It is a heterogeneous system of cells. It consists of more than one type of cell working together as a unit. It consists of parenchymatous and sclerenchymatous cells. They are also known as conducting tissues because they are involved in the conduction of water, minerals, and food material inside the plant body. The most important complex tissues are - xylem and phloem.
- Xylem (wood): Xylem is a conducting tissue that conducts water and mineral nutrients upward from the root to the leaves. The xylem is composed of different kinds of elements. They are (a) tracheids (b) fibres, (c) vessels or trcheae (d) wood parenchyma
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- Xylem tracheids
- These are elongated; tube like cell having tapering, rounded or oval ends.
- The walls are lignified.
- It is without protoplast and non-living on maturity
- The secondary wall layers are deposited in different manner, thus the wall shows various types of thickening - annular (thickening in the form of ring), spiral (thickening in the form of spiral), reticulate (thickening in the form of network), scalariform (thickening in the for of ladder) and pitted (thickening on entire inner cell wall).
- It conducts the water and minerals.
- Xylem vessels / Tracheae
- These are rows of elongated tube like dead cells.
- The characteristics are perforation plate at their ends walls and joined end to end to form chain of cells called vessels.
- Tracheae are formed by the dissolution of ends walls of developing vessel members.
- It is shorter and boarder than tracheids.
- Xylem with smaller vessels is called protoxylem and xylem with larger vessels is called metaxylem.
- Xylem parennchyma
- The parennchyma cells which frequently occur in the xylem of most plant are called xylem parenchyma.
- The walls of xylem parenchyma are thin and made up of cellulose.
- These cells are noted for storage of food in the form of starch, fat, tannin, crystals and various other substances also occur in xylem parenchyma cells.
- These cells assist directly or indirectly in the condition of water upward through the vessels and tracheids.
- Xylem fibers
- These are also called as wood fibers.
- They are common in secondary xylem.
- They are made up of sclerenchymatous cells.
- The cells of xylem fibers contain thick lignified walls and narrow lumen.
- The cells are dead as that of tracheids and vessels.
- These are the principle supporting component of xylem, which provide mechanical strength to the plant parts.
- Xylem tracheids
- Phloem (Bast): The phloem is a specialized complex permanent tissue. It is mainly concerned for the conduction of organic food synthesized by leaves to the different parts of plant. Phloem is composed of different kinds of elements. They are (a) Sieve elements, (b) companion cells, (c) phloem fibers, and (d) phloem parenchyma.
- Sieve elements:
- The conducting elements of the phloem are collectively known as sieve elements.
- They may be segregated into the less specialized sieve cells and they make specialized sieve tube.
- They are elongated tube like cells. Their walls are thin and made uo of cellulose
- Companion cells:
- These are specialized thin walled parenchymatous cells found to be associated with sieve tube.
- They are thin walled and contain dense cytoplasm and prominent nucleus.
- Nuclei of companion cells control the metabolic activities of sieve tube and pteridophyte.
- The companion cells are found in angiosperms only. In gynosperm, no companion cells are found, but there are some special parenchymatous cells called albuminous cells.
- Phloem Parenchyma
- These are thin walled living parenchymateous cells.
- They are absent in most of monocots and herbaceous plants.
- The cells contain cytoplasm and nucleus.
- They store food material in the form of starch, fat and other organic substances.
- Phloem fibers
- These are made up of thick sclerenchymatous cells.
- In many flowering plants, fibers form a prominent part of both primary and secondary phloem.
- They serve to the mechanical strength of phloem and plant body as a whole.
- Sieve elements: