side walls. Only through this conduction takes place in Gymnosperms. They are arranged one above the other. Tracheids are chief water conducting elements in Gymnosperms and Pteridophytes.
They also offer mechanical support to the plants. Annular Spiral Reticulate Scalariform Pitted thickening Figure . : Types of secondary wall thickenings in tracheids and vessels Vessels or Trachea Vessels are elongated tube like structure. They are dead cells formed from a row of vessel elements placed end to end.
They are perforated at the end walls. Their lumen is wider than Tracheids. Due to the dissolution of entire cell wall, a single pore is formed at the perforation plate. It is called simple perforation plate , Example: Mangifera .
If the perforation plate has many pores, it is called multiple perforation plate . Example Liriodendron. The secondary wall thickening of vessels are annular, spiral, scalariform, reticulate, or pitted as in tracheids, Vessels are chief water conducting elements in Angiosperms and absent in Pteridophytes and Gymnosperms. In G netum of Gymnosperm , vessels occur.
The main function is conduction of water, minerals and also offers mechanical strength. Xylem Fibre The fibres of sclerenchyma associated with the xylem are known as xylem fibres. Xylem fibres are dead cells and have lignified walls with narrow lumen. They cannot conduct water but being stronger provide mechanical strength.
They are present in both primary and secondary xylem. Xylem fibres are also called libriform fibres. The fibres are abundantly found in many plants. They occur in patches, in continuous bands and sometimes singly among other cells.
Between fibres and normal tracheids, there are many transitional forms which are neither typical fibres nor typical tracheids. The transitional types are designated as fibre- tracheids . The pits of fibre-tracheids are smaller than those of vessels and typical tracheids. Vessels are found in Gymnosperms like Ephedra, Gnetum and Welwitschia Vesselless angiospermic families Winteraceae, Tetracentraceae and Trochodendracae .
Xylem Parernchyma The parenchyma cells associated with the xylem are known as xylem parenchyma. These are the only living cells in xylem tissue. The cell wall is thin and made up of cellulose. Parenchyma arranged