Complex tissue is an assemblage of two or more types of cells performing a common function. It is heterogeneous in nature. Xylem and phloem constitute complex permanent tissues.
(a) Xylem (Xylos = wood):
Xylem is a complex tissue consisting of both living and dead cells. It forms a part of vascular bundles. It is a conducting tissue and is composed of four elements of different kinds:
(i) Tracheids (ii) Vessels (iii) Wood fibers and (iv) Wood parenchyma. The function of xylem is to conduct water and mineral salts upwards from the root to the leaf. Besides, it gives mechanical strength to the plant body.
A single tracheid is a very elongated or tube-like cell with hard, thick and lignified walls and a large cavity. They are devoid of protoplast and hence dead. The ends of the tracheids are tapering, blunt or chisel-like.
Their walls are usually provided with one or more rows of bordered pits. Tracheids may also be annular, spiral, scalar form or pitted. In transverses section they are mostly angular, either polygonal or rectangular.
In angiosperms they occur associated with vessels. Being lignified and hard, tracheids give strength to the plant body. Their main function is to conduct water and mineral salts from the root to the leaf.
(II) Vessels or Tracheae:
Vessels are rows of elongated tube-like dead cells, placed one above the other with their transverse or end-walls perforated. A vessel or trachea is thus, like a series of water pipes forming a pipe-line.
The cells are dead and without protoplast. They are shorter and broader than tracheids. Their walls are thickened in various ways. Depending on the mode of thickening versels have received names such as annular, spiral, scalar form, reticulate and pitted.
Vessels have been found in a majority of angiosperms and in some pteridophytes and gymnosperms. They help to conduct water and mineral salts. They also serve the mechanical function of strengthening the plant body.
(iii) Wood fibers or xylem fibers:
Sclerenchymatous cells associated with xylem are known as wood fibers. These are found in both the primary and secondary xylem. They have thickened walls and obliterated central lumen. They add mechanical strength to the xylem and the plant body as a whole.
(iv) Wood parenchyma:
Parenchymatous cells associated with xylem form the wood parenchyma. The cells are living and possess thin cellulosic walls. They assist in the transport of water. They store starch, oil and egoistic substances like tannins.
The first formed xylem elements are described as protoxylem and consists of annular, spiral and scalar form vessels, and lie towards the centre of the stem. The later formed xylem is described as Meta xylem and it consists of some tracheids along with reticulate and pitted vessels. In stem, it lies away from the centre and its vessels have much bigger cavities compared to those in the protoxylem.