Leaves

Chapter 7

Outline

Overview

Leaf Arrangements and Types

Internal Structures of Leaves

Stomata

Mesophyll and Veins

Specialized Leaves

Autumnal Changes in Color

Abscission

Relevance of Leaves

Overview

All leaves originate as primordia in the buds.

At maturity, most leaves have a stalk (petiole) and a flattened blade (lamina) with a network of veins (vascular bundles).

Leaves of flowering plants are associated with leaf gaps and have an axillary bud at the base.

May be simple (single blade) or compound (divided into leaflets).

Overview

Pinnately compound leaves have leaflets in pairs along the rachis, while palmately compound leaves have all the leaflets attached at the same point at the end of the petiole.

Pinnately compound leaves may be further subdivided an thus be referred to as bipinnately compound.

Overview

Green leaves capture sunlight and thus go through photosynthesis.

Lower surfaces of leaves are dotted with stomata which allow carbon dioxide to enter and oxygen and water to diffuse out.

Guard Cells control stomatal opening.

Transpiration occurs when water evaporates from the leaf surface.

Guttation - Root pressure forces water out hydathodes.

Leaf Arrangements and Types

Leaves are attached to stems at nodes, with stem regions between nodes known as internodes.

Phylotaxy (leaf arrangement) generally occurs in one of three ways:

Alternate

Opposite

Whorled

Leaf Arrangements and Types

Arrangement of veins in a leaf or leaflet blade may also be pinnate or palmate.

Pinnately veined leaves have a main midvein within a midrib.

Secondary veins branch from midvein.

Palmately veined leaves have several primary veins that fan out from the base of the blade.

Parallel in monocots

Divergent in dicots (reticulate venation)

Internal Structure of Leaves

Epidermis is a single layer of cells covering the entire surface of the leaf.

Upper epidermal cells are devoid of chloroplasts.

Waxy cuticle often present.

Different glands may also be present in the epidermis.

Stomata

Lower epidermis of most plans is perforated by numerous stomata.

Guard cells originate from the same parent cell, and contain chloroplasts.

Primary function includes regulating gas exchange between leaf interior and the atmosphere, and the evaporation of water.

Cell water pressure regulates guard cells which in turn regulate stomata.

Mesophyll and Veins

Most photosynthesis takes place in the mesophyll between the two epidermal layers.

Palisade Mesophyll - Uppermost layer Contain most of leaf’s chloroplasts.

Spongy Mesophyll - Lower layer

Veins (Vascular bundles) are scattered throughout the mesophyll.

Consist of xylem and phloem tissues surrounded by the bundle sheath.

Specialized Leaves

Shade Leaves

Leaves in the shade receive less total light, thus tend to be thinner and have fewer hairs than leaves on the same tree exposed to direct light.

Leaves of Arid Regions

Many have thick, leathery leaves and few stomata.

Some have succulent, water-retaining leaves, or dense, hairy coverings.

Specialized Leaves

Tendrils

Modified leaves that curl around more rigid objects helping the plant to climb or support weak stems.

Become coiled like a spring as they develop.

When contact is made, the tip curls around the object, and the direction of the coil reverses.

Specialized Leaves

Spines, Thorns, and Prickles

Spines - Modified leaves designed to reduce water loss and protect from herbivory.

Thorns - Modified stems arising in the axils of leaves of woody plants.

Prickles - Outgrowths from the epidermis or cortex.

Specialized Leaves

Storage Leaves - Succulents

Flower-Pot Leaves - Urn-Like Pouches

Window Leaves - Leaves buried in ground.

Reproductive Leaves - New plants at tips.

Floral Leaves - Bracts

Specialized Leaves

Insect-Trapping Leaves

Pitcher Plants

Sundew

Specialized Leaves

Insect-Trapping Leaves

Venus’s Flytraps

Bladderworts

Autumnal Changes in Leaf Color

Cholorplasts of mature leaves contain several groups of pigments.

Chlorophylls - Green

Carotenoids - Yellows

In fall, chlorophylls break down and other colors are revealed.

Water soluble anthocyanins (red or blue) and betacyanins (red) may also be present in the vacuole.

Abscission

Deciduous plants drop their leaves seasonally.

Occurs as a result of changes in an abscission zone near the base of the petiole of each leaf.

Cells of the protective layer become coated and impregnated with suberin.

Leaf Abscission Zone

Human and Ecological Relevance of Leaves

Landscaping

Food

Dyes

Ropes and Twine

Drugs

Tobacco

Marijuana

Insecticides

Waxes

Review

Overview

Leaf Arrangements and Types

Internal Structures of Leaves

Stomata

Mesophyll and Veins

Specialized Leaves

Autumnal Changes in Color

Abscission

Relevance of Leaves

. Leaves - Basic Terminology

Major Leaf Parts

Lamina or Blade - flat part of the leaf

Petiole - stem-like structure which attaches the Lamina and the Stem

Sheath - broad laminar structure which attaches monocot leaves to the stem

Axil - Upper angle where the petiole meets the stem

. Simple Leaf - has only one lamina

Compound Leaf - has several lamina attached to one petiole. Each unit is called a Leaflet. The term Rachis is used instead of Petiole to designate the structure to which leaflets are attached

Venation - pattern of veins in the leaf

. Midrib - Central part of the leaf which is usually continuous with the Petiole. It is usually elevated above or below the lamina.

Pinnate Venation - one large central vein (midrib) present with smaller lateral veins that diverge in pairs, each on the opposite side of the midrib

. Palmate Venation - several main veins of equal size diverge from the petiole

Reticulate Venation - large veins give rise to progressively smaller veins - the ultimate branches delimit small areas called "Aeroles"

Parallel Venation - major and minor veins run parallel to one another - these are interconnected by

Commisural Bundles which diverge at angles approaching 90 degrees. This is the most common term used to describe venation in Monocots.

Striate Venation - another term used to describe the complex venation patterns found in Monocots. Parallel venation is called longitudinal-striate venation.

. Dichotomous Venation - many veins of equal size which form two equal branches  at successive branch points. This is common in Ferns but is found in a few Angiosperms & Gymnosperms.

Adaxial - side facing the stem (Upper Surface)

Abaxial - side facing away from the stem (Lower Surface)

. Simple Leaf - has one lamina

Pinnate Simple - has pinnate venation

Palmate Simple - has palmate venation

Pinnate Compound - has an elongated central rachis (midrib) to which leaflets are attached opposite one another along its length - a terminal leaflet is often present at the tip of the rachis

. Palmate Compound - rachis is not elongated and the leaflets are closely attached to it at one locus.

. Simple Leaf with Pinnate Venatio

. Leaves of Olona - These are Simple Pinnate leaves.

. Sassafras leaves that are Simple & Palmate

. Immature Leaf from the "Oriental Snowflake Plant"
Simple Palmate

. Sumac in autumn - one side of a Pinnately Compound Leaf

. Adaxial surface of a leaf with Reticulate Venation

. Palmately Compound Leaves from the "Umbrella Tree"

. Lily with Parallel (Longitudinal Striate) Venation

. Terrestrial Orchid with Parallel (Longitudinal Striate) Venation

Travelers' Palm a monocot with a Large Simple Leaf - Its leaf partitions are caused by the wind. Much the same occurs with banana (Musa).

. Parallel (Longitudinal Striate) Venation in the leaf of Strelitzia (Bird of Paradise).The cross-hatch pattern is caused by the Commisural Bundles.

. The prayer plant (Maranta), Ti & Traveler's Palm exhibit several vein traits which point out the limitations of the generalization that Monocots have only Parallel Venation.

All of them have Commisural Bundles which results in a close-knit pattern that rivals the contact between veins and the mesophyll seen with Reticulate Venation.

. Furthermore, Ti and Maranta have apparent "Midribs" and there are "lateral" veins which diverge from it in a pinnate-like pattern (Maranta). These midribs do not contain extremely enlarged veins but they contain a concentration of parallel veins.

. Finally, the veins converge and join near the leaf margin forming a closed, integrated vascular system. The pattern seen in Maranta is called Pinnate-Striate.

. The term Striate is preferable to Parallel when Venation in Monocots is considered. Grasses have Longitudinal Striate Veins.

. Dichotomous Venation consists of veins which fork at their apex and produce 2 veins of the same size. The branches can develop equally or unequally. The minor veins tend to be of the same size rather than getting progressively smaller. The terminal Veins may end "Blindly" near the margins of the leaf. Dichotomous Venation occurs with "Ancestral" plants which arose before the

. Flowering Plants. It is sometimes called "Primitive" because it is not as efficient as Reticulate Venation & not observed in Flowering Plants. It can be found in some Ferns like the "filmy" fern above. However other ferns have Reticulate Venation

The Leaves of some Gymnosperms also have Dichotomous Venation. These are from the Cycad Zamia. However trees like Agathis also have Dichotomous Venation.

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. The Midrib provides the principal Vascular Connection to the Stem. Their Vascular Organization varies from Simple (one vascular bundle) to Complex..

. Midrib of Banana (Musa)
Note the numerous Fibrovascular Bundles & Bundles of pure Fibers which provide support for this incredibly large leaf

. Midrib of Monstera: Note the many Fibrovascular Bundles

. The Midrib also provides Structural Support for the Leaf. This is due to the presence of Collenchyma & Sclerenchyma & Xylem.

. Features of the Lamina (Blade)

Three Tissue Systems are present!  

Epidermal - Ground (Mesophyll in leaves) - Vascular (Veins)

Typical Dicot Leaf
Cross Section of Sugar Cane Leaf (Monocot)

. The Adaxial Epidermis generally has a thick cuticle on the upper (adaxial) surface. Secretory & Non-secretory Trichomes may be present. Stomata may occur on the upper epidermis but these are most frequent on the lower (abaxial) surface. The Abaxial Epidermis usually has a thin cuticle. However, it may have a dense layer of trichomes which increase the

. "boundary layer" effect which decreases the rate of transpiration. Stomata are routinely present on the abaxial surface and are usually more abundant than on the adaxial surface.

. Lamina from Metrosideros polymorpha showing a thick adaxial cuticle (C) and abaxial Trichomes (T)

. Mesophyll

Palisade mesophyll is composed of columnar cells that can be one to several layers thick. It is most common on the adaxial side of the leaf where sunlight is usually most abundant.

Palisade parenchyma cells are more efficient for photosynthesis and water retention because of their large surface areas (better to exchange gases) but small intercellular air spaces that limit evaporation.

. Spongy mesophyll is usually found on the abaxial side of the leaf. It may consist of stellate parenchyma or chains of elongate cells which have large, lateral intercellular spaces.The cells can have a stellate appearance and resemble the jacks used by children.

. Some plants have a uniform mesophyll which is composed of relatively isodiametric cells.

Mesophyll Cells in Wheat do not have pronounced shapes, and give a uniform appearance.

. Nerium oleander (Oleander) has pronounced Palisade and Spongy Layers

. Leaf Shape

A sphere has the lowest surface to volume ratio. This means that for a particular volume a sphere has the lowest surface area. Since evaporation takes place where a wet surface meets an atmosphere, less evaporation will occur from a spherical object compared to a flat object.

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A completely flat surface would have the maximal surface area and therefor have the largest area available for the evaporation.

A cylinder would have an intermediate surface to volume ratio.

. depiction of surface to volume ratios of a sphere and a flat object of the same volume. Which has the greater surface area

The leaves on this plant are Spherical & would have a low surface/volume ratio.

. The sea lettuce (Ulva) is only two cells thick. It would have a greater surface area compared to a spherical organism of the same volume.

The lamina of a filmy fern may be one cell thick in places-

Some ferns produce leaves with a thick lamina.-

Which of the two ferns would best survive desiccation?????.

. Conifers & Podocarpus

Most conifers have needle-like leaves which have a cylindrical shape overall.

Pinus monophylla has a cylindrical leaf. If you combine the needles of most other pines you can see that they represent subdivisions of a cylinder. Consequently, they retain a cylinder-like surface to volume ratio.

. Pinus monophylla leaves are Cylindrical & this shape reduces the Surface to Volume Ratio. This is the next best thing to being Spherical. Other Pinus needles are subdivisions of a cylinder and also have relatively low surface to volume ratios. The Anatomy of Pine needles is related to their ability to survive in stressful environments!

. If you combine the three needles of a three needle

pine you get a cylinder like Pinus monophyla. Pine needles are usually in tight clusters. Consequently, they retain a small surface to volume ratio but have greater surface area for gas exchange. Stomatal frequency is greatest on the inner surfaces of the needles. This shields them from the effects of wind currents.

. Conifer Needles have many Xeromorphic Traits. These include Epidermis with Thick Cuticle & Lignified Cell Walls. Sunken Stomata, Sclerotic Hypodermis, tightly packed Mesophyll Cells with small intercellular spaces, an Endodermis that separates the Mesophyll from the Vascular Tissue which is unbranched. The Vascular Tissue is located in the center of the organ. This places it as far as possible away from the Atmosphere

. Podocarpus leaves are long & narrow which improves the surface to volume ratio compared to plants with a wide lamina. However, they have a relatively large surface to volume ratio compared to Pinus monophylla.

. Succulence

Succulent leaves are noticeably thick or swollen compared to average leaves. They may be spherical, cylindrical or flat. They contain large amounts of water storage parenchyma that can contain mucilage in their vacuoles. This mucilage is very hydrophilic and releases water grudgingly

. Extracellular mucilage can also be present as in the Hawaiian Silversword which has large mucilage "canals".

They can also have

other Xeromorphic

adaptations like a

thick cuticle and

trichomes plus

stomatal adaptations.

The Hawaiian species Sesuvitum portulacastrum (akulikuli) grows in xeric habitats. Its leaves have a thick, flattened cylindrical appearance. They contain lots of water-storage parenchyma. Due to their contents & their relatively swollen appearance, the leaves are said to be Succulent. Sensuvium belongs to the Aizoaceae.

. This family has many succulent species. Some have very bizarre shapes like those of Lithops (stone plants). Some species are almost completely subterranean except at the tip of their leaves which are translucent. This is like a "sun roof" or skylight. This allows enough light to enter for photosynthesis but keeps most of the plant below ground where it is protected from evaporation and excess solar

. irradiation.

The Ice Plant (Mesembryanthemum) is widely planted as an ornamental. Its leaves have enlarged Epidermal cells which store water and impart a glistening sheen to their leaves. These plants produce attractive flowers as well

. Pereskia is one of the few cacti to retain its photosynthetic leaves. These are broad, but they are also thick and contain a lot of water-storage parenchyma.

. Cactus Seedlings have small Leaves which quickly wither. The Spines are actually Leaves!

. Mature Cacti can also produce photosynthetic leaves. However, these are ephemeral!

. Some families like the Portulacaceae & Crassulace may have many species with succulent leaves. These often have CAM (Crassulacean Acid Metabolism) Photosynthesis

. Leaf Shape-

Permanently altered leaf shapes are a compromise between the abilities to retard water loss and maintain an adequate rate of photosynthesis.

It would be preferable to change leaf shape in response to environmental conditions.  Some plants have developed adaptations which allow them to do this.

. Some plants have extremely large epidermal cells (Bulliform Cells). These cells are fully expanded when water is readily available. The lamina is consequently expanded, as well.

However, when plants come under water stress, the Bulliform cells loose water and shrink. This causes the lamina to change its orientation.

. In some plants, the leaf rolls into a tube, in another case the two halves of the leaf fold together as in prayer.

. The relative availability of water regulates the turgidity of the Bulliform cells which consequently control the disposition of the leaf with regards to sunlight.

. The  Bulliform cells are turgid and thus expanded when water is readily available. Consequently, the leaf is open with the adaxial surface facing the sunlight.

When water availability is low, the Bulliform Cells loose water. Consequently, they become flaccid and the leaf blade folds such that the abaxial surface is exposed to the sun.

. When water is scarce, small Bulliform Cells become flaccid and the leaf curls into a cylinder, such that the Abaxial surface is exposed to the sunlight. All of the stomata are in the crevices of the Abaxial surface. Consequently, transpirational water loss is greatly reduced.

. Amophila has several kinds of Xeromorphic Adaptations.

The Stomata occur in recesses of the adaxial leaf surface.

The Chlorenchyma has a similar disposition. This is more evident in the mature leaf seen below.

Bundle Sheaths with prominent Bundle Sheath Extensions are present.

. Sclerenchyma is abundant.

Trichomes are present on the adaxial surface at the openings of the recesses.

. Agave has thick, stiff leaves which are somewhat reflective. These plants are found in Xeric environments..

. Sun vs Shade Leaves

Cross-Section of a Shade Leaf

Note the following. The Sun Leaf is Thicker and has more Palisade Layers. The Spongy Layer in the Sun leaf appears to be more compact with less air space than in the Shade Leaf. The intense overall staining by the Sun leaf indicates that it probably contains more secondary metabolites like tannins.

. The Vascular Bundle is larger in the Sun Leaf and there is more Sclerenchyma associated with it. There is a noticeable Bundle Sheath & its extension in the Sun Leaf but not in the Shade leaf.

. Hydromorphic Leaves-

There are two kinds of Hydromorphic leaves, floating and submerged. Sometimes both can occur on the same plant.

Floating leaves generally have Aerenchyma. These are called Lacunae or Air Chambers. They contain "diaphragms" of parenchyma at regular intervals. Lacunae can be present in Petioles as well.

. Lacunae may be several cells thick but contain Pores or air spaces which allow gas exchange, but prevent water-logging of the entire column and they can also provide structural support.

Submerged leaves can be greatly simplified & dissected. In some cases the leaf can be divided into elongated hair-like cylindrical parts. This maximizes surface contact with

. water.

There is a reduction in the complexity of the Epidermis, Collenchyma, Sclerenchyma and Vascular Tissues, especially Xylem.

The Epidermis lacks a cuticle and may have Chloroplasts. Stomata may be completely absent on submerged leaves. Trichomes are NOT generally present.

. Nymphaea sp. (Water-lilies) have floating leaves. The upper part may have Xeromorphic traits while the lower layer may have Hydromorphic characteristics.

. The upper Epidermis may have a thick Cuticle.
Stomata are present. Look for the substomatal cavities in the image above. There are numerous Palisade layers which terminate above the extensive Aerenchyma. Astrosclereids originate in the Palisade layer and protrude into the Air Cavities. The Lower Epidermis lacks Stomata or Trichomes.

. Potomogeton Leaf Cross-sections.

. Submerged Leaves - Myriophyllm is a good example
of a plant that has submerged Leaves. It grows in wet habitats and can be completely submerged. The Leaves are finely dissected. They are only a few cell layers thick and the Epidermis contains chloroplasts. The midrib is minute and does not contain xylem

. Submerged Leaves - Elodea is another example of a plant with submerged Leaves. Its leaves are not dissected like those of  Myriophyllm but they are exceedingly thin and translucent. Their Midrib also lacks Xylem.

. Buds

Buds represent a collection of protective leaves called Bud Scales or Cataphylls, immature Leaves, and a Shoot Apical Meristem. Buds are not prominent in many tropical species because conditions are generally not as severs as those in Temperate climates. Buds are exceedingly important for perennial trees and shrubs in environments

. which have freezing temperatures or extremely dry conditions.

The Cataphylls are generally waxy due to a thick cuticle. Stomata are absent or sparingly present.

Cataphylls have a reduced Mesophyll lacking Palisade parenchyma.

Sclerenchyma can be abundant.

Cork can develop in some cases.

. Modified Leaves - Tendrils  

Tendrils are generally, slender elongated structures that attach plants to a substrate which can be itself, another plant or any suitable material.

Tendrils can resemble a Petiole and they are sometimes an extension of the Midrib or Rachis that extends beyond the region of the

. Lamina (simple leaf) or Leaflets (compound Leaf). The pitchers of Nepenthes develop from a Tendril.

. In many cases the tendril coils around an object. They may be modified leaves or stems.

Tendrils allow plants with weak stems to "scale the heights" using more sturdy plants for support. Many Vines produce Tendrils.

However, adventitious roots can serve a similar function. This is the case with "ie'ie.

. The tendrils of Boston Ivy (Parthenociss tricuspidata) produce Suction Cups that firmly attach it to almost any solid substrate.

Tendrils can be part of a Leaf, an entire Leaf, Stipules or a Stem.

Some tendrils may be ambiguous and difficult to classify as a particular organ.

Tendrils are characteristically found in certain families like the Fabaceae (Legume family).

. Members of the Passion Flower family (Passifloraceae) produce Tendrils.

The Tendrils of Passion Vine are modified stems. They develop from Axillary Buds.

. Modified Leaves - Spines & Thorns  

The Spines produced by members of the Cactus family (Cactaceae) are highly modified Leaves.

. Thorns may also be highly modified Leaves or Stipules or Stems. The leguminous tree Gleditsia (Honey Locust) produces a compound thorn. The Leaf is pinnately compound and the thorn's complexity is similar.

. Stipules are often leaf-like structures that occur where the Petiole joins the Stem.
Stipules are initiated at the outset of Leaf Initiation at the Shoot Apical Meristem. Some Stipules become sclerotic Thorns.

. Carnivorous Plants

Modified Leaves are involved in capturing prey for most Carniverous Plants.

We have already encountered several of these when we reviewed the Epidermis and Secretory Structures.

. Utricularia is known as

"bladderwort". It is a submersed plant that creeps along the water bottom of ponds. It has slender stems and leaves. Green leaves occur above the water but white stems and leaves bearing the bladder-like traps are found below.

The translucent submerged leaves are globe-like and have a narrow opening with protruding structures.

The traps are balloon like. The opening of the balloon is sealed by a viscous fluid. The sides of the trap are usually concave, and create a vacuum within it. When the hairs near the opening are stimulated, the sides of the trap become convex, and the prey quickly is sucked inside. The plant uses secretions of digestive juices to break down the captured prey and absorb the resultant minerals.

. Leaf Anatomy & Transpiration

Water is brought to leaves in the xylem that is present in the veins. Most cells are no more than 0.5 mm away from a minor vein. Water is transferred to the Protoplasts & Walls of Mesophyll Cells. Water evaporates from the cell walls until the atmosphere inside the leaf is saturated with water molecules.

Water molecules diffuse rapidly in the atmosphere. The term Water Vapor is applied to water molecules in the gaseous phase. Leaves need to open their stomata to let CO2 diffuse inside because CO2 levels are higher in the outside atmosphere than inside the leaf. However, CO2 levels in the atmosphere are far lower than the concentration of water molecules. 

. Water molecules are far more concentrated inside the leaf than outside. Consequently, when stomata are open, water molecules rapidly pass through the Stomatal Pore to the outer atmosphere. This creates a physiological dilemma for the leaves. If the stomata remain open too long, they will wilt and possibly die. Plants have developed physiological means to control stomatal opening and closing

. The Boundary Layer is the zone of unstirred air that lies immediately outside the Epidermis. This is a significant factor which affects the rate of water loss from the leaf. Factors like Epidermal Trichomes which increase the Boundary Layer moderate Transpiration rates.

The difference in water potential between the outside atmosphere and the intercellular spaces inside the leaf are always great enough to cause the movement of water molecules from the leaf into the air. This is true even at high relative humidities. There is a 100 fold difference in external and internal water potential at 25 C.

Guard Cell Anatomy

Guard Cells have anatomical features which are related to their function. We will consider the Guard Cells that are typical for Dicots like Beans or Peas. Grasses have a different Guard Cell structure which is a little harder to grasp.

. The individual Guard Cells resemble a Kidney Bean in Shape. They have thickened inner radial walls which are not completely joined. This part of the cell wall can have Ledges which project from the top or bottom. These create microenvironments which can reduce the rate of water loss during Transpiration. Despite the thickness of the Cell Wall, this part of the wall can retract so that a Stomatal Pore appears.. The outer Radial walls of the Guard Cells are usually thinner. These have an interface with Subsidiary Cells or Epidermal Cells.

The Guard Cells have Chloroplasts but other Epidermal Cells have small, translucent Leucoplasts

. The Cellulose Fibrils (CF) in the Guard cell Walls have a Radial orientation as seen from above. This has an important bearing on their function. The spaces between the CF are small near the inner radial walls but are wide near the outer radial walls.

. A cell with parallel CFs would enlarge evenly. If the CF were close together there would be little enlargement. Cell enlargement would be greater if the CF were more widely spaced. The CFs in Guard Cells have an asymmetric organization because they are closely spaced at the inner radial wall but widely spaced at the outer radial wall.

. If a Guard Cell enlarges, the outer radial walls can expand but the inner radial walls can not. As the outer radial walls enlarge, the inner radial walls are pulled apart. This opens the Stomatal Pore. If the Guard Cells shrink, the Stomatal Pore would close.

. Turgor Pressure regulates the opening and closing of the Stomatal Pore. There are many factors which regulate this process. These include CO2 concentration, light intensity and color, temperature & relative humidity.

Following a dark period, stomata open in response to light. Light induces an influx of Potassium ions from adjacent cells to the Guard Cells.

. It also stimulates production of an organic acid (malate) and accumulation of sucrose. This lowers the Water Potential of the Guard Cells & this causes an influx of Water. This increases the Turgor Pressure of the Guard Cells. The Inner Radial Walls can expand due to the loose organization of their Cellulose Fibrils.

. The Inner Radial Walls can not expand due to the tight organization of their Cellulose Fibrils and the overall thickness of these walls. They are deformed by the volume increase of the Guard cells, and they pull apart to form the Stomatal Pore. Guard Cells can increase their volume by 40 - 100%!

. Whole Plant Overview

The uptake and translocation of water by plants is complex and involves bulk flow, osmosis and diffusion. However, it becomes very simple if we look at the main compartments that are involved in terms of their Water Potential.

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