing in the light & another in the dark.

In the light In the dark1. Short, thick stem long, thin stem2. Green shoot yellow shoot

Four differences between a seedling growing in the light & another in the dark.

In the light In the dark3. Broad leaves small leaves4. Short internode long internode

QUESTION: MAY, 2009

Give a biological explanation for each of the following statements:

the internodes of a shoot growing in the dark are longer than the internodes of a shoot growing in the light; (3)

Differences in the response shown by:

A shoot in unilateral light A hand touching a hot object

Differences in the response shown by:

A shoot in unilateral light A hand touching a hot object

1. Response is positive. 1. Response is negative2. Slow response. 2. Rapid response3. Response involves

growth.3. No growth involved

4. Response does not involve muscles and nerves.

4. Response involves muscles and nerves.

EXPERIMENTS USING OAT COLEOPTILES

A Coleoptile is a : hollow, cylindrical sheath that surrounds the

primary leaf of a germinating monocot seed

Coleoptile First leaf

no growth grows

dark box

few days later

Conclusion :The tip is responsible for growth

cut & replaced tip

decapitated

Experiments

on Oat Coleoptiles

unilateral

light

few days later

black covers

Conclusion :the tip is the place responsible for bending

Experiments

on Oat Coleoptiles

Experiments

on Oat Coleoptiles

few days later

dark box

agar plate

Conclusion :the head must have produced a diffusible chemical substance which stimulates growth

Experiments

on Oat Coleoptiles

bending directions

Explanation of results :The side with the tip has a higher concentration of that particular chemical, the growth rate is faster than the other side

the tip is put aside

mica

plate

Can the explanation still work ?

bending directions