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.