Page 43 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

Sclerenchyma
1. Sclerenchyma is a simple tissue consisting of fibre cells or stone cells
(sclereids), which have thick walls impregnated with lignin.
2. The structural features are listed as follows:
(a) When sclerenchyma cells mature, the cells are dead and have no

2 protoplast.
(b) They have thick secondary walls impregnated with lignin. Lignin
is a branched polymer which makes the wall very hard and
impervious to water. The walls have many pits.
(c) Their lumens are very small and empty.
(d) The shapes of the cells depend on the types as shown in Figure
2.35.
Thick wall Thick wall

Small lumen
Small lumen Pits
A single fibre A stone cell
Figure 2.35 Two types of sclerenchyma
(i) Fibres
They are like fibres i.e. long and thin with two sharp ends.
They exist in bundles or layers.
(ii) Stone cells
They have a stone shape, usually exist as layers or scattered
stone cells.
3. Their distributions are as follows:
(a) Fibres are found beneath the epidermis and in the bundle sheaths
of monocotyledonous stems or leaves as shown in Figure 2.28
and Figure 2.33.
(b) Fibres are also found on the outside (Figure 2.26) or inside
(Figure 2.27) the phloem of dicotyledonous stems.
(c) Stone cells are found in fresh pears and ciku fruits, which give
them their gritty texture. They are found as hard protective
layers of plums, olive seeds and coconut shells.
4. Their functions are as follows:
(a) Fibres support and protect the plants especially in stems and
leaves.
(b) Stone cells protect seeds and prevent germination so that the
seeds can be dispersed further.

Xylem
1. Xylem is a complex vascular tissue, which is used to transport water
and support plants.
2. Xylem is divided into primary and secondary types. Primary xylem is
formed from procambium at the shoot or root tips. Secondary xylem
is formed from vascular cambium during secondary thickening or
wood formation in the stem or root.
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