A directional growth movement made by a part of a stationary plant response to unilateral stimulus.

A directional growth movement

made by a part of a stationary plant response to

unilateral stimulus

Types of TropismTypes of Tropism

shoot

root

hydrotropism

no response

positive

geotropism

negative

positive

stimulus

tropism

response

watergravity unilateral lightphototropism

positive

negative

Importance of GeotropismImportance of Geotropism

provide firm anchorage for roots

ensure the plant can get adequate supply of water & mineral salts

To show To show Geotropism in Geotropism in

SeedlingsSeedlings

What has happened to the radicles and plumules ?Ans: The radicles grow downwards, while the plu

mules grow upwards.

in complete darkness


P

Q R

P

Q R

plumule

radicle

after 2 days

(a) Beginning of experiment

(b) End of experiment

Why is it necessary to leave the apparatus in darkness ?Ans: To avoid any stimulus from light.



P

Q R

P

Q R

plumule

radicle

after 2 days



Do the radicles show positive geotropism or negative geotropism ?Ans: Positive geotropism.



P

Q R

P

Q R

plumule

radicle

after 2 days



What advantage do the seedlings have by showing such geotropism ?Ans: Roots grow downwards deep into the soil to p

rovide a firm anchorage underground and absorb water and mineral salts in the soil.



P

Q R

P

Q R

plumule

radicle

after 2 days



ClinostatClinostat for control experiments contains a cork disc which can be set to

rotate in a vertical or horizontal plane by a motor

to make factors uniform (evenly distributed)

To show the effect of GrTo show the effect of Gravity by using a Clinostaavity by using a Clinosta

tt

What is the purpose of marking the radicles at equal intervals ?Ans: To find out which region of the radicle grows f

aster.

plastic cover lined with moist blotting paper

X

Y


seedling in X

seedling in Y

Does the direction of growth of the seedlings in clinostats X and Y change ?Ans: The radicle of the seedlings in clinostat X ben

ds downwards in response to the pull of gravity …


X

Y


seedling in X

seedling in Y

Does the direction of growth of the seedlings in clinostats X and Y change ?Ans: since every side of the radicle of the seedling i

n clinostat Y is acted on by gravity, the radicle will grow straight horizontally.


X

Y


seedling in X

seedling in Y

What is the purpose of setting up clinostat Y ?Ans: Serve as a control for comparison.


X

Y


seedling in X

seedling in Y

Importance of Importance of PhototropismPhototropism

enables leaves to be in a position to receive as much sunlight as possible for photosynthesis

To show To show Phototropism in Phototropism in

ShootsShoots

What has happened to the shoots of plants A, B and C ?Ans: The shoots in pot A respond by growing towar

ds the light source. The shoot in pot B do not show any curvature but grow vertically upwards...

unilateral light

unilateral light


A B C

light-proof box clinostat

What has happened to the shoots of plants A, B and C ?Ans: the shoots in pot C grow vertically upwards wit

h slender and longer nodes, and carries smaller leaves which are yellow in colour.

unilateral light

unilateral light


A B C


What is the effect of the clinostat on the shoot of plant B ?Ans: The revolving clinostat cancel out the effect of

unilateral light on the shoot of plant B.

unilateral light

unilateral light


A B C


What is the significance of phototropism to plants ?Ans: This tropism ensures that a plant gets enough l

ight energy for photosynthesis.

unilateral light

unilateral light


A B C


Importance of Importance of HydrotropismHydrotropism

ensures that a plant gets enough water as a raw material for photosynthesis & enough mineral salts to maintain normal growth

no growth

grows

dark box

few days later

Conclusions :The tip is responsible for growth

cut & replaced tip

decapitated

Experiments Experiments on Oat Coleoptileson Oat Coleoptiles

unilaterallight

few days later

black covers

Conclusion :the tip is the place responsible for bending



few days later

dark box

agar plate

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


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

bending directions

micaplate

Can the explanation still work ?

AuxinsAuxinscommonest plant hormones

regulate growth & other physiological processes in plants

produced by root tip / shoot tip they affect the elongation region of the tip

Effect on Plant Effect on Plant PartsParts

positivegrowth

negativegrowth root

shoot

auxinconcentration

Growth ofplant

high conc. low conc. auxin concentration

plant growth

shoot

root

positive no effect

negative positive

highconc.low

conc.

Phototropism in Phototropism in ShootShoot

The shoot tip produces auxins which diffuse downward

The auxins diffuse to the shaded side due to the presence of unilateral light

High auxin concentration stimulates growth in shoot

The shaded side grows faster than the lighted side

The shoot bends towards the light

Geotropism in RootGeotropism in Root The root tip produces auxins which diffuse to th

e elongation region The auxins diffuse to the lower side due to gravit

y

High auxin concentration inhibits growth in root The lower side grows slower than the upper side

The root bends downward

~~ ENDEND

~~

A directional growth movement made by a part of a stationary plant response to unilateral stimulus.

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