A Plants Transportation System

Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport waterfrom roots to shoots and leaves, but it also transports some nutrients.The word "xylem" is derived from the Greek word ξύλον (xylon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant. The term was introduced by Nägeli in 1858.

Photo credit : T.Dominic walter

Cross section of some xylem cells

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 fibers.

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.