7. CONTROL AND COORDINATION
COORDINATION
IN PLANTS
Plants have no nervous system or muscles. But they can respond to stimuli.
Like animals, plants show 2 types
of movement:
o Dependent on
growth: E.g. When a seed germinates, root goes down and stem comes up.
o Independent of
growth: E.g. When we touch the leaves of a chhui-mui (the ‘sensitive’ or
‘touch-me-not’ of Mimosa family), they quickly fold up and droop.
Immediate Response to
Stimulus
In sensitive plant, movement
happens at a point different from the point of touch. It means information about
touch is communicated. Plants use electrical-chemical means to conduct information
from cell to cell.
In animals, some muscle proteins
help to change the shape of cells. But plant cells change the shape (swelling
or shrinking) by changing amount of water in them.
Movement Due to Growth
Some plants (e.g. pea) have tendrils
to climb up or fence. They are sensitive to touch.
When tendrils contact with a
support, the touching part does not grow as rapidly as the part away from
object. So, tendril circles around and clings to the object.
Plants respond to stimuli
slowly by growing in a direction. Because of the directional growth, it appears
as if the plant is moving.
Environmental triggers such as
light, gravity etc. change the directions of plant growth. They are called tropic
(directional) movements. These can be towards the stimulus (positive), or
away from it (negative).
Types of tropic
movements
Phototropism:
It is the tropic movement in response to light.
Shoots respond by
bending towards light (positive).
Roots respond by
bending away from light (negative).
It can be proved by the following activity.
- Fill a conical flask with water and cover its neck with a wire mesh having 2-3 freshly germinated bean seeds.
- Keep the flask in a cardboard such that its open side faces light coming from a window.
- After 2 or 3 days, it is noticed that the shoots bend towards light and roots away from light.
- Now turn the flask so that shoots are away from light and roots towards light. Leave it for a few days.
- The old parts of the shoot and root have no noticeable change in direction. But new growth parts show change in direction. i.e., shoot bends towards light and roots bend away from it.
Geotropism:
Movement in response to the
pull of earth or gravity.
Roots always grow downwards (positive).
Shoots grow upwards and away
from earth (negative).
Hydrotropism:
The movement of a plant toward
or away from water.
E.g. Roots bend towards the moist
soil.
Chemotropism:
The movement of plant towards
or away from chemicals.
E.g. growth of pollen tubes towards
ovules.
Controlled movements can be
either slow or fast. E.g.
o Sensitive plant quickly
moves in response to touch.
o Sunflowers
slowly move in response to day or night.
o Growth-related
movements of plants are slower.
In animal bodies also have controlled
directions to growth. E.g. growth of arms & fingers.
For fast responses to stimuli,
information must be transferred very quickly. Electrical impulses are an
excellent means for this. But it has some limitations:
o
They reach only the cells having connection with nervous
tissue.
o
Once an impulse is generated and transmitted, the cell
will take some time to reset its mechanism to generate a new impulse. So, cells
cannot continually create and transmit electrical impulses.
So most multicellular
organisms use chemical (hormone) communication between cells.
In this, stimulated cells
release a hormone and it diffuses around the original cell. Other cells
detect hormone using special molecules on their surfaces. Then they recognise
information and transmit it.
This is slower process but can
reach all cells and can be done steadily and persistently.
Plant hormones can coordinate
growth, development and responses to the environment. They are synthesised at some
places and diffuse to the area of action.
Some Plant
Hormones
o Auxin: Regulates growth in plants. When plants detect light, auxin, synthesised at
the shoot tip, helps the cells to grow longer. When light is coming from one
side of the plant, auxin diffuses towards the shady side of the shoot. As a
result, the cells on the shady side grow longer. Thus, the plant bends towards
light.
o Gibberellins: Help in the
growth of the stem.
o Cytokinins: Promote cell
division. They are present in greater amount in areas of rapid cell division,
such as fruits and seeds.
The above hormones help in promoting growth.
o Abscisic acid: Inhibits growth. Causes wilting of leaves.
Select Your Next Topic 👇
👉 Part 1: Animals- Nervous System
👉 Part 2: Coordination in Plants
👉 Part 3: Hormones in Animals
