The Word: Xylem
Bruce Palmer
Cutten, California

        Xylem, from the Greek xylon for wood, is one of several words for this issue of "The Word." Our Journal Editor Glen Jamieson helped me realize that the discussion of transpiration in the summer issue was incomplete as it stood. It needed a thorough overview of how substances move and are used in plants for a complete understanding of trasnpiration. If you're like the typical human, you have retained about ten per cent of the material you learned as a student years ago, so a refresher shouldn't hurt a thing. Here's a review, then, of part of that high school biology course many of you may have taken so many years ago.
        What we call vascular (Latin: vasculum, a small vessel) plants are those plants that have specialized cells for conducting water, minerals and food around their bodies. The members of the genus we're addicted to, Rhododendron, are vascular plants. The typical vascular plant has three primary parts: roots, stems and leaves (flowers are modified leaves). Let's concentrate on the stem. The stem is the part that transports the water and dissolved minerals from the roots to the leaves and carries the finished food from the leaves to the root cells, appropriating what it needs along the way. These two processes are carried out by two different types of cells. The water that you and the weather provide, along with dissolved quantities of nitrates, phosphates, potassium and trace minerals you added to the ground early in the summer, are carried upward toward the leaves by xylem cells. This upward transport is mostly passive, the result of diffusion and its subcategory, osmosis, helped along by transpiration, discussed in the previous summer issue. Xylem cells in stems are the woody part that's left after you peel off the bark. They serve not only to transport materials but also to support the plant. In older plants, the wood is made up of an outer layer of recently dead cells, called sapwood, some of which are conducting water and minerals upward. Inside the sapwood is an inner core of heartwood composed of long-dead cells whose major function besides support is to store the undesirable metabolic byproducts the plant can't excrete, as it has no excretory organs such as our kidneys. These metabolic by-products are useful, giving us such things as the wood scents of camphor and cedar and the beautiful colors of koa and walnut. Xylem cells conducting water and minerals upward get clogged up over time and so need to be replaced with new ones. This process occurs rapidly in spring and early summer, then grinds to halt in fall, producing what we see as annual rings in wood. New xylem cells are produced from non-specialized cells (meristem cells that we discussed in the Fall 2008 issue of JARS).

        The layer of meristem cells that produces the conducting tissues is called the vascular cambium (Latin: cambiare, to exchange). When you peel the bark off a live woody stem it separates at the vascular cambium layer. The xylem cells are produced on the inside and become the dead woody cells. On the outside of the cambium layer, phloem cells are produced. The term phloem derives from the Greek word phloos, for bark. The primary function of phloem cells is to conduct the food produced mainly in the leaves to all of the other cells in the plant body. Phloem cells are alive while they are performing their tasks, in part because moving large food particles often involves active transport, an energy-requiring process that allows living cells to move molecules against the force of diffusion. After phloem cells die, their remains are incorporated into the bark.
        Air-layering in plants illustrates the difference between xylem and phloem nicely. To produce a copy of a plant by air-layering you remove about an inch-long section of the bark and cambium from a small stem, leaving only the wood. You cover the area with damp sphagnum and encase it in a waterproof sheath such as a plastic bag. Wait a few weeks to a few months and, if you are lucky, the cambium will have produced roots. With some care you can now cut off your new plant and pot it up. What has happened is that the xylem cells in the wood continued to carry raw materials to the leaves but the finished food couldn't get to the roots because you removed the phloem. In response, the unspecialized cells in the cambium produced new roots.
        There is much more to this topic than we can cover here in one article. For example, special types of xylem and phloem cells perform particular tasks. Both xylem and phloem are present in roots and leaves, though they don't look the same under the microscope as those similar tissues in stems. Meanwhile, it's instructive to realize that plants are able to move materials around their bodies almost as well as we can, using a much different system.

Bruce Palmer is a member of the Eureka ARS Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii system for 25 years.