draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs

Structure of Transport Tissues

Learning Objective

Students will be able to:

1. Identify and label the main components of xylem vessel elements.
2. Identify and label the main components of phloem sieve‑tube elements and companion cells.
3. Produce accurate drawings from microscope slides, photomicrographs and electron micrographs.

Key Anatomical Features

Xylem \cdot essel Elements

• Elongated, dead cells forming continuous tubes.
• Thick secondary walls with lignified pits for lateral water movement.
• End walls are perforated (perforation plates) allowing unrestricted flow.
• Presence of vessel members (shorter cells) and vessel elements (longer cells).
• Residual ray parenchyma may be attached to vessel walls.

Phloem Sieve‑Tube Elements

• Living cells, elongated and cylindrical.
• Walls are thin; end walls are modified into sieve plates with pores.
• Contain callose deposits that can block pores in response to injury.
• Reduced cytoplasm; nucleus and most organelles are absent.
• Surrounded by companion cells that maintain metabolic activity.

Companion Cells

• Living parenchyma cells closely associated with sieve‑tube elements.
• Contain dense cytoplasm, a nucleus, mitochondria, endoplasmic reticulum and plasmodesmata linking to the sieve tube.
• Two main types: ordinary and transfer (or “phloem parenchyma”) cells.

Microscopic Observation Guides

Light Microscopy (LM) – Slides

1. Stain sections with safranin (xylem) and fast green (phloem) to enhance contrast.
2. Locate vessels in the xylem region: look for long, empty lumina with perforation plates.
3. Identify sieve tubes in the phloem region: look for strings of cells with faintly visible sieve plates.
4. Companion cells appear as slightly larger, densely stained cells adjacent to sieve tubes.

Photomicrographs (Bright‑field)

Use high‑resolution images to capture the following details:

• Perforation plates – central opening flanked by radial thickenings.
• Sieve plates – series of uniform pores; note any callose deposits (dark staining).
• Plasmodesmata connections between companion cells and sieve tubes.

Transmission Electron Microscopy (TEM)

Electron micrographs reveal ultrastructural features essential for accurate labeling:

• Secondary wall thickenings in vessel elements (spiral, annular, or pitted).
• Microtubules and actin filaments in companion cells supporting transport.
• Presence of endoplasmic reticulum and mitochondria in companion cells.
• Absence of nucleus in mature sieve‑tube elements.

Suggested Diagram Layouts

Comparison Table

Step‑by‑Step Drawing Procedure

1. Observe the slide under low magnification (×40) to locate the vascular bundle.
2. Switch to higher magnification (×400–×1000) and focus on individual cells.
3. Sketch the overall arrangement first: xylem on the inner side, phloem on the outer side.
4. Zoom in on a vessel element; draw the lumen, secondary wall thickenings and perforation plate.
5. Move to a sieve tube; outline the thin wall, indicate the sieve plate with a series of pores.
6. Draw the adjacent companion cell, adding nucleus, mitochondria and plasmodesmata connections.
7. Label each part clearly; use arrows to show direction of transport.
8. Compare your sketch with the provided photomicrograph and TEM image to verify accuracy.

Common Pitfalls and How to Avoid Them

• Confusing perforation plates with sieve plates: Remember perforation plates are large, often single openings; sieve plates consist of many uniform pores.
• Omitting companion cells: Always check the cell adjacent to a sieve tube; a dense cytoplasm indicates a companion cell.
• Misidentifying ray parenchyma as vessel elements: Ray cells are shorter, lack perforation plates, and have thin walls.
• Over‑staining: Excess safranin can obscure phloem details; use differential staining.

Assessment Checklist