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ICSE Question Bank
2023Q3(v)(b)Chemical Coordination in Plants
Short answerHard1 mark
3(v)

Study the diagram given below and answer the questions that follow:

Figure for this question

Name the plant hormone responsible for this response.

Topic: Auxins and tropic movement

Previous-year question practice database

Answer

Exam answer

AUXIN.

Explanation

Auxin causes cell ELONGATION, and tropic movements happen because it becomes UNEQUALLY DISTRIBUTED.

In a shoot lit from one side, auxin migrates to the SHADED side. Those cells elongate more than the lit ones, so the shoot bends TOWARDS the light.

THE POINT THAT SURPRISES STUDENTS: the same hormone has the opposite effect in a root. Root cells are far more sensitive to auxin, and the concentration that stimulates a shoot INHIBITS a root — so in a horizontal root, auxin gathers on the lower side, growth there is slowed, the upper side grows faster, and the root curves DOWNWARD.

One hormone, two opposite results, because the tissues differ in sensitivity.

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