Leaf_fig_labeled
leaf_fig
Above: a slice through part of a leaf, with the cells shown in section. BS,
bundle-sheaf; C, cuticle; GC, guard cell; LE, lower (abaxial) epidermis; P,
phloem; PM, palisade mesophyll; S, stoma; SA, sub-stomatal air space;
SM, spongy mesophyll; UE, upper (adaxial) epidermis; X, xylem. Note: the
lower epidermis also has a cuticle, but this is much thinner than that on
the upper epidermis.

Below: a photomicrograph of a section through a leaf, with part of a vein
shown in longitudinal section. BS, bundle sheaf;C, cuticle; E, epidermis;
P, phloem; S, stoma; X, xylem.
Internal Links:

Canopy - a discussion of the woodland canopy with pictures of  leaves, leaf venation and stomata.

Leaf-litter - the importance of shed leaves in the woodland ecosystem.

Transport in Plants - movement of water and nutrients in the plant body.
The Purpose of Leaves

The function of leaves is to provide the plant with food through photosynthesis. The leaf is
designed to intercept the optimum level of light and to absorb sufficient carbon dioxide gas from the
air. They harness the energy in the light to fix the carbon dioxide - turn into the non-gaseous form
of sugars, to begin with. Sugars are
carbohydrates and so contain carbon, in this case obtained
from the carbon dioxide, and may be converted into another carbohydrate, starch, which is a solid
sugar polymer and can be stored inside plant cells as starch grains. Some of the sugars are
converted into amino acids, the building blocks of
proteins, and others into oils (lipids) and
nucleotides, the building blocks of
nucleic acids (DNA and RNA).

Photosynthesis occurs in the mesophyll cells (especially the palisade mesophyll) and to a lesser
extent in other green parts of the plant. Note that the epidermal cells do not have chloroplasts,
except for the guard cells. The chlorophyll resides inside organelles called chloroplasts, in the
thylakoid membranes.

So, why have lots of small leaves instead of a solid canopy or dome? There are several advantages
to having lots of separate leaves:

  1. They are easily replaced.
  2. They are less crowded and so avoid competition for light and carbon dioxide.
  3. Smaller leaves have smaller boundary layers and so can obtain fresh carbon dioxide more
    easily.
  4. Smaller boundary layers allow leaves to cool more easily through transpiration.
  5. Small leaves can be easily moved about to position them for maximum light absorption.
  6. Small leaves can be easily vibrated by gentle winds, further reducing boundary layers.
  7. The leaves are thin and plate-like to reduce weight and to reduce the amount of tissue that
    will not receive adequate amounts of light for photosynthesis in the centre of the leaf.

Boundary layers

In a fluid (gas or liquid) every solid surface is surrounded by a layer of still-moving or stagnant fluid,
called the boundary layer. As fluid moves around in mass (mass flow, e.g. by advection or wind)
and sweeps past solid surfaces, the fluid tends to stick to the surface (due to its viscosity) and gets
dragged back by it. Where the boundary layer meets the surface, there will be a molecular layer of
fluid covering the surface which is stationary, this is the non-slip boundary condition. Above this is
the boundary layer, over which successive 'layers' of fluid move progressively faster and faster, but
are still dragged back as they stick to the slower moving layers of fluid beneath them. Eventually, at
a certain distance from the surface the fluid will be moving with its free-flowing velocity.

Boundary layers have lots of important implications in biological systems. In the case of leaves,
carbon dioxide must diffuse across the boundary layer to the pores in the leaf, called
stomata
(sing. stoma). Once inside the leaf the carbon dioxide diffuses throughout the leaf in an inter-
conneceted network of air spaces which are most developed in the
spongy mesophyll which
makes up the bulk of the packing tissue of the leaf, but the air spaces also connect to the
palisade
meosphyll
. Photosynthesis, in which the carbon dioxide is used, occurs in the mesophyll cells, both
spongy and palisade, but especially in the palisade mesophyll. The epidermis, with the exception of
the guard cells is not photosynthetic. The mesophyll cells are green, since they contain
chloroplasts
- descendants of photosynthetic bacteria which contain the green pigment
chlorophyll in special
membranes called thylakoid membranes. It is the chlorophyll that taps the light energy and converts
it into chemical energy to make food, fuel and chemical building blocks for the plant.

The palisade mesophyll cells are column or plate-like and all arranged in a close array with their
chloroplasts aligned and stacked in such a way as to intercept as much light as possible. The
palisade meosphyll is the greenest tissue in the leaf, since these cells contain the most chloroplasts
and carry out most of the photosynthesis. Light not intercepted by the palisade cells may be
intercepted by the spongy mesophyll lining the air spaces. Failing that, some of the light passing
through the leaf may be intercepted by leaves in lower layers of the tree canopy.
Shade-leaves
occur lower down on a tree and typically have only a single layer of palisade cells and are set to
photosynthesise at lower light levels, but they can be partially bleached and damaged by very
intense sunlight.
Sun-leaves higher up in the canopy may have 2-3 layers of palisade cells and
can photosynthesise at higher light intensities.
The Leaf - Solar Power!
Photosynthesis is the process whereby plants, some bacteria and algae use light energy from
sunlight to convert (or fix) inorganic carbon, typically in the form of carbon dioxide, into more complex
organic carbon molecules like sugars, fatty acids and amino acids. The light energy is therefore
converted into chemical bond energy.

The Biochemistry of Photosynthesis

Photosynthesis is completed in two stages: the light-dependent stage and the light-independent
stage.

1. Light-dependent Reaction:

This reaction uses the energy of sunlight to generate NADPH2 (a reduced coenzyme) which carries
hydrogen from NADP (nicotinamide adenine dinucleotide phosphate) - essentially it carries the
electron from hydrogen and is an
electron/hydrogen carrier:
And also generates ATP (adenosine trisphosphate), and produces oxygen from water. The hydrogen
comes from water, which is split into hydrogen and oxygen. Some of the oxygen is used by the plant
in
respiration (burning fuels for energy) but more is generated than needed and the rest is given up
to the atmosphere as it diffuses out of the stomata. ATP is the energy currency of the cell - like
tokens for an electricity meter, ATP molecules carry small packets of energy that are used in most of
the cell's energy-requiring processes (that is processes requiring energy that cannot be obtained
from the ambient heat energy, since all processes really require energy in one form or another).

The physical reaction of light with chlorophyll

Plants contain two types of chlorophyll, chlorophyll-a (chl-a) and chlorophyll-b (chl-b). Each pigment
molecule can absorb a single photon of light. The likelihood that a photon will be absorbed depends
upon the wavelength of the photon (the wavelength of the photons is what gives light its colour).
When a pigment molecule, say a molecule of chl-a absorbs a photon, the molecule enters an excited
state (a high-energy state). These excited states are very short-lived and must be ‘captured’ by the
cell and converted into a more stable and useful form of energy.

Some light energy is also collected by accessory pigments (chl-b and carotenoids) and transferred (in
small packets called quanta) to chl-a, which in turn transfers this energy to a special molecules of chl-
a (the primary pigment molecules) that form the reaction centres. The accessory or antenna pigments
increase the range of photon wavelengths that can be absorbed. This arrangement of pigments is
called a
photosystem and is diagrammed below.
Photosystem
Photosystems

Chlorophylls a and b are physically and functionally grouped together into photosystems. There are
two different photosystems, photosystem I (PSI) and photosystem II (PSII).

The most abundant chlorophyll is chl-a, the primary pigment which forms the reaction centre of each
photosystem.

Note that there are several different modified forms of chl-a and chl-b, each with a slightly different
absorption spectrum (they absorb photons with different ranges of wavelengths).

The chl-a in PSII has an absorption peak at 680 nm and so its reaction centre is called P680.
The chl-a in PSI has an absorption peak at 700 nm and so its reaction centre is called P700.

Accessory pigments consist of chlorophyll b, and the carotenoids. The carotenoids are a group of
pigments, including two sub-groups: the carotenes, which are hydrocarbons, e.g. b-carotene and
the xanthophylls, which are oxygenated organic molecules, e.g. lutein in green plants and
fucoxanthin in brown algae. Carotenoids absorb mostly in the blue-violet and also in the green part
of the spectrum.

Q. What colour would you expect carotenoids to be?
A. As they absorb blue-violet light and reflect/transmit red-yellow light we would expect them to be
yellow, orange or red. Carotenoids give carrots their orange colour.

Q. What gives autumn leaves their characteristic hues?
A. Just before leaves are shed from a deciduous plant in autumn, the plant reabsorbs many useful
nutrients from the leaf, including chlorophyll. Carotenoids remain and these are partly responsible
for the colours of autumn leaf fall.

Accessory pigments broaden the range of wavelengths that can be used for photosynthesis.
(Compare the action spectrum with the absorption spectrum of chlorophyll a). They also help protect
the chlorophyll from being oxidised and broken down in the light. (Other photosynthetic organisms,
such as blue-green
cyanobacteria and red algae have different accessory pigments. Green algae
have very similar pigment systems to green plants).

Q. Which plant organs are photosynthetic?
A. All the green parts - leaves and green stems and branches. this may include the young stems of
woody plants, but not the older non-green woody parts.

Q. Where are the photosystems/chlorophyll located?
A. In the thylakoid membranes of the chloroplasts in photosynthetic cells.

There are two different photosystems that work together, photosystem I (PS I) and photosystem II
(PS II). These photosystems have slightly different types of chl-a in their reaction centres. The chl-a
in PS I has a peak absorption of light at 700 nm and is called P700.

Q. The chl-a in the reaction centre of PS II is also called P680, why?
A. Because its peak absorption of light is at a wavelength of 680 nm (nanometres).


The conversion of energy from P700 and P680 into useful chemical energy

Now here comes the hard part! Here is a diagram called the z-scheme, which shall be explained
below:
Z-scheme
Cells harness chemical energy, whether by respiration or photosynthesis, by exchanging electrons
between chains of molecules which are called electron transport chains (ETCs) or electron transport
systems (ETSs). When molecules or atoms react chemically, it is movement and changes in energy of
the electrons in the atoms that bring about the reaction. When chlorophyll absorbs a photon of light,
one of its electrons gains the energy of the photon and the molecule and this electron are said to be in
a high-energy or
excited state (indicated by an asterisk *). In order to maximise the amount of this
energy that is harvested, the electron is de-excited, or made to give up its gained energy, by a chain of
chemical reactions, which extract its energy in a series of small steps. Chemical reactions in which
electrons are exchanged are called redox (reduction-oxidation) reactions. When a chemical molecule
gains an electron it is said to be reduced, and when it loses an electron it is said to oxidised.
These electrons, released from the splitting of water molecules, reduce the oxidised chlorophylls back to
energy. This splitting of water, which is ultimately driven by the energy in the absorbed light, is called
photolysis.

Q. What happens to the oxygen released from the water?
A. Some of it is used by the plant in respiration, the rest passes out through the stomata and into the air.

The H+ ions (protons) are released into the lumen of the thylakoid, which thus becomes high in proton
concentration. Other protons (2 per electron) are pumped from the stroma into the thylakoid membrane
by cytochrome (and its associated molecules in the ETC). The stroma is thus kept low in proton
concentration. The pH of the thylakoid is much lower than that of the stroma. These factors establish an
electrochemical gradient across the thylakoid membrane. The thylakoid membrane is imperemeable to
protons, except where there are protein pores, each leading to an ATPase. One molecule of ATP is
synthesised for every 3 or 4 (probably 4) protons that flow through the ATPase-pore protein complex.

Q. How many photons must be absorbed to produce one molecule of O2 by splitting water?

Q. What are the products of non-cyclic photophosphorylation?
A. ATP (but no NADPH).

The protons that diffuse through the ATPase-pore complex end-up in the stroma, where they may
combine with NADP+, along with electrons from the ETC, to reduce NADP+ to NADPH2:
Cyclic Photophosphorylation

The only product of cyclic photophosphorylation is ATP.

This process is an alternative route to non-cyclic photophosphorylation for electron flow.

The P700 chl-a molecule in the reaction centre of PSI absorbs a photon to become excited P*700 as
before. This electron is then passed on to the first electron-carrier of the ETC as before, and flows down
the ETC to reduce ferredoxin (Fd) to Fdr, but rather than reducing NADP+, this electron returns
immediately to PSI via cytochrome pigments. (Remember in non-cyclic photophosphorylation, the
electron lost by P700 would be replaced by an electron flowing down the ETC from P680 in PSII).
The cytochromes then utilise the electron’s energy to pump protons from the stroma into the thylakoid
lumen, for ATP synthesis. (Again 2 protons are pumped per electron, requiring two electrons to give the
4 protons required to synthesise one molecule of ATP).
Sometimes an electron will flow via the cyclic route, sometimes via the non-cyclic route.


2. Light-Independent Reaction:

Q. What happens to the ATP and NADPH2 produced by the light-dependent reaction? Where does the
carbon dioxide come into photosynthesis?

A. The light-independent reaction uses the energy from the ATP made in the light-dependent reaction
and the reducing power of the NADH2 to fix carbon dioxide gas into organic carbon.
Initially the carbon dioxide is fixed by adding it to 5-carbon pentose CO2-acceptor molecule (called
ribulose bisphosphate (RuBP), to form a 6-carbon compound. The enzyme that catalyses this CO2-
fixation reaction is called ribulose bisphosphate carboxylase (RuBisCo).
Rubisco is the most
abundant protein on the Earth!
(There are about 10 kg of rubisco per person). About 20% of leaf dry
weight is Rubisco.
The Calvin Cycle

Also called the Benson-Calvin cycle, although should more properly be called something like the
BCB-cycle (Benson-Calvin-Bassham cycle) after its discoverers. It is a biochemical cycle - a chain of
chemical reactions in which the final reaction in the chain links back to the first reaction, forming a circle.
These reactions take place in the stroma of the chloroplast. The sugar ribulose, a sugar containing 5
carbon atoms is phosphoryl;ated - a phosphate group is added to it, forming ribulose phosphate.
Another phosphate is added, forming ribulose bisphosphate (also called ribulose diphosphate or
ribulose biphosphate, the 's' is inserted when ambiguity may arise in nomenclature). The carbon dioxide
(CO2) is added to ribulose bisphosphate to form an unstable six-carbon intermediate with a carboxylic
acid group added to it. This unstable intermediate rapidly splits into two 3 carbon molecules of
phosphoglyceric acid (phosphoglycerate, PGA); (a carboxylic acid group is -COOH and when ionised to
carboxylate we have -COO with a minus charge, so CO2 gives us the -COO carboxylate group tagged
onto the ribulose bisphosphate).
The Z-scheme illustrates the transfer of the electrons from molecule to molecule. The blue arrows
indicate the flow of electrons through an electron transport chain (ETC). Each photon absorbed by
P680 pumps an electron up the z, that is the electron gains energy in the excited state of P680, which
is indicated by an asterisk: P*680 and this gain in energy is shown by an orange arrow. This electron
(Fd becomes reduced ferredoxin, Fdr) when it also absorbed a photon to become excited (P*700).
The electron lost by P680 is replaced by ‘stealing’ an electron from a hydrogen in water, splitting the
water into oxygen and protons or hydrogen ions, H+). As the electron loses energy in each successive
step, it gradually loses its power to bring about redox reactions - its
redox potential falls.

The Z-scheme illustrates a process called
non-cyclic photophosphorylation. Phosophorylation is
the addition of the element phosphorus, P (as phosphate) to a molecule. Photophosphorylation uses
energy derives from photons to do the phosphorylating. In this case P is being added to ADP
(adenosine diphosphate) to make the vital ATP. It is non-cyclic because the electrons travel in a Z-
shape (in redox potential space) and do not end where they started.

Q. What do you think that ‘cyt’ is an abbreviation for? What does it do?
A. 'Cytochrome' - an iron (haem) containing protein that transports electrons in redox reactions.

Q. How is the H+ from water is used in ATP production?
A. It is pumped into the intermembrane space of the chloroplast and then its electropotential energy is
harvested to make ATP in the same manner as in
respiration in mitochondria.
OIL RIG
Oxidation is loss of electrons, reduction is gain
Above: the Z-scheme represented in physical space, showing the arrangement of the participating
molecules in the thylakoid membrane of the chloroplast.
Q. What happens to the NADPH (NADPH + H+ or NADPH2) synthesised in non-cyclic photophosphorylation?
A. The hydrogen (and its still energetic electron) is used in redox reactions in the Calvin cycle, which synthesises
This carbon fixation or carboxylation is the first stage and is shown in detail below. This reaction is one of the most
important on Earth, second only to the reactions of respiration.
Note that of the two PGA molecules produced for each carbon dioxide molecule fixed, only one of these will contain the
new carbon atom (the other carbon atoms came from previous rounds of the cycle). The second stage is to convert this
fixed carbon into useful organic molecules - the lipids, carbohydrates, nucleic acids and amino acids. These
organic
building blocks are based on hydrocarbon skeletons (hydrocarbons are molecules of carbon and hydrogen only), with
various functional groups (chemical groups containing more reactive elements such as O, S, P, N and halogens) added.
Thus, adding carbon will not give us organic molecules - we need hydrogen too. Addition of hydrogen is also defined as
reduction (since hydrogen tends to donate electrons, so reduction is also addition of hydrogen; in like manner oxidation
is also defined as addition of oxygen, since oxygen tends to remove electrons). This is where the electron/hydrogen
carrier NADP comes in. The light-dependent reactions used light energy to reduce NADP to NADPH (in addition to using
some of the light energy to make ATP). This NADPH (also written NADPH2) carries the hydrogen to the PGA, which is
itself energised by phosphorylation by ATP to diphosphoglycerate which reacts with the NADPH, picking up the
hydrogen to form glutaraldehyde phosphate (GALP).

Some of this GALP is used to regenerate ribulose phosphate to complete the cycle. Some is siphoned off from the cycle
and drawn into other complex biosynthetic pathways where it is converted ultimately into sugars (which maybe stored as
starch) amino acids and other organic building blocks as needed.

The light-independent reactions are sometimes called 'dark reactions' though this terminology was disfavoured, since it
implies the requirement for darkness. In fact plants that use the cycles we have described (C3 plants since PGA, the
product of carbon-fixation is a 3 carbon compound) fix 99% of their carbon during the daytime.

C4 and CAM Pathways

Some 99.6% of plants, including all eukaryotic algae, bryophytes and gymnosperms, undergo the C3 light-independent
reaction mechanism outlined above. However, some tropical plants, many of them grasses (including sugarcane, maize
and sorghum) have a different mechanism which is more efficient in high light intensities and at high temperatures.
These fix carbon in a different way. They fix carbon dioxide (actually in the form of bicarbonate ions) using an enzyme
called phosphoenol pyuruvate carboxylase. This adds the carbon to a three carbon-containing molecule called
phosphoenol pyruvate, forming a four-carbon compound (hence C4) called oxaloacetate, which is rapidly converted into
malate (4C). This reaction occurs in the cytosol. The malate is then shipped into the chloroplasts, where it reacts to
release its carbon into the calvin Cycle.
The Structural Mechanics of Leaves

Many textbooks consider the important biochemistry in depth, but neglect to discuss the mechanical adaptations of
leaves, which are also important.

The photosynthetic parenchyma cells are swollen when turgid (that is when their vacuoles are full with sap), their cell
walls stretch and they push against one-another. This gives stiffness to the leaf and supports it. The parenchyma are
functioning as what engineers call a 'pressurised cellular solid'. Polystyrene is another example, in this case the cells
are tiny polystyrene balls, spongy balls filled with air spaces. When polystyrene is stressed the air inside the air
spaces within each ball is squeezed and pressurised. This combination of solid struts and pressurised fluid confers
stiffness. In parenchyma, we have water instead of air and the cellulose cell walls form the tiny struts and columns.
When a leaf loses excess water, the pressure within the parenchyma cells drops as their vacuoles shrink and the leaf
loses stiffness and wilts.

The veins of the leaf also provide support. In addition to the vascular tissues, especially the xylem and more so
sclerenchymatous fibres frequently underlie the main vascular bundles, such as that in the midrib. Sclerenchyma
contributes about 25 times more stiffness to a leaf than the conducting vascular cells. In some grasses, 90-9% of the
leaf's stiffness may be due to these sclerenchyma fibres. Sclerenchyma form tough cords that are especially good at
resisting tension. Additionally, some of the parenchyma in the midrib, especially those cells just beneath the
epidermis, may develop into collenchyma, especially in young leaves. (In older leaves the parenchyma/collenchyma
develop (secondary) lignified walls and become somewhat 'woody'). Young collenchyma cells are plastic and
viscoelastic, meaning that their cell walls permanently stretch as the leaf grows and collenchyma is excellent tissue
for providing support in rapidly growing organs. The epidermis and mesophyll of the leaf act like webbing, and the
sclerenchyma as taught cords or cables under tension. (This is reminiscent of the construction of an aircraft wing).
Calculations show that the optimum spacing of these supportive cables is about 30 times their width, which is about
the spacing between main veins in say the parallel-veins of a monocot leaf. Leaves are light, reducing the loading on
the branch and reducing the amount of non-photosynthetic tissue needed to support them. Some leave scan be
remarkably tough for such thin and light structures.

Many leaves have a corrugated appearance. Corrugated sheets are much stiffer than simple flat sheets. The fan
palm (
Corypha umbellata) produces the World's largest leaves and these are corrugated.
Above: The parenchyma (P) in region P1 tend to become sclerified (transforming into sclerenchyma fibres), though
the extent of sclerification depends on the age of the leaf and the forces it is subjected to (I can see little signs of it in
this section). Similarly parenchyma in region P2 tend to develop thickened walls (especially at the corners) and
transform into
collenchyma (there is some evidence that this process has started in this leaf). LE, lower epidermis; Ph,
phloem; X, xylem; UE, upper epidermis. Bundles of sclerenchyma fibres also tend to develop in the phloem (a normal
feature of this tissue).

It has been shown experimentally that the non-photosynthetic cells (sclerenchyma, or sometimes lignified (woody)
cells) that develop from the parenchyma above the vascular bundles, as in the midrib, actually enhance
photosynthesis. At first one might think that the loss of photosynthetic parenchyma cells, which have transformed into
non-photosynthetic sclerenchyma, would reduce the photosynthetic activity of the leaf. However, the sclerenchyma
act as lenses and not only emit light but to some extent magnify it and channel it to the parenchyma deep within the
leaf. This occurs in leaves in which the sclerenchyma in the bundle sheath extensions extends all the way from the
epidermis to the vascular bundle, such as in the common grapevine (Karabourniotis
et al., 2000).

Divided or 'dissected'  leaves, in which the leaf blade is deeply lobed or the leaf divided completely into leaflets
(compound leaves, e.g. horse chestnut) may have reduced loading, since wind will tend to pass through the leaf more
easily. Both simple leaves and the leaflets of compound leaves are flexible enough to tend to align their axes in the
direction of wind and also fold upon themselves in high winds. This flagging or lateral collapse reduces the surface
area of leaves exposed to the wind, reducing drag and wind loading on the branch. Leaflets also tend to oscillate at
different
natural frequencies, which collectively damps the system and dissipates kinetic energy more quickly.
Contrast this with the 1940 Tacoma Narrows Bridge, nicknamed 'Galloping Gertie', a suspension bridge that famously
collapsed. It lacked enough gaps to allow wind through and was finally brought down by wind
resonance. Any object
has a natural frequency and if a periodic force or vibration is applied to the object at this frequency, then resonance
occurs and the object vibrates particularly strongly. (Think of pushing a swing, to get it to go higher you have to push
it at the right time in each swing, according to the swing frequency). Resonance is also one of the main factors
causing tree failure in high winds, and tends to bring down younger trees more easily (young trees put their energies
into upwards growth, to reach the light before their competitors, before emphasising girth and so are generally more
vulnerable).

Keeping Cool

Another problem leaves face is overheating in the Sun! The pulvinus hinge causes leaves to flutter easily in the
breeze, which mixes the air around the leaves, removing carbon dioxide depleted air and supplying the leaf with fresh
carbon dioxide. This will also increase cooling. Dissected leaves will also cool more easily if the lobes are large
enough to break-up the boundary layer of still air that coats the leaf. However, this will also enhance evaporation and
water loss. A leaf has to strike a balance between cooling and water-conservation.

Leaves on the same plant respond to the stresses placed upon them as they grow and so we have sun leaves and
shade leaves, which in some plants (like ivy) may have completely different shapes. Shade leaves tend to be larger
and less dissected and so have greater boundary layer resistance and so cool by convection less easily and may
overheat in full sunlight. (Something to consider when pruning plants!).

Sun Leaves and Shade Leaves

Sun leaves, those that grow in direct sunlight, tend to have a smaller surface area and are thicker because they have
more palisade mesophyll (typically 2-3 layers) with longer (taller) palisade cells. This does not hinder carbon dioxide
diffusion enough for the supply of carbon dioxide to be aversely affected, and light can reflect of the cylindrical
palisade cells deeper into the leaf, so the spongy mesophyll still receive adequate light. These changes are
stimulated mainly by the amount of light striking the leaf. Warmer leaves also tend to produce smaller cells, as do
leaves which experience more water stress. (Note, however, that increased salinity stimulates leaves to grow thicker
with larger cells).

Shade leaves typically have only a single layer of palisade mesophyll and are less dissected. They have higher
specific leaf areas (the specific leaf area or SLA is the ratio of leaf area to leaf weight) since they are thinner, whilst
dry leaf weight does not change very much. Leaves tend to adapt by changing their area and morphology whilst
keeping the dry weight of the leaf more-or-less constant.

Leaf Orientation

Leaves will reorient as they grow, positioning themselves to catch the optimum amount of sunlight. In some plants the
leaves move on a daily or hourly basis in response to changing light. Light intensity (more specifically the density of
photons within the right frequency range to be usable in photosynthesis, the photosynthetic photon flux density or
PPFD) obviously falls deeper within the canopy as overlying leaves absorb much of the down-welling light. You may
have also noticed that the angle of leaves changes with height up a stem, or in a tree canopy. The topmost leaves
tend to be approximately vertical, this angle changes down the stem and the basal leaves are often almost horizontal.
This means that the vertical leaves at the top of the plant, exposed to full sunlight, do not absorb more light they can
use. When the noonday Sun is directly overhead, these vertical leaves will absorb what they need whilst reflecting
more light to the leaves lower down. This arrangement ensures that most leaves absorb neither more light than they
can utilise (the photosynthetic machinery saturates in very bright light) nor too little. If a leaf receives too little then it
produces more carbon dioxide by respiration than it produces by photosynthesis and the leaf is operating at a net
loss. The light intensity that causes photosynthesis to exactly balance respiration in the leaf is called the
light
compensation point
and is the lowest level of illumination at which the leaf breaks even.

This positioning of leaves can even be seen in plant communities. In grassland, grass is often the tallest plant and
has vertical leaves that intercept a smaller fraction of the bright sunlight, whilst shorter plants (like dandelions) tend to
have flat leaves to intercept more of the dimmer light that passes through the grass layer.

The take home message is that plants have to optimise their 'design' in response to various needs: light, carbon
dioxide, and engineering constraints: temperature, water loss and mechanical strength. This is achieved by a
combination of hardwired genetic pre-programing and adaptive response to environmental signals that the plant and
its cell sense and react to in an 'intelligent' manner. Often this involves an optimum compromise. It is this adaptability
of life and its ability to extract useful information from the environment and select (compute) an appropriate response,
and so maximise survival, that makes living things so remarkable!
A transverse section (T.S.) through the
midrib of a privet leaf (
Ligustrum).
Leaf Margins

If you examine the margin of a leaf, you will often see that the margin is usually not smooth but variously sculpted in a
pattern depending on genus, species or variety. The margin is often serrated or divided into a few rounded or pointed
lobes, or with lots of small lobes or teeth. Various technical names are given to these leaf margin shapes as they are
important in taxonomy. Sometimes the margin bears spines, as in holly. Under the microscope additional features
become visible - typically leaf vessels run toward the margin and sometimes breach it, in which case they may end in
spines, or form a toughened edge to the leaf by following along the course of the margin. In the soft leaves of elm
(
Ulmus) several veins typically join together at the bases of the clefts, forming conspicuous junctions (anastomoses),
areas of tough vascular tissue, between the serrations and one vessel often follows the contour of the leaf, forming a
tough leaf edge and linking the vessel anastomoses together. The purpose of this arrangement has eluded
explanation, but one function is surely mechanical. Leaves without strengthening veins would be prone to tear as they
twist about in the wind or upon dehydration. These tearing forces will tend to focus at the pointed clefts between the
serrations, but here they encounter the tough groupings of vascular tissue where the anastomoses are and this will
resist tearing, as will the toughened vascular edge characteristic of many leaves.

The spines of holly are possibly a defence mechanism against herbivory, making it harder for herbivores to eat these
leaves. This might explain why leaves lower down on the plant, in range of browsers, tend to be more spiky. However,
experiments have failed to show that herbivores avoid spiny holly leaves when given a choice. It is also possible that
the spines serve a mechanical function, possibly taken up the stresses when a leaf tends to twist. In evergreen holly,
leaves may freeze at winter and are subjected to harsh mechanical stresses when this happens, but the toughness of
the leaves maintains their shape and so reduces damage when the leaves freeze and thaw. Perhaps the spines assist
in this.

Leaf Hairs

Trichomes are highly variable appendages of the epidermis, and are often useful in taxonomy - distinguishing genera,
species and even varieties or subspecies. They may be extensions of single epidermal cells, or consist of several cells.
They may be non-glandular or glandular (secretory), scale-like, wart-like or hair-like. Some are beautiful and elaborate
umbrella-like or scale-like structures on stalks, and some are branches and tree-like in shape, others star-shaped.
Again the functions of these leaves is often not clear. They possibly create a thicker boundary layer of still air over the
surface of the leaf, reducing water-loss, though experimental evidence seems unclear on this. They may insulate the
mesophyll from excessive heat, especially when one considers that many of the hairs are hollow (air-filled?) and silvery
in appearance and so quite reflective. Although they may occur on both surfaces of the leaf, they are often densest on
the undersurface (where the stomata are located and where water-loss will be greatest).

Leaves with a rough texture (said to be 'scabrid', especially when the finger is gently rubbed against the 'grain' of these
hairs from the base of the leaf towards the tip typically have hard and rigid spike or hook-like hairs, curved slightly and
pointing towards the leaf base. These hairs have, at least in some cases, been shown to impale insect grubs crawling
up the leaf to feed, and so serve a clear protective function. Some plants, however, have very thin, flexible and soft
hairs, the function of which is not clear, but when in great density probably create a significant boundary layer of still air
across the surface of the leaf. Sometimes the hairs can be so dense as to appear cobweb-like (arachnose hairs).

Hairs on the undersurface of leaves may occur exclusively or more commonly along the veins, again suggesting a
protective function as many insects target the veins to reach the plant sap. Glandular hairs may also serve a protective
function. The minor elm,
Ulmus minor, has many minute red glandular hairs on the lower leaf surface, each only about
0.1 mm long, each consisting of a basal cell, a stalk cell and a pair of red spherical-ovoid glandular apical cells at the
tip. These spherical capsules seem to explode on contact, releasing their contents, and it has been suggested that
they may serve a protective function against mites or herbivorous insects. These glands are especially dense in young
leaves. Elm leaves have up to three prominent hair types (scabrid, soft and curly and glandular) and tuft of hairs may
occur in the axils where the main veins branch off from the midrib (to protect against sap-sucking insects?) and these
hairs are useful in distinguishing some species of elm from one-another (see:
http://www.s231645534.websitehome.co.uk/elm_hairs_pictures.htm for an excellent description).

In some cases, the trichomes are thought to remove salt from the leaf tissue and so prevent damaging salts
accumulating (dehydration/water-loss might cause salts to accumulate, especially in salty habitats, and these can
damage tissues by drawing the water out from cells by osmosis).

The trichomes on the leaves of
stinging nettles (Urtica urens) have an obvious defensive function by deterring large
herbivores. These trichomes are glass-like (impregnated by
silica), sharp and brittle, and contain acid. They pierce the
skin and snap on touch, releasing painful acid into the tiny wound! The sticky glandular hairs of certain
insectivorous
plants are used to trap insects. In sundews (Drosera) each hair is tipped by a very sticky globule and when an insect
gets caught, its struggling activates neighbouring hairs which bend towards it, trapping it further with their sticky tips,
before finally the entire leaf rolls around the victim! Some trichomes produce proteolytic insects to digest prey in
insectivorous plants. Butterworts (
Pinguicula) for example, have both sticky hairs and enzyme-secreting
trichomes/glands. The enzyme-secreting glands also serve to absorb the digested materials in this case.

In buds, glandular trichomes and more complex multicellular glandular appendages called
colleters secrete the sticky
materials that protects buds from insects in many woody plants.


Bibliography and References

Karabourniotis, G., Bornman, J.F. and Nikolopoulos, D, 2000. A possible optical role of the bundle sheath extensions of
the heterobaric leaves of
Vitis vinifera and Quercus coccifera. Plant, Cell and Environment 23: 423-430.

Section under construction...


The shapes of leaves


Article updated:

25/5/2013
17 Oct 2016