Everything Life Sciences Grade 10

FACT

In addition to

transporting water

and mineral salts

from roots to leaves,

xylem also provides

support to plants

and trees because of

its tough lignified

vessel elements.

We will now examine the complex permanent tissues. Remember the difference between
simple and complex permanent tissues is that simple permanent tissues are made up of cells
of the same type whereas complex permanent tissues are made up of more than one cell
type that combine to perform a particular function. We will examine thevasculartissues,
xylem and phloem tissues next.

Xylem tissue DUMMY

Xylem has the dual function of supporting the plant and transporting water and dissolved
mineral salts from the roots to the stems and leaves. It is made up of vessels, tracheids, fibres
and parenchyma cells. The vessels and tracheids are non-living at maturity and are hollow to
allow the transport of water. Both vessels and tracheids have lignin in their secondary walls,
which provides additional strength and support.

Xylem vesselsare composed of a long chain of straight, elongated, tough, dead cells known
as vessel elements. The vessel elements are long and hollow (lack protoplasm) and they make
a long tube because the cells are arranged end to end, and the point of contact between two
cells is dissolved away. The role of xylem vessels is to transport water from roots to leaves.
Xylem vessels often have patterns of thickening in their secondary walls. Secondary wall
thickening can be in the form of spirals, rings or pits.

Tracheidshave thick secondary cell walls and are tapered at the ends. The thick walls of
the tracheids provide support and tracheids do not have end openings like the vessels. The
tracheids’ ends overlap with one another, with pairs of pits present which allow water to pass
through horizontally from cell to cell.

Diagram Micrograph

Figure 5.15: Longitudinal section through a

xylem vessel to show hollow lumen to allow

for transport of water and nutrients.

Figure 5.16: Xylem vessel fibres with rings of

lignin thickening.

Chapter 5. Plant and animal tissues 127