CONTROL AND CO-ORDINATION
(PART 4)
 Although plants lack nerves and muscles, they too have well-coordinated and controlled movements.
In fact, movements are the basic features of all living organisms.
 Plants do not have nervous system so, they use chemicals for control and coordination. These
chemicals are known as plant hormones or phytohormones.
 Phytohormones are the chemical compounds which are released by stimulated cells and are diffused
around the cell.
Different types of hormones present in plants are:
1. Auxins
2. Gibberellins
Growth promoters
3. Cytokinins
4. Abscisic acid
Growth retarder
 Auxins cause growth in plants especially in the roots. They are responsible for phototropism, apical
dominance, flowering and help fruits in attaining large size.
 Gibberellins cause stem elongation and make genetically dwarf plant tall. It also induce onset of
germination of seeds.
 Cytokinins promote cell division and they are present in roots, developing fruits and in the embryo.
They work in association with auxin.
 Abscisic acid is a plant hormone which functions in plants as a growth retarder or growth inhibitor.
It is responsible for wilting of leaves and inducing dormancy of seeds and buds.
Auxin is involved in phototropism. Auxin helps in bending the shoot towards the source of light
which occurs due to the diffusion of auxin (synthesized at the shoot tip) towards the shady side of the
shoot which stimulates the cells to grow abundantly.
Movement in plants :
Plant shows two types of movements- one dependent on growth (tropic movements) and the
other independent of growth (nastic movements).
Tropic Movements: Movements due to growth

A growth movement of a plant part in response to an external stimulus in which the direction
of a stimulus determines the direction of response is called tropism. Thus tropism is a
directional movement of the part of a plant caused due to its growth.

The tropic movements can be either towards the stimulus or away from it e.g. the shoot of a
growing plant bends towards the light, while roots of a plant move away from light
Types of tropism:
1. Phototropism: The movement of a plant part in response to light is called phototropism. In
general, the stems usually show positive phototropism, while roots show negative
phototropism.
2. Geotropism: The movement of a plant part in response to gravity is called geotropism. The roots
are positively geotropic whereas the stems are called as the negatively geotropic as they grow
away from the centre of gravity.
3. Chemotropism: The movement of a plant part in response to chemicals is called chemotropism.
Example- the growth of a pollen tube down the style during fertilization
5. Hydrotropism: The movement of a plant part in response to water is called hydrotropism. Roots
show the positive hydrotropic response, as they move and grow towards the water.
5. Thigmotropism: The directional growth movement of a plant part in response to the touch of an object
is called thigmotropism. These type of movements are usually seen in tendrils and twiners
Nastic movements: Independent of growth.

When the direction of response in a plant is not determined by the direction of stimulus, it
is called nastic movement. Nastic movement is not the directional movement of the plant
part.

In nastic movement, whatever be the direction of stimulus, all the parts of the plant
equally move in the same direction. This type of movement is generally seen in leaves,
flower petals etc.
Types of nastic movements:
(1) Seismonastic movements These movements are brought about by mechanical stimuli such
as contact with a foreign body, fast wind and raindrops etc. movements in the leaves of sensitive
plant (Mimosa pudica) caused by ‘touch’.
(2) Photonastic movements These movements are induced by fluctuations in the intensity of
light. Dandelion and moonflower are examples of photonasty.