Q.19 The anamalous distribution of vascular tissues in older stem of Bougainvillea is
due to
(A). formation of accessory cambial rings
(B). formation of interxylary phloem
(C). abnormal behaviour of normal cambium
(D). abnormal behaviour of abnormal cambium
Choose the correct answer from the options given below:
1. (A) only.
2. (A), (B) and (C) only.
3. (A), (B), (C) and (D).
4. (B), (C) and (D) only.
Bougainvillea exhibits anomalous secondary growth in older stems, leading to scattered vascular tissues instead of a single ring. This woody climber from Nyctaginaceae family shows a unique distribution pattern due to specific cambial activity.
Correct Answer: Option 1 – (A) only.
Why Option 1 is Correct
In Bougainvillea, the older stem develops successive accessory cambial rings that produce new vascular tissues in concentric or wavy patterns. These rings form from pericycle or cortical cells outside primary bundles, creating multiple layers of secondary xylem and phloem. This results in the characteristic anomalous (scattered, polycyclic) vascular distribution, distinct from normal dicot stems with one cambium ring. Studies confirm this via serial sections showing continuity between these rings.
Explanation of All Options
Option (A): Formation of Accessory Cambial Rings
Correct. Primary feature causing anomaly. Successive cambia arise centrifugally, each producing vascular bundles embedded in conjunctive tissue. This matches observed T.S. with crumpled patches and rings.
Option (B): Formation of Interxylary Phloem
Incorrect. Interxylary (included) phloem occurs inside xylem (e.g., in Aristolochia), not Bougainvillea. Here, phloem forms externally to xylem in each cambial ring, without internal embedding.
Option (C): Abnormal Behaviour of Normal Cambium
Incorrect. Normal vascular cambium forms from procambium between xylem/phloem. In Bougainvillea, anomaly stems from additional accessory cambia, not irregular activity of the original one.
Option (D): Abnormal Behaviour of Abnormal Cambium
Incorrect. Term “abnormal cambium” is vague/non-standard. Literature specifies successive accessory cambia with regular bidirectional activity (xylem inward, phloem outward), not “abnormal” behavior.
Key Features Comparison
| Feature | Normal Dicot Stem | Bougainvillea Older Stem |
|---|---|---|
| Cambium Type | Single vascular cambium | Successive accessory rings |
| Vascular Arrangement | Single ring | Multiple concentric/wavy rings |
| Phloem Position | External only | External to each ring |
| Cause of Anomaly | N/A | Accessory cambial formation |
Exam Tips
Standard NEET/CSIR question on anomalous secondary growth in climbers/monocots. Memorize: Bougainvillea/Dracaena = successive cambia; Beta vulgaris = interxylary phloem. Focus on precise terminology for Assertion-Reasons or MCQs.
