3.4.5 Plant Excretion
Objectives – What you will need to
know from this section
 Outline the role of leaves and lenticels as excretory
organs of plants.
 Students should appreciate however that excretion is an
animal function and that secretion or loss from plant
would be more appropriate.
3.4.3
The Structure of an Exchange System
in Flowering Plants
 Plants absorb water through
their roots, but lose water
from their leaves by
evaporation (transpiration).
 Over 90% of the water that
enters a leaf goes right on
through it and evaporates
into the surrounding air.
 The evaporation of water
from leaves [and stems] is
called transpiration.
 In plants, cells become specialised into one of three tissue
types—dermal, ground and vascular.
LEAF TS
 Dermal Tissue forms and
protects the surfaces of the
plant.
 Waxes coat the outermost
cell wall, forming a
cuticle—this cuts
down water loss and is a
barrier to attacks by germs
or insects.
Four things are needed for photosynthesis:
SUNLIGHT
Gives the plant energy
CHLOROPHYLL
WATER
Travels up
from the roots
CARBON DIOXIDE
Enters the leaf through small
holes on the underneath
The green
stuff
where the
chemical
reactions
happen
LEARNING CHECK
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Plants absorb water from where?
Plants can lose water from where?
What is evaporation?
What is transpiration?
How do plants try and reduce water loss?
What do plants need for photosynthesis
Where does most photosynthesis take
place?
Structure of the Leaf
Lots of
chlorophyll
Large
surface area
Transparent
Thin
structure
Packed with
chloroplasts
Network of
veins
Lots of air
spaces
Holes
 Covering the
epidermis of the
leaf is a cuticle,
which reduces the
loss of water.
 A leaf’s veins are
vascular tissue—
xylem transports
water and phloem
carries sugars.
 Ground Tissue is
photosynthetic, with
large air spaces
between the cells, to
allow rapid diffusion
of gases
Leaf structure:
 The epidermis of stems and leaves have many holes [stomata]
to allow for gas exchange.
 O2 is excreted as a by-product of Photosynthesis
 CO2 and H2O are excreted as by-products of Respiration
 The loss of water is
reduced by a waxy
cuticle and by the
closing of the stomata
[pores] on the lower
surface of the leaf at
night-time.
Water loss
Water loss through the stomata is biggest on a hot,
dry, windy day. Plants that live in these conditions
often have a thicker waxy layer.
 The stomata stay open during the day so that CO2 can
enter, for photosynthesis. But, water can escape at this
time too. Stomata can stay closed at night, when there is
no light for photosynthesis. This allows plants to
conserve water.
Controlling water loss
Carbon dioxide enters a leaf through the ________. These
cells are also responsible for controlling the ______ content
of the leaf…
The ______ cells control how wide the
stomata opens (if at all). If too much
water is being lost through the stomata
then the guard cells will _____ to prevent
further loss.
Water and carbon
dioxide pass through
here
No more water
and carbon
dioxide allowed
through
What about trees and woody plants ?
 How do gases [oxygen and
carbon dioxide] get through
the waterproof, corky
surface of bark?
 They do so through
lenticels, which are areas
where the packing of bark
cells is loosened up a bit.
 Lenticels often look like tiny
raised blisters on a branch
or twig.
Winter twig – showing lenticels
LEARNING CHECK
• Explain the roles of the cuticle and stomata
in a leaf.
• What environmental conditions result in a
high transpiration rate?
• Why are stomata open during the day, but
closed at night?
• What are lenticels?
• How are lenticels different from stomata?