In woody stems the stomata are blocked by the presence of cork cells
The epidermis of woody stems breaks up to form tiny pores called lenticles which allow gaseous exchange.
Xylem tissue is made up of four types of cells
- xylem vessels
- xylem tracheids
- xylem sclerenchyma
- xylem parenchyma
Xylem vessels and xylem tracheids are highly specialised cells.
Xylem vessels are dead cells. They are long, cylindrical cells.
Their cross walls are perforated or completely absent and form continuous tubes from roots to leaves.
Xylem vessels have thick walls made up of lignin.
Lignin is laid down in various patterns
Annular thickening ( lignin laid in circles)
Spiral thickening ( lignin laid in spiral)
Scalariform thickening (laid in ladder form)
Pitted thickening (laid in pits)
Xylem tracheids are similar to xylem vessels except that the ends are tapered and cross walls are always present
Cross walls are perforated or completely absent – this forms xylem roots forms continuous tubes with the xylem stems and leaves.
Xylem vessles have no living contents – allows the water to flow freely inside.
The walls of the xylem vessels and tracheids are not completely thickened – the unthickened portions and bordered pits allow water to move across the root and stem
Adaptation of xylem to provide strength
the vessels and tracheids are thick-walled – they have a secondary wall of lignin
the vessels and tracheids are round in cross-section – round structures give additional strength
Phloem tissue is made up of four types of cells:
Sieve tubes and companion cells are highly specialised cells
Sieve tubes are living cells with long cylindrical cells.
Their cross walls are perforated to form sieve plates
They are thin walled cells
They contain strands of cytoplasm which run through the sieve plates from one cell to the next
Companion cells lie next to the sieve tubes – they are thin walled, with cross walls and has a well-defined nucleus