What is xylem? Explain its structure. Which one of its component is very important and why?

Xylem is a complex plant tissue or vascular tissue. Xylem consists of four elements that are-:

(i) Tracheids. A tracheid is an elogated hollow cell with its both ends tapering. The walls of these cells are thick by the deposition of lignin. At certain spots lignin is not present.These spots are termed as pits. These cells are arranged in such a fashion so as to form a system of long tubes and channels in which water can move readily. The tracheids are dead cells. The walls show various kinds of thickenings.

(ii) Vessels are tube like structures formed by a number of cells placed end to end with their transverse walls dissolved or absorbed. The side walls of these tubes also have deposition of lignin. The thickening of the walls show various kinds of patterns. They are also dead cells.

(iii) Wood (xylem) Parenchyma. These are parenchymatous, thin walled living cells. They help in short term conduction of materials. They also store starch and fatty materials.

(iv) Wood Fibres. Xylem sclerenchyma consists of lignified dead fibres. They provide mechanical support.

The most important element of xylem is vessel because they conduct most part of water and mineral salts upward from roots to different parts of shoots.

Fig. 6.10. Tracheids of xylem.