IGCSE Biology Paper-4: Specimen Questions with Answers 149 - 150 of 279

Passage

An image of a germinating seed showing the growing root

Cell Membrane

Question 149 (1 of 4 Based on Passage)

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There are situations, either in wild plants or in laboratory experiments, where roots do not grow downwards.

Suggest and explain one situation.

Explanation

Clinostat/Klinostat is an instrument which can eliminate the effect of gravity and allow a plant to grow horizontally by slowly rotating it.

The main axis of which is attached to a rod. On the top of the rod is attached a flower pot. The clinostat is kept in a horizontal position as shown in fig. When the clock axis rotates the flower pot also rotates. As a result of this the plant grows horizontally as the effect of gravity is nullified by clinostat. If the clock of the clinostat is stopped the rotation of the plant stops, the shoot apex moves upward (negative geotropism) and the root apex moves downwards (positive geotropism) .

Structure of Plant

Question 150 (2 of 4 Based on Passage)

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The root shown in Fig. is growing downward into the soil.

Name this response seen in roots.

Explanation

Geotropism (Gravitropism)

  • Growth of movements induced by the stimulus of gravity is known as geotropism.
  • Generally, the primary root grows towards the force of gravity and hence is positively geotropic. The stem coloptile and phematophores grows away from the force of gravity and is negatively geotropic. The secondary roots and stem branches arise at angle less than 90o. They are thus plageotropic. Certain underground stems such as rhizomes, stolon՚s of potato are oriented at right angle to the direction of force a gravity and are called diageotropic. Some of the lateral organs (e. g. coralloid roots of Cycas) possess little or no geotropic sensitivity, they are called ageotropic.
  • If some seedlings are kept in a dark chamber in different directions, root always move downwards and shoot away from the gravitational force.