Everything Life Sciences Grade 10

epidermis

epidermal hair

collenchyma of the cortex

pith (parenchyma)

cortex (parenchyma)

endodermis

xylem

sclerenchyma of the vascular bundle

cambium vascular bundle

Cross-section of a Dicotyledonous Stem

phloem

medullary ray

(parenchyma cells in gaps

between vascular bundles)

Figure 6.12: Cross-section of a dicotyledonous stem showing tissue distribution.

Vascular cylinderorstele: comprised of thepericycle,vascular bundlesandpith

• Pericycle: made up mainly of lignified, dead, fibrous cells known as sclerenchyma
  cells. Sclerenchyma cells have end-to-end connections (tapering ends), and have ex-
  tremely thick walls that consist of lignin and/or cellulose. These thickened, woody cell
  walls are very hard and play an important role in strengthening the stem, and providing
  protection for the vascular bundles.


• Vascular bundles: characteristically organised in a ring inside the pericycle of the di-
  cot plant. Mature vascular bundles are made up of water-conducting xylem, cambium,
  and food-conducting phloem. The phloem is located on the outside of the bundle and
  the xylem towards the centre (see Figure 6.12). The phloem and xylem is separated by
  meristematic tissue known as cambium, which is responsible forsecondary thicken-
  ing. Xylem has lignified cell walls which helps it fulfil its two important roles, namely;
  strengthening and supporting the stem, and transporting water and minerals from the
  root system to the leaves. The function of phloem is to transport synthesised food from
  the leaves to other parts of the plant.


• Pith (or medulla): occupies the large, central part of the stem. The pith is made up of
  thin-walled parenchyma cells containing intercellular spaces. Where the parenchyma
  extends between vascular bundles as thin bands it is known as medullary rays, and
  can be continuous with the pith and cortex of the parenchyma. The cells of the pith
  store water and starch, while the intercellular spaces allow for gaseous exchange. The
  medullary rays facilitate transport of substances from xylem and phloem to the inner
  and outer parts of the stem.

Investigation: Examining the structure of the root and stem

Aim:

To examine the structure of the root and stem

Chapter 6. Support and transport systems in plants 165