Coordination And Response In Plants Flashcards
Do plants show the MRS H GREN?
Plants are living organisms, and therefore show all 8 of the characteristics of living things including sensitivity.
How do plants respond to stimuli?
By growing. This is known as tropism
Definition of tropism
The growth response of a plant to a stimulus
What are the two main tropisms?
Phototropism and Geotropism
What is phototropism?
Positive and negative phototropism
Shoots and roots
It is the growth by plants in response to the direction of light.
Growth towards light is called positive phototropism
Growth away from the light is called negative phototropism
Shoots are shown by positive phototropism
Roots are shown by negative phototropism
What is geotropism?
Positive and negative geotropism
Shoots and roots
Geotropism is the growth by plants in response to the pull of gravity
The response of growth in the directions of towards the force of gravity is called positive geotropism.
The response of growth away from the force of gravity is called negative geotropism
Shoots are shown by negative geotropism
Roots are shown by positive geotropism
What do these responses do to help plants?
These responses help plants to produce leaves where there is the most light for photosynthesis and help roots to grow in the right direction to find the water that the plant needs.
How are tropisms controlled? Where is this hormone made?
Tropisms are controlled by a plant hormone called auxin.
It is made in the tips of shoots and roots but then diffuse away from the tips.
Response to auxin by shoots
In shoots, auxin causes cells to elongate (grow longer) so the shoots get longer. The number of cells stays the same, but those cells affected by auxin elongate.
Auxin moves through the shoot according to the position of the light.
When all sides of the shoot receive equal amounts of light, the auxin diffuses equally down the shoot. All cells are equally stimulated to elongate. Therefore, it would grow straight up.
However, when the light comes from one side only, the auxin diffuses away from the light towards the shaded side of the shoot. The cells on the shaded side, therefore receive more auxin than those on the light side. The shaded cells, therefore, elongate more than the cells in the light and THE SHOOT CURVES TOWARDS THE LIGHT.
This is called positive phototropism
Response to auxin by roots
In roots, auxin has the opposite effect, it reduces cell elongation.
In roots, auxin moves due to the effect of gravity and accumulates at the lower side of the root.
If the root is pointing straight down, all sides of the root receive the same amount of auxin and are affected equally and so the root grows straight down.
If the root is growing at an angle to the force of gravity, the auxin accumulates on the lower side of the root causing cells on that side to elongate less and so the root grows downwards.
This is called positive geotropism.
Summary of the effect of auxin
In Roots it Reduces cell growth
In Shoots it Stimulates cell growth
