Biology – Structure of transport tissues | e-Consult

Identification of Xylem and Phloem:**

To identify xylem and phloem, I would look for the following features:

• Xylem: Xylem vessels will appear as relatively large, circular or oval structures with thick, lignified walls. The walls will be opaque and appear dark. The presence of perforation plates (visible as pores) is a key indicator. The lumen will be large and open.
• Phloem: Phloem will appear as smaller, more rounded structures. The walls will be thinner and less opaque than xylem. The sieve plates (perforations in the end walls) will be visible. The cytoplasm will appear less dense and may show the presence of companion cells adjacent to the sieve tube elements.

Functional Significance of Structural Differences:

Xylem: The lignified walls provide strength and prevent the vessel from collapsing under negative pressure (tension) caused by transpiration. The perforation plates allow for efficient water flow through the vessel. The large lumen facilitates the bulk flow of water. These features are essential for the long-distance transport of water and minerals from the roots to the rest of the plant.

Phloem: The sieve plates allow for the flow of phloem sap between sieve tube elements. The companion cells provide metabolic support to the sieve tube elements, which lack a nucleus and other organelles. The phloem proteins in the cytoplasm help maintain turgor pressure within the sieve tubes, facilitating the flow of sugars from source to sink. These features are crucial for the long-distance transport of sugars produced during photosynthesis.