Support and transport in plants

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Slide 18

Annual Rings

Slide 19

Formation of Lenticles

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.

Slide 20

XYLEM

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

Slide 21

Xylem Vessels

Annular thickening

Scalariform

Thickening

Spiral

Thickening

Pitted Thickening

Xylem Tracheids

Slide 22

Adaptation of xylem to transport

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

Slide 23

Phloem

Phloem tissue is made up of four types of cells:

Sieve tubes

Companion cells

Phloem parenchyma

Phloem sclerenchyma

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