A xylem cell is a cell which is responsible for providing support to a plant. These cells also make up the vascular system of plants, conducting water throughout the plant and providing circulation. These cells can be both alive and dead, and there are several different types of xylem cell which can be found within the parts of a plant known collectively as xylem.
Plant cells start out as undifferentiated parenchyma cells. These cells can store energy for the plant, and they can also differentiate and mature into various cell types, including xylem cells. The xylem of plants usually contains a number of parenchyma cells, leading some people to classify them as a type of xylem cell, although this is technically incorrect.
Support is created through trachieds and fibers, cells which contain a great deal of lignin in their cell walls. The lignin makes the cell walls rigid, making the xylem as a whole very stiff so that it will support the plant and keep it upright. Trachieds are also involved in conduction, as are cells known as vessel members. Vessel members are tubular xylem cells which are designed to force water up against the pull of gravity so that it can circulate into the upper reaches of the plant.
When xylem cells die, they are still useful to the parent plant, unlike dead animal cells, which are usually broken down and discarded because they no longer serve a function. Although a dead xylem cell is no longer able to perform complex biological functions, it can still act as part of a support network for the plant, because the lignin in the cell walls is intact. These cells can also continue to conduct water through the xylem after death, because their conductive properties are purely mechanical, created by the shape of the cell, rather than being biological in nature.
Without xylem cells, a plant would have no vascular system. Vascular plants are able to be much more complex than their non-vascular counterparts, and they could be considered an evolutionary step up from nonvascular plants. Vascular plants may also be referred to as “woody plants,” because their xylem gives them a woody texture and the ability to grow large, upright, and complex. Woody plants serve a number of important ecological functions, and they are also highly prized as ornamentals in gardens all over the world.