Why xylem and phloem are called complex tissues?

Xylem and phloem are complex tissues because it consist of various elements which are given below: (a) Xylem:It is a conducting tissue composed of four elements of different kinds:(i) tracheids, (ii) vessels, (iii) wood fibres and (iv) wood parenchyma.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.(i) Tracheids:A single 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 are hence dead. The ends of the tracheids are tapering, blunt or chisel-like. These are constituents of the xylem of primitive plants. The cell wall is hard moderately thick and lignified. The secondary wall layers possess various kinds of thickenings in them and may be distinguished as annular, spiral, retuculate, scalar form or pitted. In a transverse section, they appear circular, polygonal or polyhedral in outline. On account of these structural adaptations, besides the mechanical support to the plant body, they transport water, hormones and solutes from the root to the stem, leaves and the floral parts. In gymnosperms, these are the chief water transporting elements.(ii) 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 are perforated and as such the entire structure looks like a water pipe. The perforations may be simple or multiple. The latter may be arranged in reticulate, scalariform or foraminate manners. Vessels have been found in a majority of angiosperms. They are also found in a few pteridophytes and gymnosperms. They serve as a more efficient mode of transportion of water and minerals as compared to tracheids, due to the presence of perforation plates. These also give mechanical support to the plant body.(iii) Xylem fibres:These are found in both the primary and secondary xylem and may possess simple or bordered pits, highly thickened walls and obliterated central lumen. These may either be septate or aseptate.(iv) Xylem parenchyma:The cell walls of the primary xylem parenchyma are thin and made up of cellulose. These store food materials in the form of starch or fat and sometimes tannins and other substances. The ray parenchyma cells also participate in the radial conduction of water.The first formed xylem elements are described as protoxylem and consist of annular, spiral and scalariform vessels and lie towards the centre of the stem. The latter formed xylem is described as metaxylem and it consists of some tracheids along with reticulate and pitted vessels. In the stem, it lies away from the centre and its vessels have much bigger cavities when compared to those of the protoxylem.(b) Phloem:The phloem is yet another conducting tissue.It is composed of four elements: (i) sieve cells or sieve tube elements, (ii) companion cells, (iii) phloem parenchyma and (iv) Phloem fibres.The main function of the phloem is to conduct prepared food materials from the leaf to the storage organs and growing regions of the plant.(i) Sieve tube elements:These occur as long, slender tube-like structures, arranged in longitudinal series and are associated with companion cells. Their end walls are perforated in a sieve-like manner to form the sieve plate. These may either be simple or compound and at maturity they become impregnated with cellulose. They are devoid of nucleus at maturity. However, they possess a peripheral cytoplasm as well as a large vacuole. The uniqueness of the sieve tube is that although without nucleus, it is living and the nucleus of the companion cells controls its functional activities. Distinct proteinaceous inclusions, the P-proteins are seen evenly distributed throughout the lumen of the sieve tube. During wounding, along with cellulose, P-protein helps in sealing.(ii) 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 the same meristematic cells that give rise to the sieve tube elements.The sieve tube elements and companion cells are connected by pit field presenting their longitudinal walls, which is a common wall for both and with the death of one, the other cell also dies. The companion cells play an important role in the maintenance of a pressure gradient in the sieve tubes.(iii) Phloem parenchyma:This is made up of elongated, tapering to broadly cylindrical, living cells, which have a dense cytoplasm and nucleus. The cell wall is composed of cellulose, with pits, interconnection axial parenchyma cells and ray cells. The phloem parenchyma stores organic food materials and other substances such as resins, mucilage, latex, etc.(iv) Phloem fibres:These are much elongated, unbranched and have pointed, needle-like apices. Their cell wall is quite thick with simple or slightly bordered pits. At maturity, these fibres lose their protoplast and die. These occur in groups, as sheets or cylinders.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 metaphloem.