Transport - The movement of liquids and materials in solution from one place to another. There are 2 basic types of transport; passive and dynamic, and 2 basic directions; axial or longitudinal, and radial. Axial transport is mostly downward in phloem, and mostly upward in xylem or wood. Radial transport is mostly out- ward early in the growing season and mostly inward later. These are the basic themes, and the variations are numerous. In newly forming tissues that have not yet become woody, transport may go in either direction in the xylem and phloem. The photosynthate-sugars-are forming in the leaves-source-and moving to a place where they are used for energy or being convened to energy reserves-sink. Materials can move only if they are soluble. Water and essential elements from the soil move upward in the xylem-vessels, tracheids. There are several forces and factors acting in the upward transport: transpiration "pull" when leaves or needles are functioning, the capillary action of the vessels and tracheids, the force of cohesion of water in the capillaries, and root pressure. These are all passive because they do not involve living processes. Active or dynamic transport involves movement of materials from one living cell to another through membranes. Chemical changes and electrical charges on each side of the membrane are part of the forces and factors involved. Energy is required for dynamic transport. As cells use food for energy, carbon dioxide and water are produced. The pressure of carbon dioxide may also drive some of the transport system, and not only the dynamic, but add to the passive system. The 2 systems do relate to each other because the water and elements in the vessels and tracheids must get to the living cells. So the 2 systems are not isolated from each other. As substances are dissolved in tree liquids or sap, the osmotic pressure of the liquid increases. Liquids with a high osmotic pressure will not move against a gradient into a cell with a lower osmotic pressure. And, again there are exceptions to this where high amounts of energy are used. Phloem transport is still not understood clearly. The liquids with a high osmotic pressure move from leaves to roots. The pathway is not always in a downward direction because tree branches normally bend downward. So, the photosynthate must move in an upward direction first and then downward when it reaches the trunk. The materials move in the cells called sieve tubes, and companion cells surround the sieve tubes. Gravity is not the answer to the movement because the cells are so small and the osmotic pressure is so high. The materials must be actively "pumped" downward. In a sense, the cells "squeeze" the materials downward. The subject of transport in trees is one that always elicits controversy. We are aware of the many pitfalls on this subject and do not mean to make the subject appear simple. Yet, it does happen. Materials go up, and materials go down; and in and out. Most of the xylem transport takes place in the current growth ring, or the last few rings. Some trees have very large earlywood vessels -oaks, elm- and much smaller vessels later. Vessels do not become functional for transport until they lose their protoplasm and die, and after the end- walls between them break. Vessels do end. Vessels are made up of many smaller single-celled units that connect at their axial tips. Some vessels may be as long as a meter. Then, by pores or pits the vessel connects with another long vessel.
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