The most distinctive xylem cells are the long tracheary elements that transport water. Tracheids and vessel elements are distinguished by their shape; vessel elements are shorter, and are connected together into long tubes that are called vessels.
Xylem also contains two other cell types parenchyma and fibres.
Xylem can be found:
- in vascular bundles, present in non-woody plants and non-woody parts of woody plants
- in secondary xylem, laid down by a meristem called the vascular cambium in woody plants
- as part of a stelar arrangement not divided into bundles, as in many ferns.
In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only.
The branching pattern exhibited by xylem follows Murray's law.
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