Structure of transport tissues

Diagram should show a circular arrangement with the epidermis as the outermost layer, cortex filling the space between the epidermis and vascular bundles, vascular bundles arranged in a ring, and pith in the center. Xylem should be located towards the inside of the vascular bundle, and phloem towards the outside.

Xylem forms a central core (often a star shape) in the root. Phloem is located in discrete groups between the arms of the xylem core, often alternating with the xylem.

Xylem and phloem are located together in vascular bundles within the leaf veins. Xylem is usually positioned towards the upper surface of the leaf, and phloem towards the lower surface.

Diagram should show elongated, hollow cells with thickened walls containing lignin. Pits (small unlignified areas) should be visible in the walls. End walls are absent or heavily modified (perforated).

Lignified walls provide strength and prevent collapse under tension. The hollow lumen and absence of cell contents allow for efficient water transport. Pits allow lateral water movement between vessels. Perforations or absent end walls allow unimpeded vertical flow.

Sieve tube element should be elongated with sieve plates at the ends. Companion cell should be smaller and adjacent to the sieve tube element, containing a nucleus and dense cytoplasm. Plasmodesmata should be visible connecting the two cells.

Sieve plates allow for the flow of assimilates (sugars) between cells. The lack of a nucleus and other organelles maximizes space for translocation. They rely on companion cells for metabolic support.

Companion cells provide metabolic support to sieve tube elements. They have a nucleus and many mitochondria to carry out active transport of sugars into and out of the sieve tube elements, maintaining the concentration gradient for translocation.