Roots, Stems and Leaves
Part 2
Leaves
• The structure of the leaf is optimized for
absorbing light and carrying out
photosynthesis. The major structures of the
leaf include:
• The blade – thin, flattened sections where
most of the photosynthesis takes place
• The petiole – a thin stalk that connects the
blade to the stem
Leaf Types
• Simple leaf – a single blade, which can be a
variety of shapes
• Compound leaf – the blade is divided into
many separate leaflets
Tissue Layers
• Leaves are covered on the top and bottom by
a layer of epidermis made of tough, irregularly
shaped cells. In many leaves the epidermis is
covered by the cuticle. These two layers form
a waterproof barrier to protect tissues and
limit the loss of water through evaporation
Stomata & Guard Cells
• The bottom layer of epidermis contain
stomata, which are pore-like openings to
allow carbon dioxide and oxygen to diffuse
into and out of the leaf
• Specialized epidermal cells called guard cells
control the opening and closing of the
stomata by responding to changes in water
pressure
Veins
• The xylem and phloem are gathered together
into bundles that run from the stem into the
petiole. Once in the blade of the leaf, these
vascular bundles are known as veins.
Mesophyll
• The bulk of the leave consists of a specialized
type of ground tissue known as mesophyll
(meso means middle, phyllon means leaf),
which is specialized for photosynthesis
– Palisade mesophyll is found below the epidermis
layer, tightly packed cells that absorb light
– Spongy mesophyll are the next layer, a loose
tissue with air spaces between the cells
Photosynthesis
• The main purpose of the leaves is as the
primary site of photosynthesis. In particular,
most of the photosynthesis takes place in the
mesophyll cells.
• They absorb light from the top of the leaf, and
carbon dioxide from the stomata.
• These leaves contain many chloroplasts,
which are the organelles that carry out
photosynthesis.
Transpiration
• The surfaces of the spongy mesophyll cells are
moist to increase the ability for gases to diffuse in
and out.
• Unfortunately, this also means that water easily
evaporates from the surface of these cells.
• Transpiration is the term that refers to this type
of evaporation from the leaves of a plant.
• This lost water is then replaced by water drawn
into the leaf through the vascular tissues.
Gas Exchange
• Leaves take in carbon dioxide and give off
oxygen during photosynthesis. Also, when
plants use cellular respiration to turn food
into quick energy (ATP), they use up oxygen
and give off carbon dioxide (just as animals
do).
Gas Exchange
• Plant leaves are specialized to allow gas
exchange.
• The spaces between the spongy mesophyll
cells and their moist surfaces allow optimal
conditions for gas exchange.
• The stomata are openings to the outside of
the plant, which is necessary for gas exchange,
since the rest of the leaf is essentially
waterproof.
Stomata & Guard Cells
• The opening and closing of the stomata is
controlled by the guard cells:
– Specialized epidermal cells on the underside of
the leaves
– Control the stomata to regulate the flow of gases
Guard Cells
• Respond to changes in water pressure
– High water pressure  guard cells curve, opening the
stomata
– Low water pressure  guard cells relax, closing the
stomata
• Generally stomata are open during the day to
allow photosynthesis and gas exchange to occur,
and closed at night
• Stomata can also be closed during hot, dry
conditions when water conservation is critical for
survival
Adaptations of Leaves
• The leaves of certain plants have specific
adaptations for dry or low-nutrient conditions
Pitcher Plant
• The pitcher plant has modified leaves to
attract and then digest insects and other
small prey. These plants usually live in
nutrient-poor soils, so they use the insects to
supplement their source of nitrogen.
• Venus Fly Trap & Pitcher Plant video
– https://www.youtube.com/watch?v=ktIGVtKdgwo
(3:30 min)
Cactus
• Cactus leaves are non-photosynthetic spines
or thorns that help to protect against being
eaten by herbivores. The cactus carries out
most of its photosynthesis on its stem.
Rock Plant
• The rock plant is adapted for hot, dry
conditions. These plants have round leaves
with very few stomata, and they have clear
tissue that allows light to penetrate into the
leaf.
Structure Video – Bozeman Science
• https://www.youtube.com/watch?v=zHp_voy
o7MY (14 min – stop at 10:50)
Water Transport
• Root pressure created by water entering the root
tissues is enough to push the water into the
vascular system and out of the root, however
root pressure alone isn’t enough to lift water
throughout the entire plant (especially a tall tree
like a giant redwood!)
• Plants must take advantage of some of the
unique properties of water in order to move
water throughout the plant body and to such
great heights.
Capillary Action
• There are 2 major forces that go into making
capillary action:
– Water molecules are attracted to each other by a
force called cohesion (the attraction of molecules
of the same substance to each other).
– Water molecules are also attracted to other
substances through a force called adhesion
(attraction between molecules of different
substances)
Capillary Action
• Capillary action occurs when water molecules
stick to the sides of a small tube through
adhesion, and then cohesion causes other water
molecules to be pulled up after them. The
thinner the tube, the higher the water will rise in
the tube.
• Both the tracheids and the vessel elements form
hollow connected tubes, and capillary action in
these tubes causes water to rise well above the
ground.
Transpiration
• For trees and other tall plants, root pressure
and capillary action together don’t provide
enough force to lift water to the top branches
and leaves. The remaining major force in
water transport is transpiration:
– Water is lost from the leaves through transpiration
– Osmotic pressure moves water from the vascular
tissue and into the leaf to replace the lost water
Transpiration
• A combination of osmotic pressure and
cohesion among the water molecules causes
water from lower in the plant to move higher,
all the way up from the roots.
– This is known as transpirational pull
Impact of Transpiration
• On a hot day, a small tree might lose as much
as 100 litres of water to transpiration
– Water loss increases with heat, dryness, and
windiness of the weather conditions
– As water loss increases, the plant must draw up
even more water from the roots
Controlling Transpiration
• The plant must control this rate of
transpiration to maintain optimal conditions
• When water is abundant, it flow into the leaf,
raising the water pressure in the guard cells
and keeping the stomata open
• When water is scarce, the water pressure in
the leaf and guard cells falls, and the stomata
closes, preventing further loss of water
Wilting
• Osmotic pressure also keeps plant stems and
leaves rigid and stiff by keeping the central
vacuole full of fluid, producing the turgor
pressure the plant cells need to keep their shape.
• When a plant loses too much water through
transpiration, the turgor pressure drops, the cell
walls bend inward, and the plant starts to wilt.
• Wilting also causes the guard cells to relax,
closing the stomata and reducing further water
loss. Therefore wilting actually helps a plant to
conserve water.
Water Movement Video
• https://www.youtube.com/watch?v=qvAG91p
yNKc&index=18&list=PL4LEUrNDNoyRQqVGW
Y-pMS_lbg2HLQjcm (3:30)
Functions of Phloem
• Most nutrients, including sugars, minerals, and
complex organic compounds are pushed through
the phloem. This process is used to move sugars
and other nutrients to where they are needed,
including:
– Plants push sugars and nutrients into fruits in order to
make them more attractive to animals, so they
increase their chances of spreading their seeds
– In cold climates, plants often pump food down into
their roots for winter storage, and then this food must
be pumped back into the stems and branches before
growth begins again in the spring.
From Source to Sink
• Sugars are moved through the phloem from a
source to a sink.
• A source is any cell where sugars are
produced by photosynthesis.
• A sink is any cell that uses sugars or stores
them.
• What causes this type of phloem transport?
Pressure Flow Hypothesis
One possible explanation is the pressure-flow
hypothesis.
• Sugars are pumped into the phloem at the source
(often a leaf)
• As concentrations of sugars increase in the
phloem, water moves in by osmosis
• This increases the water pressure at that point in
the phloem, forcing the fluid to move through
the phloem away from the source and toward a
sink
Pressure-Flow Hypothesis
• When a part of a plant (sink) actively absorbs
nutrients from the phloem, this reduces the
concentration - osmosis causes water to also
move away, reducing the water pressure
• This reduction in water pressure causes a
movement of sugar-rich fluid toward this area
Summary
• When nutrients are pumped into or out of the
phloem, the change in concentration causes a
movement of fluid in the same direction – this
allows phloem to transport nutrients in any
direction to meet the needs of the plant.
Summary Video
• https://www.youtube.com/watch?v=xGCnuXx
bZGk (5:30 min)
• If time, can also watch this summary video
(Bozeman Science) while you start studying
• https://www.youtube.com/watch?v=bsY8j8f5
4I0 (14 min)