Page 113 - NCERT Science Class 10 English Medium

Electrical impulses are an excellent means for this. But there are
limitations to the use of electrical impulses. Firstly, they will reach only
those cells that are connected by nervous tissue, not each and every cell
in the animal body. Secondly, once an electrical impulse is generated in
a cell and transmitted, the cell will take some time to reset its mechanisms
before it can generate and transmit a new impulse. In other words, cells
cannot continually create and transmit electrical impulses. It is thus no
wonder that most multicellular organisms use another means of
communication between cells, namely, chemical communication.
If, instead of generating an electrical impulse, stimulated cells release
a chemical compound, this compound would diffuse all around the
original cell. If other cells around have the means to detect this compound
using special molecules on their surfaces, then they would be able to
recognise information, and even transmit it. This will be slower, of course,
but it can potentially reach all cells of the body, regardless of nervous
connections, and it can be done steadily and persistently. These
compounds, or hormones used by multicellular organisms for control
and coordination show a great deal of diversity, as we would expect.
Different plant hormones help to coordinate growth, development and
responses to the environment. They are synthesised at places away from
where they act and simply diffuse to the area of action.
Let us take an example that we have worked with earlier [Activity 6.2].
When growing plants detect light, a hormone called 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.
This concentration of auxin stimulates the cells to grow longer on the
side of the shoot which is away from light. Thus, the plant appears to
bend towards light.
Another example of plant hormones are gibberellins which, like
auxins, help in the growth of the stem. Cytokinins promote cell division,
and it is natural then that they are present in greater concentration in
areas of rapid cell division, such as in fruits and seeds. These are examples
of plant hormones that help in promoting growth. But plants also need
signals to stop growing. Abscisic acid is one example of a hormone which
inhibits growth. Its effects include wilting of leaves.

Q U E S T I O N S

1. What are plant hormones?
2. How is the movement of leaves of the sensitive plant different from the
movement of a shoot towards light? ?
3. Give an example of a plant hormone that promotes growth.
4. How do auxins promote the growth of a tendril around a support?
5. Design an experiment to demonstrate hydrotropism.

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