The Leaf - Solar Power!
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 pigmentchlorophyll 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.

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. (It is important to realise that the O2 evolved comes from water not carbon dioxide). 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 color). 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 centers. 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 center 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 center is called P680.
The chl-a in PSI has an absorption peak at 700 nm and so its reaction center 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 color 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 color.

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 colors 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

Notice there are two photon absorption steps: photons activating PSII (photosystem 2 or P680) and PSI (photosystem 1 or P700). Specifically, these photons are absorbed by one of several hundred chlorophyll molecules in each photosystem, making up either the core or the light-harvesting antenna (which transfers energy to the core). The absorption of the photon excites a bonding electron from a bonding molecular orbital into a higher energy molecular orbital. In essence photon absorption has imparted energy from the photon to the molecule which is now in a less stable and more energetic state. This energy is transferred to a pair of chlorophyll molecules in the reaction center of each photosystem. This transfer of energy occurs by a process called resonance energy transfer (RET) in which electric charges in the excited molecule excite electrons in a neighbouring molecule by direct interactions. This energy transfer is thought to possibly involve quantum coherence, a purely quantum mechanical effect that would allow the energy flow to select the most efficient pathway by simultaneously comparing different routes to the reaction center. The excited electron in the reaction center chlorophyll detaches from the molecule in a process of oxidation, by flowing down an energetic hill of coupled reactions making up an electron transport chain (ETC).

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.

OIL RIG
Oxidation is loss of electrons, reduction is gain

Ultimately, the final chlorophyll in the reaction chain of each photosystem (called the trap chlorophyll) gives up an electron, the chlorophyll is oxidized, and the electron enters the ETC of the Z-scheme. 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 eventually replaces one lost from P700, when P700 absorbs light. Both photosystems must absorb photons to complete the reaction chain. (PS II can actually transfer surplus excitation energy directly to the electrons in PS I if necessary, to keep the two systems in optimum balance). When P700 absorbs a photon to become excited (P*700), the electron that is lost from the trap chlorophyll is passed to another electron carrier called ferredoxin, Fd and Fd becomes reduced ferredoxin, Fdr. 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 along the reaction chains represented by the two blue arrows, 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 energy storage molecule ATP. It is non-cyclic because the electrons travel in a Z- shape (in redox potential space) and do not end where they started. ATP is the chief store of immediately usable chemical energy in the cell.

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 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.

These electrons, released from the splitting of water molecules, reduce the oxidised chlorophylls back to their non-excited absorptive state. 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 (which is the logarithm of H+ concentration) of the thylakoid is much lower than that of the stroma. These factors establish an electrochemical gradient across the thylakoid membrane. This gradient is essentially a capacitance that is storing electrical energy in the form of proton-motive force (PMF). The thylakoid membrane is impermeable 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 protein complex.

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

A. It takes 4 electrons to liberate each O2 molecule. One absorbed photon liberates one electron (from the trap chlorophyll). However, to complete the reaction chain, electrons must also be released from photosystem I. To complete the reaction this photosystem must reduce 2 molecules of NADP+ which requires a further 4 electrons (2 per NADP+) and hence 4 more photons. Thus it takes 8 photons to remove one molecule of O2 from two molecules of 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:

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 base organic compounds which are used in other synthetic reactions.

Above: the Z-scheme represented in physical space, showing the arrangement of the participating molecules in the thylakoid membrane of the chloroplast.

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 center 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).

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 energized 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.



A transverse section (T.S.) through the midrib of a privet leaf (Ligustrum).

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!

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 defense 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.

Studies suggest that the venation pattern (pattern of veins) in the leaves of most plants is optimized for both mechanical support and distribution of water and nutrients to the cells of the leaf. Indeed, both problems seem to require a similar venation pattern. Veins ending near the margin of the leaf, often in a protruding tooth of the leaf margin, may transfer water to a secretory hydathode (water gland). Water drops can be seen to drip from hydathodes in very humid atmospheres and at night (when stomata are closed), in a process called guttation, and rather than simply removing excess water, the hydathodes allow the transpiration stream to continue, albeit at a reduced rate, in the absence of evapotranspiration, ensuring that leaves receive necessary mineral nutrients. This mechanism possibly requires parenchyma cells in the leaf to expend energy to actively pump the water (by osmotic means) but is generally thought to involve active pumping in the roots to generate root pressure. Root pressure, however, is thought to be insufficient in tall woody plants.


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 May 2013
17 Oct 2016

13 Oct 2020