What do you understand by secondary medullary rays?

Secondary medullary rays (also called vascular rays or xylem/phloem rays) are radially oriented parenchymatous bands that traverse through secondary vascular tissues.

Formation: During secondary growth in dicot stems:

• Vascular cambium produces secondary xylem (inward) and secondary phloem (outward)
• At certain locations, cambial cells do not differentiate into xylem or phloem
• Instead, they repeatedly divide to form parenchymatous rays

Structure:

• Narrow bands of parenchyma cells
• Oriented radially (perpendicular to vascular bundles)
• Extend from secondary xylem through cambium to secondary phloem
• May be uniseriate (1 cell wide) or multiseriate (several cells wide)

Functions:

1. Radial Transport:
   • Conduct water, minerals, and nutrients laterally
   • Connect xylem to phloem (metabolic exchange)
2. Storage:
   • Store starch, proteins, tannins
   • Seasonal nutrient mobilization
3. Structural Support:
   • Provide lateral strength to woody stems
   • Prevent splitting along growth rings

Contrast with Primary Medullary Rays:

Feature	Primary Medullary Rays	Secondary Medullary Rays
Origin	From ground meristem	From vascular cambium
Location	Between primary vascular bundles	Within secondary vascular tissues
Timing	Primary growth	Secondary growth

NEET Application: Wood anatomy questions often involve identifying rays in cross-sections.