Lignin staining reveals xylem in dicot stem vascular bundles

Steps:

• Dicot stem has vascular bundles arranged in a ring, with phloem outside and xylem inside each bundle.
• Lignin, a key component of xylem cell walls (vessels and tracheids), is absent in phloem.
• The stain targets lignin, so only xylem appears stained.
• W is the outer, unstained tissue (phloem); Z is the inner, stained tissue (xylem).

Why B is correct:

• Xylem contains lignified elements that bind the stain, distinguishing it from non-lignified phloem.

Why the others are wrong:

• A: Wrongly assigns staining to W (phloem lacks lignin).
• C: Reverses tissue positions; xylem is inner, not outer W.
• D: Reverses both tissues and staining; phloem is outer and unstained.

Final answer: B