FACT
The chemicals in
trichomes make
plants less easily
digested by hungry
animals and can
also slow down the
growth of fungus on
the plant. As such
they act as a form of
protection for the
plant against
predation.
Structure Function
Layer of cells covering surface of entire
plant.Acts as a barrier to fungi and other
microorganisms and pathogens.
Layer is thin and transparent. Allow for light to pass through, thereby
allowing for photosynthesis in the tissues
below.
Epidermal tissues have abundant
trichomeswhich are tiny hairs projecting
from surface of epidermis. Trichomes are
abundant in some plant leaves.Leaf trichomes trap water in the area
above the stomata and prevent water
loss.Root hairs are elongations of epidermal
cells in the root.Root hairs maximise the surface area
over which absorption of water from the
soil can occur.
Epidermal tissues in leaves are covered
with a waxycuticle.The waxy outer layer on the epidermis
prevents water loss from leaves.
Epidermal tissues containguard cells
containing chloroplasts.Guard cells control the opening and
closing of the pores known asstomata
thus controlling water loss in plants.
Some plant epidermal cells can secrete
poisonous or bad-tasting substances.The bitter taste of the substances deter
browsing and grazing by animals.Figure 5.6: Scanning electron microscope image ofNicotiana alata(tobacco plant) upper leaf surface,
showing trichomes (also known as ‘hairs’) and a few stomata.Guard cells and StomataAstomais a pore found in the leaf and stem epidermis that allows for gaseous exchange.
The stoma is bordered on either side by a pair of specialised cells known asguard cells.
Guard cells are bean shaped specialised epidermal cells found mainly on the lower surface
of leaves which are responsible for regulating the size of the stoma opening. Together, the
stoma and the guard cells are referred to asstomata.The stomata in the epidermis allow oxygen, carbon dioxide and water vapour to enter and
leave the leaf. The guard cells also contain chloroplasts for photosynthesis. Opening and
closing of the guard cells is determined by the turgor pressure of the two guard cells. The
turgor pressure is controlled by movements of large quantities of ions and sugar into the
guard cells. When guard cells take up these solutes, the water potential decreases causing
osmotic water to flow into the guard cells. This leads to an increase in the swelling of the
guard cells and the stomatal pores open.120 5.3. Plant tissues