Xylem is a type of tissue in vascular plants that transports water and some nutrients from the roots to the leaves. Phloem is the other type of transport tissue; it transports sucrose and other nutrients throughout the plant. Xylem and phloem give vascular plants their classification; they are the vascular tissues that transport substances throughout the plant.
Function of Xylem
The main function of xylem is to transport water, and some soluble nutrients including minerals and inorganic ions, upwards from the roots to the rest of the plant. Xylem cells form long tubes that transport materials, and the mixture of water and nutrients that flows through the xylem cells is called xylem sap. These substances are transported through passive transport, so the process doesn’t require energy. The phenomenon that allows xylem sap to flow upwards against gravity is called capillary action. This occurs when surface tension makes liquid move upward. Water is also aided in moving up through the xylem by adhering to the xylem cells. However, it gets harder to work against gravity to transport materials as a plant grows taller, so xylem sets an upper limit on the growth of tall trees.
Xylem evolved in plants over 400 million years ago. In order to make food through photosynthesis, plants need to absorb carbon dioxide from the atmosphere and water from the soil. However, when the stomata—small holes in a plant’s leaves—are open to allow CO2 in, a lot of water evaporates, much more than the amount of CO2 taken in. Plants that developed systems to transport water to the sites of photosynthesis on leaves had a better chance of survival.
Structure of Xylem
Xylem is made up of several types of cells. Tracheids are long cells that help transport xylem sap and also provide structural support. Vessel elements are shorter than tracheids, but also help conduct water. They are found in flowering plants, but not in gymnosperms like pine trees. Vessel elements have perforation plates that connect each vessel element to form one continuous vessel. Xylem also contains parenchyma, a tissue that makes up most of the soft parts of plants, and long fibers that help support the plant. In a cross section of a plant, under a microscope, xylem appears star-shaped.
Details of Xylem Development
The first xylem that develops in a growing plant is called protoxylem, and it contains narrow vessels as the plant is not yet big. Metaxylem develops later on and has larger vessels and cells. There are four ways protoxylem and metaxylem can be arranged in a plant: centrarch, exarch, endarch, and mesarch.
- Centrarch: xylem forms one chamber in the middle of the stem, with metaxylem surrounding the protoxylem. This formation is not found in any living plants today.
- Exarch: xylem develops in multiple strands, and each strand develops inward towards the center of the root. Xylem in roots of vascular plants develops this way.
- Endarch: xylem develops in multiple strands, and each strand develops outward towards the periphery of the stem. Xylem in stems of vascular plants develops this way.
- Mesarch: xylem develops in multiple strands, and each strand develops from its middle both toward the center of the stem and in the opposite direction toward the periphery. Xylem in leaves and stems of ferns develop this way.
Types of Xylem
The two types of xylem, primary and secondary, perform the same function but are categorized by the type of growth that they are formed with.
Primary xylem forms with primary growth of a plant. This is the growth that occurs at the tips of stems, roots, and flower buds. It allows the plant to grow taller and the roots to grow longer. This growth is called primary because it occurs first in the growing season, before secondary growth. Both primary and secondary xylem transport water and nutrients.
Secondary xylem is formed with a plant’s secondary growth; this is the type of growth that allows the plant to get wider over time. Wide tree trunks, for example, show a lot of secondary growth. It occurs each year after primary growth. Secondary xylem is what gives the inside of tree trunks dark rings that are used to determine the age of the tree.
Differences Between Xylem and Phloem
Xylem and phloem both make up the vascular system of the plant, and work together to form vascular bundles that provide mechanical strength to the plant, but they have important differences. While xylem transports water, phloem transports food and nutrients. (One way to remember this is that phloem and food both begin with an “F” sound.) Xylem is unidirectional; its job is to make sure water flows upward. However, phloem is bidirectional and transports food and nutrients to all of the plant. Mature xylem is made up of dead cells that do not have cell contents, while phloem contains living cells (albeit without nuclei). The structure of xylem and phloem is also different. While xylem is made up of tracheids and vessels, phloem is made up of sieve tubes which have many holes for transporting nutrients. Xylem is star-shaped, while phloem is round and actually surrounds the xylem.