In the secondary growth of a dicotyledonous stem, how is the complete, continuous vascular cambial ring formed?

Answer and explanation

Correct answer: A. By the joining of intrafascicular cambium with interfascicular cambium formed by dedifferentiated medullary ray cells

  1. Step 1: In young dicot stems, strips of primary meristem called intrafascicular cambium exist between primary xylem and primary phloem within vascular bundles.
  2. Step 2: During the onset of secondary growth, parenchyma cells of the medullary rays adjacent to the intrafascicular cambium undergo dedifferentiation to become meristematic.
  3. Step 3: These newly meristematic cells form the interfascicular cambium, which joins with the intrafascicular cambium to establish a continuous cylindrical ring of vascular cambium.
Answer
By the joining of intrafascicular cambium with interfascicular cambium formed by dedifferentiated medullary ray cells
Why not the others: 'By the direct division of outer collenchymatous hypodermis...' describes cork cambium (phellogen) origin in some periderms, not vascular cambium; 'By the division and elongation of endodermal...' is false because Casparian strips are suberized impermeable bands in roots, not stem cambia; 'By the fusion of xylem vessels with sieve tube...' is physiologically impossible since both are differentiated conducting elements.

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