Anomalous secondary growth occurs in Dracaena (D).​​

Option Analysis

Triticum (wheat): This monocot lacks vascular cambium, showing only primary growth through scattered vascular bundles without secondary thickening.​

Oryza (rice): As a typical grass monocot, it exhibits no secondary growth, relying on tillering for expansion rather than cambial activity.​

Zea (maize): Monocot stem anatomy features closed vascular bundles without cambium, preventing any secondary growth.​

Dracaena: Shows anomalous secondary growth via a secondary thickening meristem in the cortex, forming successive cambial rings that produce amphivasal vascular bundles and conjunctive tissue internally first.​​

Anomalous secondary growth in Dracaena represents a rare adaptation in monocots, enabling stem thickening absent in typical grasses like Triticum, Oryza, and Zea. This process, crucial for CSIR NET Life Sciences, involves extrastelar cambium formation.​​

Mechanism in Dracaena

Cells in the parenchymatous cortex dedifferentiate into a secondary thickening meristem (STM), forming a ring outside primary bundles. Initially, it cuts xylem (amphivasal bundles with phloem inside) and conjunctive tissue inward; later, some parenchyma outward, creating concentric vascular rings. This differs from dicot vascular cambium, which produces xylem outward and phloem inward.​​

Why Absent in Other Options

Monocots like Triticum (wheat), Oryza (rice), and Zea (maize) lost vascular cambium genes early, limiting growth to primary tissues and tillers. No cambial activity occurs, unlike Dracaena’s adaptive anomaly for tree-like habit.​

Exam Relevance

• Matches CSIR NET patterns on monocot anomalies (e.g., Dracaena, Yucca).​​

• Key diagram: T.S. shows scattered primary bundles, STM ring, successive secondary bundles with sclerenchyma sheaths.​​