Vascular Bundles of the Stem and the Leaf

Chapter 7LEAVES

Most leaves are specialized for photosynthesis.

VASCULAR BUNDLES OF THE STEM AND THE LEAF

A primordium is an organ at its earliest stages of development; it usually consists of a group of several hundred cells.

A leaf primordium may consist of fewer than 200 cells.

The primordium begins to grow in response to environmental factors like temperature, day length, and availability of water.

Hormones are produced that stimulate cell division.

Procambial strands of the stem arise behind the apical meristem just below the leaf primordia.

As the leaf primordium grows in length, the procambial strands differentiate within them.

From the beginning, the procambial strands of the leaf are continuous with those of the stem.

The strand of vascular tissue that goes into the leaf is called leaf trace.

The region of ground tissue in the vascular cylinder above the diversion of the leaf trace is called a leaf trace gap.

Buds develop in the axil of leaves and their vascular system is connected with that of the main stem by branch traces.

BASIC STRUCTURE OF THE LEAF

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

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 of deciduous plants normally live through one growing season.

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

Leaves may have a simple or compound blade.

Compound leaves have the lamina divided into leaflets.

Leaves may be simply pinnate, bipinnate, tripinnate, or rarely quadripinnate.

The central vein of pinnate leaves is an extension of the petiole, ant it is called the rachis.

Leaflets can be distinguished from leaves by

1. Buds are found in axil of leaves but are absent in the axil of leaflets.
2. Leaves extend from the stem in various planes but leaflets lie in the same plane.
3. A branch with leaves has an apical bud; compound leaves usually end on a leaflet and never have an apical bud.

Most monocots have parallel veins that run the length of the leaf blade.

Dicots have a network of veins.

Stomata or pores allow CO2 to enter the leaf but water then escapes by a process called transpiration.

In some plants there special openings called hydathodes at the tips of leaf veins.

Root pressure forces water out of the hydathodes usually at night when transpiration does not occur. The loss of water through the hydathodes is called guttation.

LEAF ARRANGEMENT AND TYPES

The arrangement of leaves on the stem is called phyllotaxis.

Leaves are attached to the stem at the nodes, and the stem region in between the nodes is called internode.

Alternate

• Opposite.
• Whorled.
• Helical or spiral.
• Distichous.
• Decussate.

LEAF TISSUES

1. Epidermis

There are upper and lower epidermises that form the surface of the leaf.

• Made of living parenchyma cells.
• Lack chloroplasts.
• Covered with a waxy layer, the cuticle.
• Glands may be present in the epidermis or at the tip of trichomes.
• Have small openings for gas exchange called stomata (sing. stoma).
• Guard cells flank each stoma.
• Bulliform cells may be present in the epidermis of some monocots.
• Trichomes or hairs may be present.

2. Mesophyll

The photosynthetic tissue found in between the two epidermises is called mesophyll.

It consists of...

• Living parenchyma cells.
• Abundant chloroplasts; the tissue with chloroplasts is often called chlorenchyma.
• Usually loosely arranged with many air spaces.
• Often arrange in two regions: palisade and spongy mesophyll.
• Monocots usually don’t have the mesophyll differentiated into palisade and spongy.

3. Venation

Veins and diffusion cooperate in the movement of materials in veins.

• Veins or vascular bundles extend through the mesophyll.
• Each vein contains xylem and phloem tissue.
• Xylem is usually restricted to the upper side of the vein, and phloem to the lower side of the vein.
• A non-vascular parenchymatous layer of cells called the bundle sheath surrounds veins.
• Monocot leaves usually have parallel venation.

SPECIALIZED LEAVES

On the basis of water requirement plants can be classified as mesophytes, hydrophytes and xerophytes.

The structure of the leaf is related to the habitat requirement for water.

SUN AND SHADE LEAVES.

Leaves that grow exposed to direct sunlight are smaller and thicker than those that grow in the shade.

Several layers of palisade parenchyma develop in sun leaves, with more extensive vascular tissue thicker wall epidermis.

Great variation can be found on one plant.

LEAVES OF ARID REGIONS - XEROPHYTES

Leaves of xerophytes are usually thick and leathery. Some have succulent leaves that store water.

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

These layers under the epidermis may be thickened, and then called hypodermis. The hypodermis is different from the underlying tissue.

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

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

Epidermal hairs may occur in the sunken stomata of xerophytes.

LEAVES OF AQUATIC AREAS - HYDROPHYTES

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

Submerged leaves usually have little xylem and phloem.

The mesophyll is undifferentiated and has large air spaces to help in buoyancy.

TENDRILS

Tendrils are leaves modified into whip-like structures.

Tendrils are sensitive to tact.

Tendrils curl around the stem and branches of other plants and help the plant in climbing or supporting weak stems.

Some tendrils are modified stems!

SPINES, THORNS AND PRICKELS

Thorns, Spines and Prickles are designed for defense and protection. They aremodifications of different plant parts.

• Thorns are branch systems typically found in leaf axils e.g. hawthorn.

• Spines are modified leaves or in some cases, the stipules of leaves, e.g. barberry, cactus.

• Prickles are extensions of epidermis and cortex tissue, e. g. roses, blackberry.

STORAGE LEAVES

The succulent leaves of xerophytes are modified to store water.

Water storage cells are large parenchyma cells without chloroplasts interior to the chlorenchyma tissue just beneath the epidermis.

These parenchyma cells have large vacuoles which can store large amounts of water.

The water storing cells of Peperomia are part of the multiple epidermis derived from the protoderm.

The storage leaves of onions and garlic store large amounts of carbohydrates that are used by the plant in the next growing season.

FLOWER PLOT LEAVES

Some epiphytes develop leaves that form a pouch around the stem where debris accumulates and forms humus.

WINDOW LEAVES

Window leaves-are common in many desert plants, are shaped like tin ice-cream cones and grow mostly underground, with only a small transparent "window" tip protruding above the soil level.

Most of the leaves are buried allowing light coming through the “window” to penetrate the chloroplasts in the mesophyll.

REPRODUCTIVE LEAVES

Leaves modified for reproduction-form tiny plants at the edges of their leaves. These plants become new individuals when they are shed from parent leaves.

FLORAL LEAVES OR BRACTS

Bracts are floral leaves that form at the base of a flower or flower stalk. They are usually small and scale-like, and protect developing flowers.

In some plants like poinsettias and dogwoods, the bracts are colored to resemble petals and attract pollinators.

INSECT-TRAPPING LEAVES

Insect-trapping leaves-in carnivorous plants, leaves modified for attracting, trapping, and digesting animals. These adaptations range from sticky flypaper surfaces to vat-like leaves.

Pitcher plants

Sundews

Venus’ flytraps

Bladderworts

AUTUMNAL CHANGES IN LEAF COLOR

There are three types of pigments in the leaf:

• Chlorophyll, which gives leaves their basic green color. It is necessary for photosynthesis, the chemical reaction that enables plants to use sunlight to manufacture sugars for their food. Trees in the temperate zones store these sugars for their winter dormant period. Chlorophyll is not very stable and sunlight decomposes it; it has to be continuously synthesized by the plant.

• Carotenoids, which produce yellow, orange, and brown colors in such things as corn, carrots, and daffodils, as well as rutabagas, buttercups, and bananas. Carotenes absorb blue and blue-green light and reflect yellow. Carotenes are more stable than chlorophyll and persist in the leaf longer than chlorophyll.

• Anthocyanins, which give color to such familiar things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. They are water soluble and appear in the watery liquid of leaf cells. Anthocyanins absorb blue, blue-green, and green light. Therefore, the light reflected by leaves containing anthocyanins appears red. Unlike chlorophyll and carotene, anthocyanins are not attached to cell membranes, but are dissolved in the cell sap.

When carotene and chlorophyll occur in the same leaf, together they remove red, blue-green, and blue light from sunlight that falls on the leaf. As a result of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down, the green color disappears. The yellow color of carotenes then shows in the leaf.

Graph from

LEAF ABSCISSION.

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 are 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.

HUMAN AND ECOLOGICAL RELEVANCE OF LEAVES

Leaves are a source of food: cabbage, spinach, parsley, lettuce, etc.

Petioles of celery and rhubarb.

Some spices are derived from leaves: oregano, marjoram, peppermint, basil, dill, cilantro, etc.

Cordage fibers for ropes: agave, Manila hemp.

Oils: lemon grass, citronella, lavender, Eucalyptus oils, camphor, etc.

Drugs: cocaine, belladonna, opium, etc.