Contrast the structure and function of xylem and phloem. Discuss loading of sugars into the phloem. Describe the process of phloem transport. V. Discuss the differences between transport in the xylem and phloem. Assigned reading Purves et al.
Phloem sap is much different in composition that xylem sap.
Flux varies seasonally. Different sieve tubes may carry different things, even in the same vascular strand. The direction of flow can change. A chemiosmotic mechanism is responsible for movement of sucrose into companion cells and sieve-tube members. Sieve tube members contain 2-3 times more sucrose than mesophyll cells.