Leaves

Chapter 6                                                         

THE LEAF

The shoot system has both stems and leaves. Stems and leaves must function together if the plant is to survive and reproduce.

Leaves should be flat and thin for maximum absorption of light and carbon dioxide. These tissues must be alive and differentiated into chlorophyll-rich chlorenchyma to carry out photosynthesis.

Leaves have many functions. The morphology of the different leaf types allows the organ to function properly and is, therefore, advantageous.

The shoot system contains both stem and leaves, which must function together.

MORPHOLOGY OF THE LEAF - EXTERNAL SRUCTURES

Leaves normally consist of a blade or lamina and a petiole or stalk.

The blade’s lower side is its dorsal surface; the larger veins protrude like backbones. This side is also called the abaxial side; the upper side is the ventral surface and is usually rather smooth. This is the adaxial side of the leaf.

Leaves without petiole are called sessile.

The petiole allows the leaf to flutter in air helping to cool the plant and to bring carbon dioxide closer to the stomata.

Scale-like stipules develop at the base of the some leaf petioles.

In many monocots and some eudicots, the base of the leaf is expanded into a sheath that covers the internode, e.g. grasses.

Leaves may have a simple or compound blade.

Leaves may be palmately compound, with all leaflets attached at the same point. Simple leaves may also be palmate in their shape but they have lobes rather than pinnae.

Compound leaves have the lamina divided into leaflets. The leaflets are attached to the rachis by petiolules.

Leaflets can be distinguished from leaves by…

• Buds are found in axil of leaves but are absent in the axil of leaflets.
• Leaves extend from the stem in various planes but leaflets lie in the same plane; they are never arranged in whorls, decussate or spirally.
• A branch with leaves has an apical bud; compound leaves usually end on a leaflet and never have an apical bud.

In some species, the juvenile leaves are different from the adult leaves. Seedlings may have the first few leaves different in size and shape.

Other species produce two types of leaves simultaneously; one type in long shoots and other in short shoots. In cactus, the spines are short shoot leaves, while the long shoot leaves are usually microscopic.

 

The vascular bundles of the leaf, veins, are continuous with the vascular bundle of the stem.

Magnoliids and eudicots have a netted venation.

Most monocots have a parallel venation with veins extending along the long axis of the leaf.

Small veins may connect the larger parallel veins in monocots.

The abscission zone is perpendicular to the petiole. Its function is to detach the leaf from the stem when the function of the leaf is no longer needed.

Chemical changes in the abscission zone at the base of the petiole precede abscission of the leaf.

There are two layers in the abscission zone: the separation layer and the protective layer.

The separation zone is made of small cells with thin cell wall that make this layer structurally weak.

Before abscission, reusable ions, amino acids and carbohydrates are returned to the stem.

Enzymes break down the cell wall in the separation layer including hydrolysis of cellulose and materials in the middle lamella.

Cell division may occur in the separation zone prior to abscission. If so, these new cells are the one affected by hydrolysis.

Beneath the separation layer, a layer of highly suberized cells is formed further isolating the leaf from the stem.

Tyloses may form in tracheary elements.

Tylose is balloon-like structures produced from ray parenchyma that tend to clog and seal wounded tissue.

The separation layer forms the leaf scar after the leaf falls off.

 

INTERNAL STRUCTURES OF FOLIAGE LEAVES

Epidermis.

Water loss through the epidermis is called transpiration.

Leaf and stem epidermises are very similar, consisting of a large percentage of flat tabular cells. Guard cells and trichomes may be present and abundant or not. The trichomes may be glandular or not.

It is compact and provides strength to the leaf.

Some xerophytes have a multiple epidermis made two or three layers of cells.

It is covered with a cuticle made of cutin and wax in mesophytes and xerophytes. Cuticle is absent in most hydrophytes.

The upper and lower epidermises exist in different microclimates. Convection currents on the upper epidermis remove water if there are open stomata.

The number of stomata is usually grater in the lower epidermis or may be completely absent from the upper epidermis.

The epidermis on both sides of the leaf lacks chloroplasts except for the guard cells of the stomata.

Stomata (sing. stoma) may occur on the lower surface of the leaf or on both sides in mesophytes.

Hydrophytes with floating leaves have their stomata on the upper side only, and those that are entirely submerged usually lack stomata entirely.

Xerophytes have numerous stomata only on the lower side of the leaf often sunken in pits or depression of the leaf surface (crypts). These are called sunken stomata.

Epidermal hairs may occur in the sunken stomata of xerophytes.

Leaves of many plants have epidermal hairs on one or both sides of the leaf. Some of these hairs may be glandular and secrete resin-like substances.

Trichomes may shade the epidermis of the leaf and reflect excessive sunlight; they prevent rapid air movement on the lower epidermis and slow water lass. Trichomes make walking and chewing difficult for insects, and some secrete powerful stinging compounds

In magnoliids and eudicots, stomata are usually scattered over the surface.

In monocots, the stomata are arranged in rows along the long axis of the leaf.

Mesophyll.

The mesophyll is the ground tissue of the leaf found in between the lower and upper epidermis.

It is specialized for photosynthesis with large number of chloroplasts and air spaces.

In mesophytes the mesophyll is commonly differentiated into palisade and spongy parenchyma.

The palisade parenchyma consists of a layer or two of columnar cells located right under the upper epidermis and with their long axis perpendicular to the epidermis.

There are intercellular spaces in between the palisade cells.

In some leaves the palisade surface may be two to four times greater than the spongy surface.

Chloroplasts are more abundant in palisade cells and most of the photosynthesis probably occurs in this layer.

The spongy parenchyma is made of irregular shaped cell and is found on the lower side of the leaf above the lower epidermis.

In some plants like corn all cells are more or less alike and there is no distinction between palisade and spongy parenchyma.

In many xerophytes there is palisade parenchyma on both side of the leaf.

Vascular tissue

Veins contain primary xylem and primary phloem, which is primary in origin in most cases.

The coarser midvein (midrib) and some secondary veins (lateral veins) may undergo limited secondary growth in magnoliids and eudicots.

Commonly the xylem occurs on the upper side of the vein and phloem on the lower side.

The xylem usually extends to the end of the vein but the phloem may or may not extend to the end of the vein.

Veins have different degree of thickness and the minor or thinner veins play the principal role in the collection of photosynthates.

Larger veins transport what the smaller veins collected.

• As veins increase in size, their primary function changes from collection of photosynthates to transport of photosynthates out of the leaf.

 

Parenchyma cells with very few chloroplasts normally surround larger veins.

A compact layer of fiber cells called the bundle sheath surrounds the smaller veins.

The bundle sheath extends to the end of the veins and completely surrounds the vascular tissue so it is never exposed to the air in the intercellular spaces of the mesophyll.

All substances entering and leaving the vascular tissue must pass through the bundle sheath cells.

In C3 grasses a second layer of small sclerified parenchyma cells, the mestome layer, lies between the much larger bundle sheath cells and the vascular bundle.

Bundle-sheath extensions connect the bundle sheath of many leaves with lower and upper epidermis.

The bundle-sheath extension provides mechanical support to the vein.

Collenchyma and sclerenchyma cells may be present beneath the epidermis of the vein ribs or larger veins in eudicots and magnoliids.

In monocots, the veins may be bordered by fibers.

Collenchyma and fiber cells may found along the leaf margins of eudicots, magnoliids and monocots.

Other types of non-conducting cells like mucilage, tannins and starch storing may be present in larger veins

Petiole

Petioles are considered part of the leaf. They are the transition between the stem and the blade.

Petioles vary in size from being non-existent in sessile leaf plants to massive like in palms.

The epidermis usually contains fewer stomata and trichomes.

Petioles have a considerable amount of collenchyma.

The cortex is compact and has few if any air spaces.

The number may vary from one to five traces. The traces may number into the hundreds in large palm leaves.

The leaf traces may fuse before entering the petiole. The leaf traces in the petiole fuse into one to form the midvein of the blade.

Stipules are sometimes found at the base of the petiole. These modified leaves may protect the young developing leaf or may be wide enough that can contribute to photosynthesis or may help to protect the plant by becoming spines.

INITIATION AND DEVELOPMENT OF LEAVES

Leaf primordia are initiated by groups of cells in the peripheral zone of the apical meristem:

• It includes cells in the three layers L1, L2 and L3.
• It ranges from about 5-10 cells per layer in Arabidopsis, to 50-100 cells per layer in cotton and corn.
• These cells are called founder cells.

 

The founder cells divide and change orientation to produce bulges called leaf buttresses.

Leaf buttresses develop into leaf primordia.

Dense bands of cells form on opposite sides of the primordium and give rise to the blade.

The central region of the primordium gives rise to the midrib.

The L1 layer gives rise to the epidermis and the L2 and L3 layers form the internal tissues.

Expansion and increase in length of the leaf takes place by intercalary growth that is by cell division and cell enlargement in the various layers.

Differences in the rate of growth produce the spaces and other features characteristic of different leaves.

The primordium consists of leaf protoderm and leaf ground meristem, and all cells are meristematic at this stage.

A strand of cells in the center differentiates into provascular also known as procambium, tissue and then into primary xylem and phloem.

Normally growth stops first at the tip and last at the base.

Maturation of the leaf is from tip to base.

The characteristic growth exhibited by the leaf is said to be determinate as compared with the indeterminate growth of the apical meristems.

The procambium differentiates upward into the primordium as an extension of the leaf-trace procambium.

All coarse and major veins develop upward and outward toward the leaf margin.

Minor veins, however, are initiated at the tip of the leaf and develop from the tip to the base.

Monocots

In monocots, the lateral bands of the primordium quickly grow and encircle the apex forming a hood around it.

The length increases by the addition of cells at the base, the basal intercalary meristem.

As more cells divide, the hood becomes more like a cylinder that surrounds the shoot apical meristem.

The tubular portion becomes the basal sheath of the leaf, and the original conical portion now located on one side of the tube becomes the leaf blade.

The outer surface of the tube is the abaxial side of the leaf, and inner surface is the adaxial side of the leaf.

The shoot apex enlarges, forms a new stem tissue, and initiates the next leaf primordium.

Elongation of cells is restricted to a zone above the basal meristem, and maturation follows as cells are pushed upwards.

The difference between the sheath and the blade does not become distinct until late in the development of the leaf when the ligule appears at the top of the sheath.

In grasses, the lamina grows by a meristem that is located at its base where it attaches to the sheathing base. This is the intercalary meristem.

Differentiation occurs very rapidly right above the intercalary meristem. The tissue above is mature and differentiated.

MORPHOLOGY AND ANATOMY OF OTHER LEAF TYPES

Succulent leaves

Succulent leaves and thick and fleshy and reduce the surface to volume ratio and favors water conservation.

Some are cylindrical and others are spherical, the optimal surface to volume shape.

These leaves have fewer air spaces and are more transparent allowing deeper penetration of light.

Sclerophyllous foliage leaves

Leaves should produce more sugars than they consume in their construction and metabolism.

Some leaves have evolved so they are perennial living more than two years. With this extended lifetime and prolonged productivity, sclerenchymatous leaves are feasible, and their hardness makes them ore resistant to animals, fungi, more resistant to freezing and ultraviolet light.

Sclerenchyma appears as layer below the epidermis and bundle sheaths.

The cuticle is usually very thick and waxes are abundant.

“Sclerophylly is often used to describe the material properties of evergreen leaves in certain shrubs and trees growing primarily in xeric habitats. These include the chaparral of the western United States, the macchia of Chile and the Mediterranean basin, the fynbos of the western Cape in South Africa, and the heath of western Australia. The leaves of sclerophyllous species are characterized as being thick, tough, and leathery, with a high fiber (sclereid) to protein ratio.”
Leaf biomechanics, morphology, and anatomy of the deciduous mesophyte Prunus serrulata (Rosaceae) and the evergreen sclerophyllous shrub Heteromeles arbutifolia (Rosaceae). Ronald A. Balsamo, Aaron M. Bauer, Stephen D. Davis and Benita M. Rice. American Journal of Botany. 2003;90:72-77.

Leaves of conifers

Almost all leaves of conifers are sclerophyllous, having a thick cuticle, and their epidermis and hypodermis cells have thick walls.

Conifer leaves are always simple, never compound

They may be needles, scales sometimes broad, or awl-shaped.

Leaves are perennials.

Three general have deciduous leaves: larches (Larix), bold cypress (Taxodium), and dawn redwood (Metasequoia).

Bud scales

Dormant shoot apical meristems are protected by bud scales from low temperatures and the drying effect of wind during winter.

Bud scales are not photosynthetic, and are simple; their petiole is either short or absent; they often produce a corky layer on the exposed areas that adds protection.

Spines

Cactus spines are modified leaves of axillary buds.

• Their spines have a structure related to their function:
• No mesophyll or vascular tissue is present;
• The mesophyll consists of closely packed fibers.
• After the fibers mature, they deposit lignin in their walls.
• The cells then dry and die out, hardening even further.

 

Thorns are modified branches used in protection of the plant, e.g. hawthorn.

Prickles are modifications of the epidermis, e.g. roses.

Tendrils

Tendrils are leaves modified into wire-like structures, and aid in support, e.g. peas.

Their cells can sense contact with other surfaces.

The side in contact with an object ceases to grow but the other side  continuous growing thus forming the coil

Leaves with Kranz anatomy

This special anatomy of the leaf is found in plants that have C4 photosynthesis.

They lack palisade parenchyma and spongy mesophyll., and have prominent bundle sheath cells surrounding the vascular bundle.

Insect traps

Insectivory evolved in plants that grow in habitats poor in nitrates and ammonia.

Traps can be active traps that move during capture, or passive traps that do not move.

Pitcher leaves are anatomically similar to foliage leaves and are capable of photosynthesis.

These leaves are tubular and secrete a watery digestive fluid.

Active traps close around the insect due to an electrical stimulus produced by hairs on the trap surface that cause cells involved in the movement mechanism to loose water rapidly. These traps also secrete a digestive fluid.