Loading and Unloading of Sieve Tubes

In order to understand the loading of food from manufacturing leaf cells to sieve tubes we must examine the anatomy of a minor vein shown in Figure. We can see

Figure: A cross-section of a minor leaf vein.  (MC-Mesophyll  Cell, XE -Xylem Elements, SE-Sieve Element, TC-Transfer  Cells, PP-Phloem  Parenchyma  cell.

a xylem vessel (X), two sieve elements (SE) which are relatively smaller than other cells and without cytoplasm, the modified companion cells or transfer cells (TC) with dense cytoplasm and phloem parenchyma (PP) which is less dense and vacuolated.