Xylem Structure and Function

The earliest land plants were short, herbaceous plants that evolved from primitive, water-living ancestors. For these plants, the change from a predominantly aquatic to a terrestrial environment was accompanied by the need for additional structural support to keep the plants upright and the need for more efficient transport of water to the aboveground parts of the plants. Larger plant sizes also increased the need for co-ordination between remote plant parts. The development of specialized vascular tissues to fulfil these requirements played an important role in the evolution and adaptation of plants to the terrestrial environment. As the early land plants filled more and more terrestrial niches, the selective advantage of increased propagule dispersal associated with increase in height, and later competition for sunlight, increased the selection pressure for plants that could grow taller than other plants. The most successful plants were able to support more weight, transport water further and sustain growth for more than one season. The dramatic result of this evolutionary process is evident in the rapid increase of plants with secondary vascular tissues and arborescent growth form in a rather short evolutionary timespan (380–350 million years ago). The stems and roots of modern plants are highly specialized conductive organs that can transport water, nutrients, photosynthetic products and chemical regulatory signals. These organs contain two types of conductive tissue: phloem and xylem. Phloem is the tissue that transports photosynthetic products and plant growth regulators (phytohormones) mainly from the leaves to the rest of the plant. Xylem is the tissue that transports water, mineral nutrients and phytohormones from the roots to the leaves and other plant organs. While herbaceous plants do contain xylem, it is a tissue that is most prominent in woody plants, especially trees. Most of our knowledge of xylem structure and function is based on woody plants. The most important functions of xylem include: (1) transport of water and mineral nutrients, (2) mechanical support and (3) storage of nutrients and water.