Xylem

Xylem is a type of vascular tissue in higher plants. It transports water and dissolved minerals from the roots to the stems, branches and leaves. It also provides mechanical support to the plant, because the cell walls of xylem tissue contain lignin, a substance that strengthens them.

Xylem is paired with phloem, the vascular tissue that transports sugars produced by photosynthesis. Together, these two tissues form the transport systems that run through the body of the plant.

Component cells

Xylem contains elongated cells called tracheids and vessel elements joined to one another to form a long channel. When mature, these cells die and lose their walls, so water can flow through them easily. There are also supporting cells and fibers that strengthen the structure.

How water moves

Water movement in xylem does not depend on an active force directly generated by the plant. As water evaporates from the leaves (transpiration), it creates a negative pressure that pulls water up the stem. Water molecules stick to each other (cohesion) and to the walls of the vessel cells (adhesion), making the water column continuous and unbroken. Roots can also generate root pressure.

Growth and rings

In woody plants, the cambium layer produces new xylem on the inside each year. The older xylem in the center becomes heartwood. In regions with distinct seasons, the difference in xylem produced creates annual rings visible in the stem, which can be used to estimate the age of a tree.