Transpiration: Factors and Mechanism · NCERT Biology

Transpiration is often called a 'necessary evil' because:

  1. It kills the plant if it happens too rapidly
  2. It is needed for sap ascent but wastes most absorbed water
  3. It only occurs in weed species, not crop plants
  4. It blocks the plant from absorbing carbon dioxide
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Correct answer: It is needed for sap ascent but wastes most absorbed water

Transpiration is a 'necessary evil' because it leads to massive water loss — over 99% of absorbed water is lost. However, it is essential for the ascent of sap, mineral distribution, and thermoregulation of the plant.

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