In the trunk, the rays deliver water to the phloem tissue; they extract it from the vessels (where else?), move it to the outside edge of the VC where the phloem tissues are, and deliver it to the phloem sieve tube cells and their companion cells. In the leaves it’s a little different, with the xylem and phloem tissues lying side-by-side in a bundle in the leaf veins—not so much of a reach for the water.
That explains the phloem’s water source, but there is more to it. We need to return to the chloroplasts, innumerable in the leaves. As sugars are produced in the chloroplasts, they are exported into the phloem sieve tube elements in the vasculature of the tiniest leaf veins. As sugar concentrations rise, an imbalance occurs, creating an osmotic pull of water, mostly from the companion cell, to rush into the sieve tube element, a phloem cell. The pressure flow hypothesis states that sugars are loaded into the phloem tubes at the sources, causng a high-concentration sugar solution to build up. This draws water in, water transported by the rays from the xylem to the companion cells. The increased pressure pushes the sugar solution in that sieve tube element (STE) through its pores and into the next STE. The entire phloem element becomes pressurized and flows. At the sink end, the sugars are unloaded. The sugars are unloaded from the STE and water moves out of the phloem cells into adjacent tissue, restoring normal pressure. The sugars have been delivered to where they are needed.
Sinks are the recipients of phloem sugars. Chloroplasts in leaves are sourcses; they produce sugars. Hungry tissue needing sugar is a sink. Sinks vary; wherever energy is currently needed is a sink. Locations change through the season. First thing in the spring is non-woody root growth—that’s a sink. Cambial startup and first growth are early sinks, shortly followed by bud break and juvenile leaf growth, which are also sinks. Leaves still growing, costing more than they produce, are a sink. The phloem is not a strict leaf-to-root downward journey; at times it works this way, but phloem action can move in both directions and there is always a lot of radial movement of sugars from the phloem to wherever sinks are working. It is no coincidence that the phloem is right beside the VC—the VC is one of the hungriest, hardest-working tissues of the whole tree complex.
The phloem is responsible for the movement of sugars produced in leaves to wherever there is a demand for energy or sink. Phloem cells are alive and usually only live for one growing season, sometimes for just days or weeks. New phloem cells are constantly being generated by the VC. Phloem sap is so valuable that when STE are broken or fed upon (by aphids) there is a backup response that plugs the holes. This is accomplished by P-proteins, phloem proteins, which are filamentous and attach to the hole and plug it.
