In plants xylem and phloem are the complex tissues which are the components parts of conductive system.
In higher plants xylem contains tracheids, vessels (tracheae), xylem fibres(wood fibres) and xylem parenchyma (wood parenchyma).
Tracheids These are elongated narrow tube like cells with hard thick and lignified walls with large cell cavity. Their ends are tapering or pointed. They are dead empty cells with their walls provided with one or more row of bordered pit.
Trachied occur alone in the wood of ferns and gymnosperms. But in Angiosperm they occur associated with vessels.
- Conduction of water and minerals
- Mechanical support to the plant body
Vessels (Tracheae) are cylindrical tubular structure. They are formed of row of cells placed end to end fro which the cross wall breaks down. The vessel members are connected by means of plates with pores and are known as perforation plates through which water moves up ward. Their walls are thickened in various ways.On the basis of these thickening, vessels are named as annular, spiral, scalariform, reticulate and pitted.
The conduction of water and minerals from root to the leaves and giving mechanical support are the important function of vessels.
Difference between Tracheids and Vessels
1. Main conducting elements of pteridophytes and gymnosperms
2. Tracheids originate from single cells
3. Tracheids have a comparatively narrow diameter
4. They have large pits which are less in number
5. They are imperforated cells
6. They are less efficient in water conduction due to the absence of perforations
7. Walls are more thickened with narrow lumen
8. Average length upto 1 m. m (rarely 12 cms.)
1. Main conducting elements of angiosperms
2. Vessel members, which are joined into long continuous tubes, originate from longitudinal file of cells
3. Vessels have a comparatively wide diameter
4. They have small pits which are larger in number
5.They are perforated cells
6. They are more efficient in water conduction due to the presence of perforations
7. Walls are less thickened with large lumen
8. Average length upto 10 cms or more( Rarely 2-6 metres)