Chapter 6 · Control and Coordination

NCERT Class 10 Science Chapter 6 Intext Questions (Page 108)

All 5 questions of NCERT Class 10 Science Chapter 6 Intext Questions (Page 108) — Control and Coordination — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   What are plant hormones?
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   Plant hormones are chemicals made in very small amounts that coordinate a plant's growth, development and responses.

   Plant hormones, also called phytohormones, are chemical compounds produced by plant cells in very small amounts which help to coordinate the growth, the development and the responses of the plant to its environment. Plants have neither a nervous system nor muscles, so these chemicals do the work of control and coordination. They are synthesised at places away from where they act and simply diffuse to the area of action, which is why the responses of plants are slow. Different plant hormones have different effects: auxin, made at the shoot tip, helps cells to grow longer and makes a shoot bend towards light; gibberellins help in the growth of the stem; cytokinins promote cell division and are present in greater concentration in fruits and seeds; while abscisic acid inhibits growth, and its effects include the wilting of leaves.

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2. Q2
   How is the movement ​of ​leaves of the ​sensitive plant different from the ​movement of a ​shoot towards ​light?
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   The leaf folds quickly and reversibly on touch without growth; the shoot bends slowly and permanently towards light through auxin-driven growth.

   The two movements differ in almost every respect. The leaves of the sensitive plant move in response to touch, whereas the shoot bends in response to light. No growth is involved in the leaf movement: the plant moves the leaves themselves, the cells at the base of the leaf changing shape by losing water and shrinking, and the information about the touch is passed from cell to cell by electrical-chemical means, so that the movement happens at a point different from the point of touch. The bending of a shoot, on the other hand, is caused by growth — if the shoot is prevented from growing there is no movement at all — and it is brought about by auxin, which is made at the shoot tip, diffuses towards the shady side and makes the cells there grow longer than those on the lit side. The leaf movement is very quick, over in a few seconds, and it is reversible, the leaves opening again later; the bending of the shoot takes days and is permanent. Finally, the leaf folds in the same way whichever side is touched, so the movement is not directional, while the shoot always bends towards the light: the shoot's movement is a tropic movement, phototropism, and the sensitive plant's is not.

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3. Q3
   Give an ​example ​of ​a ​plant hormone that ​promotes ​growth.
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   Auxin is a plant hormone that promotes growth by making cells grow longer.

   Auxin is an example of a plant hormone that promotes growth. It is synthesised at the tip of the shoot and helps the cells to grow longer, which is what makes a shoot bend towards light. Gibberellins, which like auxins help in the growth of the stem, and cytokinins, which promote cell division and are present in greater concentration in regions of rapid division such as fruits and seeds, are also correct answers. Abscisic acid is not, because it inhibits growth.

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4. Q4
   How do auxins promote the ​growth of a ​tendril ​around ​a support?
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   Auxin collects on the side away from the support and speeds its growth, so the tendril curves around the support.

   A tendril is sensitive to touch. When it comes into contact with a support, the auxin present in it moves away from the side that is touching the support and collects on the side away from it. Auxin stimulates cells to grow longer, so the cells of the free side, which now have more auxin, elongate faster, while the part of the tendril in contact with the object does not grow as rapidly. Because one side of the tendril grows longer than the other, the tendril cannot remain straight: it curves towards the side that is growing less, that is, towards the support. As growth goes on, the tendril circles around the support and thus clings to it. This directional growth in response to touch is called thigmotropism.

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5. Q5
   Design ​an ​experiment ​to ​demonstrate hydrotropism.
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   Roots bend towards a water-filled porous pot in dry soil but grow straight down in the control, showing positive hydrotropism.

   Aim: to show that roots grow towards water, that is, to demonstrate hydrotropism. Materials: two troughs, dry sandy soil, germinated gram or bean seedlings, a small porous unglazed earthen pot and water. Method: (i) Fill the first trough with dry soil and sink the porous pot, filled with water, into the soil near one end, keeping it filled throughout; water seeps slowly through its wall so that only the soil near it stays moist. (ii) Plant two or three germinated seedlings in the middle of this trough, at a distance from the pot. (iii) Set up a second, control trough with the same soil and the same seedlings, but water it evenly all over and use no porous pot, keeping light and temperature the same for both. (iv) Leave both undisturbed for about a week, keeping the pot filled. (v) Dig the seedlings out carefully without breaking the roots and wash off the soil. Observation: in the test trough the roots have bent sideways and grown towards the porous pot instead of growing straight down, while in the control trough the roots have grown straight down. Conclusion: the roots grow in the direction in which water is available, so roots show positive hydrotropism. The control rules out gravity as the cause of the bending, since the only difference between the two troughs was where the water lay.

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