Xylem is a type of vascular tissue that is responsible for moving water through a plant from its roots to its leaves. Columns of xylem tissue begin in the root hairs and rise all the way up through the leaves of a plant.
A process known as transpiration is responsible for pulling water up from the roots to the leaves through the xylem tissue. In transpiration, water evaporates from the leaves, causing more water to be pulled upwards. Xylem vessels are comprised of dead cells. Only the rigid cell walls of these cells remain, and these walls surround a narrow cavity, or lumen, through which water is drawn.
Xylem is only one of two types of vascular tissue found in vascular plants. The other type is known as phloem, and it is responsible for carrying food substances, made through photosynthesis in the chloroplasts of leaves, to the stems and roots where they can be used for growth or stored until they are needed. Phloem columns stretch from the roots to the leaves in a similar manner as xylem, but they are comprised of living, active cells.
The ability of vascular plants to transport water and nutrients throughout their structures allows them to grow much larger than non-vascular plants. Most well-known plants, including corn, trees, tomatoes, daisies and sunflowers, are vascular plants. Non-vascular plants include mosses and lichens.