Page 111 - NCERT Science Class 10 English Medium

There is no growth involved in this movement. On the other hand, the
directional movement of a seedling is caused by growth. If it is prevented
from growing, it will not show any movement. So plants show two different
types of movement – one dependent on growth and the other independent
of growth.

6.2.1 Immediate Response to Stimulus

Let us think about the first kind of movement, such as that of the sensitive
plant. Since no growth is involved, the plant must actually move its leaves
in response to touch. But there is no nervous tissue, nor any muscle
tissue. How does the plant detect the touch, and how do the leaves move
in response?

Figure 6.46.4
Figure 6.4
Figure
6.4
Figure
Figure 6.4 The sensitive plant
If we think about where exactly the plant is touched, and what part
of the plant actually moves, it is apparent that movement happens at a
point different from the point of touch. So, information that a touch has
occurred must be communicated. The plants also use electrical-chemical
means to convey this information from cell to cell, but unlike in animals,
there is no specialised tissue in plants for the conduction of information.
Finally, again as in animals, some cells must change shape in order for
movement to happen. Instead of the specialised proteins found in animal
muscle cells, plant cells change shape by changing the amount of water
in them, resulting in swelling or shrinking, and therefore in changing
shapes (Fig. 6.4).

6.2.2 Movement Due to Growth

Some plants like the pea plant climb up other plants or fences by means
of tendrils. These tendrils are sensitive to touch. When they come in
contact with any support, the part of the tendril in contact with the object
does not grow as rapidly as the part of the tendril away from the object.
This causes the tendril to circle around the object and thus cling to it.
More commonly, plants respond to stimuli slowly by growing in a
particular direction. Because this growth is directional, it appears as if
the plant is moving. Let us understand this type of movement with the
help of an example.

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