The xylem is the vascular tissue responsible for the upward conduction of water and nutrients from the roots. The xylem tissue moves water and nutrients to various parts of the plant such as shoots and leaves. Its major components include:
- Xylem parenchyma
- Xylem fibers
- Xylem vessels
What Are Tracheids?
Tracheids are cells in the xylem of vascular plants that serve in the transportation of water and mineral salts. Tracheids are elongated tube-like cells with tapering ends and chisel like in appearance. The cells are non-living at their maturity and the mature cells are empty without protoplast.
In angiosperms, tracheids occur with other xylem elements, though in some pseudo-angiosperms (primitive angiosperms) such as drimys, tetracentron, the xylem is composed of only tracheids whereas in gymnosperms, major portion of the secondary xylem is composed of tracheids.
The average length of tracheid is 5-6 mm. Major portions of the cell wall of the tracheids are perforated with pits. Pits may be simple circular pits or advanced bordered pits. Tracheids also contain pit pairs along two adjacent tracheids at their common walls. The tracheids have highly lignified secondary cell wall and the cells angular and polygonal in cross-section. Tracheids provide most of the structural support in softwoods where they are the major cell type.
Characteristics Of Tracheids
- They are shorter cells (about 1mm long).
- Tracheids are present in all vascular plants (Pteridophytes and Gymnosperms).
- Tracheids originate from a single cell.
- They are more lignified and therefore have a narrow lumen.
- They are often considered to be primitive cells.
- The cells are long and tapering at both ends.
- The cells are dead and have bordered pits.
- They contain polygonal cross sections.
- They prevent air embolism in plants due to its high adhesion force in the narrow tube.
- They contain thin cell walls.
- They are less efficient in water conduction due to the absence of perforations.
- They consist of less number of large pits.
- Tracheids consist of a high surface area to volume ratio.
- The partition walls of the cells do not break down.
What Are Vessels?
The xylem vessels also referred to as trachea are long tubes which help in transportation of water and provide mechanical support. Each xylem vessels is arranged by end to end union of large number of short, wide, lignified dead cells to the long axis of the organ in which they occur. In these cells both the nucleus and cytoplasm are absent. The end walls of these cells are often completely broken to form a long distance channel for water transport. Components of the vessel are called vessel segments or vessel elements.
Each vessel element is shorter than the tracheids in their length; however, the diameter of the vessel lumen is much larger than that of tracheids. The cells are non-living and they do not contain protoplast at their maturity. The cell wall of the xylem vessels contain non-lignified thin areas referred to as pits.
Characteristics Of Vessels
- They are longer cells (about 10 cm long).
- Vessels are only present in angiosperms.
- Vessels originate from a longitudinal file of cells. They therefore produce continuous tubes.
- They are less lignified and therefore have a wide lumen.
- They are often considered as advanced type of cells.
- The cells are long, tubular and swollen at both ends.
- Simple pits are present in the walls of dead cells.
- They contain circular cross sections.
- They do not play any role in prevention of air embolism in plants.
- They contain highly thickened cell walls.
- They are more efficient in water conduction due to the presence of perforations.
- They contain a large number of small pits.
- A vessel consists of a low surface area to volume ratio.
- The partition walls form a tube after being broken down.
Difference Between Tracheids And Vessels In Tabular Form
|BASIS OF COMPARISON||TRACHEIDS||VESSELS |
|Length||They are shorter cells (about 1mm long).||They are longer cells (about 10 cm long).|
|Presence||Tracheids are present in all vascular plants (Pteridophytes and Gymnosperms).||Vessels are only present in angiosperms.|
|Origin||Tracheids originate from a single cell.||Vessels originate from a longitudinal file of cells.|
|Lignification||They are more lignified and therefore have a narrow lumen.||They are less lignified and therefore have a wide lumen.|
|Type Of Cells||They are often considered to be primitive cells.||They are often considered as advanced type of cells.|
|Structure||The cells are long and tapering at both ends.||The cells are long, tubular and swollen at both ends.|
|Pits||The cells are dead and have bordered pits.||Simple pits are present in the walls of dead cells.|
|Cross-section Shape||They contain polygonal cross sections.||They contain circular cross sections.|
|Role In Air Embolism||They prevent air embolism in plants due to its high adhesion force in the narrow tube.||They do not play any role in prevention of air embolism in plants.|
|Cell Walls||They contain thin cell walls.||They contain highly thickened cell walls.|
|Water Conduction Efficiency||They are less efficient in water conduction due to the absence of perforations.||They are more efficient in water conduction due to the presence of perforations.|
|Number Of Large Pits||They consist of less number of large pits.||They consist of less number of large pits.|
|Surface Area To Volume Ratio||Tracheids consist of a high surface area to volume ratio.||A vessel consists of a low surface area to volume ratio.|
|Partition Walls||The partition walls of the cells do not break down.||The partition walls form a tube after being broken down.|
What Are Similarities Between Tracheids And Vessel?
- Both tracheids and vessels are components of the xylem.
- Tracheids and vessels are both involved in conduction of water and providing mechanical support.
- Both xylem and tracheids form a secondary cell wall in between the primary cell wall and the plasma membrane that is lignified.
- They both have pits on their lateral walls.
- They both contain lignified cell wall.
- Both xylem and tracheids lose their protoplast at maturity and therefore become non-living components of the xylem eventually.
- They both contain tubular cells.