Xylem and phloem are complex tissues, which are also called conducting or vascular tissues xylem is composed of four elements of different kinds: (i) Tracheids (ii) Vessels (iii) Xylem Fibers (iv) Xylem Parenchyma.
Tracheids: A tracheid is a highly 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 tracheids are topering blunt or chisel like. These are constituents of the Xylem of primitive plants.
Vessels: A vessel is a long, cylindrical, tube like structure with lignified walls and a wide central cavity. The cells are dead and.without protoplast. These are arranged in longitudinal series in which the transverse walls (the end plates) are perforated and as such the entire structure looks like a water pipe. These give mechanical support to the plant body.
Xylem Fibers: These are found in both the primary and secondary xylem and may posses simple or bordered pifs, highly thickened walls and obliterated central lumen.
Xylem Parenchyma: The cell wall of the primary xylem parenchyma are thin and made up of cellulose. These store food material in the form of starch or fat, and sometimes tannins and other substrate.
The first formed xylem elements are described as protoxylem and consists of annual, spiral and scalariform vessels and lie towards the center of the stem. The latter formed xylem is described as metaxylem which consist of some tracheids. The function of xylem is to conduct water and mineral salts upwards from the root to the leaf and to give mechanical strength to the plant body. While the main function of phloem is to conduct prepared food materials from the leaf to the storage organs and growing regions of the plant. Phloem is composed of four elements
Sieve tube elements: These occur as long slender tube like structures arranged in longitudinal series. And associated with the companion cells. They are devoid of nucleus at maturity. However, they possess a peripheral cytoplasm as well as a large vacuole.
Companion cells: These are specialized parenchyma cells which are closely associated with the sieve tube elements in their origin, position and function. These originate from same meristematic cells that give rise to the sieve tube elements.
Phloem parenchyma: This is made up of elongated, tapering to broadly cylindrical, living cell which have a dense cytoplasm and nucleus.
Phloem fibers: These are much elongated, unbranched (rarely branched) and have pointed, needle like apices. At maturity, these fibers lose their protoplast and become dead.
The outer portion of the phloem consisting of narrow-tube elements constitutes the protophloem. The inner portion is made up of broader sieve tube elements which make the metaphtoem.