When a plant tissue is composed of more than one type of cells, it is called a complex tissue. There are two types of complex plant tissues, viz. xylem and phloem.
Xylem is composed of four elements:
- Xylem fibres
- Xylem parenchyma
These are elongated or tube-like cells. They have thick and lignified walls and tapering ends. The tracheid cells are dead and protoplasm is absent. Tracheids and vessels are the main conducting elements in flowering plants.
These are long, cylindrical tube-like structures. These are made up of many cells which are called vessel members. Each vessel member has lignified walls and a large central cavity. Protoplasm is absent in vessel cells. The vessel members are interconnected through perforations in their common walls. The presence of vessels is a characteristic feature of angiosperms. Vessels are absent in gymnosperms.
The xylem fibres have highly thickened walls and obliterated central lumens. Septa may be present or absent in xylem fibres.
The xylem parenchyma is composed of living cells which are thin-walled. The cell wall of xylem parenchyma is made up of cellulose. Xylem parenchyma stores food in the form of starch or fat, and some other substances like tannins. Parenchymatous cells facilitate the radial conduction of water in plants.
Primary xylem is of two types, viz. protoxylem and metaxylem. Protoxylem are the first formed primary xylem elements, while the metaxylem are formed later. Based on the positions of protoxylem and metaxylem vis-a-vis each other, there are two types of xylem.
- Endarch: The protoxylem lies towards the pith in stem and the metaxylem lies towards the periphery. This type of primary xylem is called endarch.
- Exarch: The protoxylem lies towards the periphery in roots and the metaxylem lies towards the pith. This type of primary xylem is called exarch.
Transportation of water and minerals from roots to different plant parts is the main function of xylem. Xylem also provides mechanical strength.
The phloem (in angiosperms) is composed of four elements, viz. sieve tube, companion cells, phloem parenchyma and phloem fibres. In case of gymnosperms, the phloem has albuminous cells and sieve cells. Sieve tubes and companion cells are absent in the phloem in gymnosperms.
The sieve tube elements are long, tube-like structures. They are arranged longitudinally. The end walls of sieve tubes are perforated to form sieve plates. A mature sieve element has a peripheral cytoplasm and a large vacuole but no nucleus. The nucleus of the companion cell controls the function of sieve element.
These are specialized parenchymatous cells. The companion cells are closely associated with sieve tube elements. There is a common pit field in the common longitudinal wall between the sieve tube element and companion cell. These piths connect both of them. The companion cell helps in maintaining the pressure gradient in the sieve tubes.
The cells of phloem parenchyma are elongated, tapering cylindrical cells. The cells have dense cytoplasm and nucleus. Cell wall is composed of cellulose and has pits. Plasmodesmatal connections exist between the cells through these pits. Phloem parenchyma stores food and other substances like resin, latex and mucilage. Phloem parenchyma is usually absent in monocotyledonous.
Plasmodesmata: Tiny pores on cell wall in a plant cell are called plasmodesmata. They serve as channel for communication between cells.
These are also called bast fibres and are made up of sclerenchyma. Phloem fibres are usually absent in primary phloem but present in secondary phloem. The phloem fibres are much elongated, unbranched and have pointed, needle-like apices or tips. Phloem fibres of jute, flax and hemp are commercially used.
The primary phloem which is first formed consists of narrow sieve tubes and is called protophloem. The later formed phloem has bigger sieve tubes and is called metaphloem.