Passive transport of fluid up the plant stem. The xylem cells form a continuous tube through which water with dissolved nutrients (sap?) is pulled up at a rate of mm/h by a mixture of capillary action (mainly for small stems) and the turgor pressure generated by transpiration. Considering that most trees can reach many meters in height, some sort of pumping might be thought necessary. However, as long as the column of water is unbroken (by bubbles, for example) the distance it travels is not too important. On the other hand, the pressure necessary to move water 100 m up a tree is 3 MPa. There are mechanisms to maintain such an unbroken column, such as removing bubbles.

Xylem is one of the the main types of vascular tissue in a plant (another main type is phloem). Xylem tissue carries water and nutrients from the roots to the leaves. Xylem tissue is vital to transpiration.

Xylem cells are found in lines with each cell touching ends with its neighbor. Immature xylem cells are long and elongated. They grow very thick woody cell walls on their two longer sides. Once the cell walls reach sufficient thickness, the xylem cells can no longer get nutrients or remove wastes and die. This process is known as lignification. Xylem cell contents (cytoplasm) breaks down, leaving a hollow tube of wood. A xylem tube looks something like this:

[  ] 
[  ] 
[  ] 
[  ] 
[  ] 
[  ] 
[  ] 

Each
'[  ]'
is a xylem cell

Xylem also provide structural support for the plant and make up most of the tissue you think of as 'wood' in the trunk and roots of a tree.

Just thought I'd better correct the other node on this topic to avoid confusion.

Xy"lem (?), n. [Gr. xy`lon wood.] Bot.

That portion of a fibrovascular bundle which has developed, or will develop, into wood cells; -- distinguished from phloem.

 

© Webster 1913.

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