Physical defences rely on barriers preventing
pathogens from getting into undamaged plants.
The leaves and stems of most plants have an outermost
layer of the covering that acts as a barrier to inhibit pathogens entering
from getting through the epidermis and damaging the leaves or stem.
The tough waxy cuticle also stops water accumulating on the leaves, so
reducing infection by pathogens that are transferred between plants via
(Note: The waxy cuticle also prevents excessive loss of water
from the leaves - it reduces the rate of evaporation so the plant does
not become dehydrated.)
Unlike animal cells, plant cells have a
stronger cell wall made of
This acts as a physical barrier against intrusion by a
pathogens that get through the waxy cuticle.
If pathogens do get past these physical defences,
their presence can trigger the cell to produce a chemical called callose. The callose is deposited between the plant cell walls and
the inner cell membranes to reinforce the cell wall.
Around their stems,
plants have layers of dead cells that acts as
a physical barrier to pathogens e.g. the bark on trees is he obvious
Pests cannot get to the healthy living cells
underneath the bark.
Plants show specific
adaptations to deter animals from even
touching, as well as not eating them.
You might call them mechanical defence systems! e.g.
Thorns and hairs
Thorns will prick insects and deter them from
egg laying. The sharp thorns will also deter grazing herbivore
animals from munching the plant and maybe wear down their teeth!
Hairs stop larvae from reaching and feeding on
the outer epidermis layers of leaves.
Some plant leaves droop or curl when touched by an animal.
When touched, some plants release chemicals
that releases water from vacuoles causing cells to collapse and the
leaves to curl.
help prevent being eaten by having the insects knocked off
Certain plants can
mimic other organisms
e.g. adaptations to inhibit animals like insects
from feeding or laying eggs on them.
Mimicry is displayed by the passion flower has bright yellow spots on its leaves that look
like butterfly eggs, deterring other butterflies from laying their eggs
on the leaves.
Some grasses have evolved anthers that look like
aphids and hollyhocks have stem markings that also look like aphids -
these visual signals deter aphids from attacking and feeding off the
Some species of plant in the 'ice plant family' in southern Africa
look like stones and pebbles, and so don't look very tasty to some
predatory animal, so they are far less likely to be eaten!
of biology notes on plant diseases
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