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I The Parts I Print this section

of a Typical Leaf

The typical green leaf is called a foliage leaf. It usually consists of two basic parts: a petiole and a blade. The petiole is a stalklike structure that supports the leaf blade on the stem. It also serves as a passageway between the stem and the blade for water and nutrients. Another function of the petiole is to move the leaf into the best position for receiving sunlight. Most petioles are long, narrow, and cylindrical. Many plants, such as grasses and corn, do not have petioles. In these plants the base of the blade is attached directly to the stem—the base encircles the stem as a sheath. Such leaves are called sessile leaves. The leaf blade is usually a thin, flat structure. Its margins, or edges, may be smooth, as in the dogwood; jagged or toothed, as in the elm; or lobed, as in the oak and maple. The surface of the blade may be smooth, fuzzy, sticky, dull, or shiny. In most plants the leaves have a single blade and are referred to as simple. In other plants, such as clover, the blade is divided into separate leaflets. This kind of leaf is called a compound leaf. Most of the functions carried on by leaves take place in the blade.

A Epidermis The blade consists of an upper and lower epidermis and a spongy layer of tissue, called the mesophyll. Running through the mesophyll is a branching system of veins. The epidermis is the leaf blade's skin. It is a thin, usually transparent, colorless layer of cells that covers both the upper and lower surfaces of the blade. The epidermis prevents the leaf from losing excessive amounts of water and protects it against injury. In most plants the epidermis is covered with cutin, a waxy substance secreted by the epidermal cells. The layer of cutin, called the cuticle, is responsible for the glossy appearance of some leaves. The cuticle gives the leaf additional protection by slowing down the rate at which water is lost. Generally, the cuticle is thinner on the epidermis on the underside of the leaf than on the upper epidermis, which is exposed to the sun. In many kinds of leaves, hairs grow from the epidermis. The soft hairs of plants such as the mullein give the leaves a woolly or feltlike texture. In some plants the epidermal hairs secrete fluids. For example, in geraniums and petunias the hairs secrete a fluid that gives the leaves a clammy texture. The strong-smelling oils of the peppermint and spearmint plants come from epidermal hairs. In other plants, such as the nettle, the epidermal hairs are stiff and contain a poisonous fluid that produces a skin irritation when a person is pricked by them.

B Guard Cells Scattered throughout the epidermis are pairs of bean-shaped cells, called guard cells. Guard cells contain chloroplasts, which are tiny granules filled with the green pigment chlorophyll. Chlorophyll gives leaves their characteristic green color. Chloroplasts enable leaves to carry on photosynthesis because they are able to absorb carbon dioxide and sunlight, which are required for the food-making process. In response to heat and light, each pair of guard cells pulls apart, and a tiny pore forms between them. The pores, called stomata, open to the outside atmosphere. When the stomata are open, carbon dioxide and oxygen pass either in or out—when carbon dioxide enters, it takes part in photosynthesis, the food-making process that releases oxygen as a waste product. This oxygen passes out of the leaf. At the same time, oxygen also enters the leaf, where it takes part in respiration, a process that forms carbon dioxide as a waste product. This carbon dioxide passes out through the stomata. Water also passes out of the open stomata in the form of a vapor. This process is called transpiration. Generally, there are more stomata on the under surface of a leaf than on the upper surface. This prevents water from evaporating too quickly or in excessive amounts from the leaf's upper side, which is exposed to the sun. Stomata close at night, providing another level of water conservation.

C Water Pores In addition to the stomata, many kinds of leaves have large specialized water pores in their epidermis. These pores, called hydathodes, permit guttation, the process by which a plant loses liquid water. Unlike the stomata, hydathodes remain open all the time. Guttation takes place only when water is being rapidly absorbed by the roots, such as after a heavy rainfall, and when transpiration slows down, as on cool, humid nights. When these conditions occur together, droplets of water can be seen on the leaf early in the morning before they evaporate in the heat of the day. Unlike dew, which condenses on leaves from water vapor in the air and covers the entire leaf surface,

guttation droplets form only on the edges and tips of leaves. Generally, the droplets are noticeable only on the leaves of strawberries and a few other kinds of plants.

D Mesophyll The mesophyll, sandwiched between the upper and lower epidermis, consists of many thin-walled cells that are usually arranged in two layers. The palisade layer is next to the upper epidermis. It consists of cylindrical cells that are packed closely together. Next to the palisade layer and making up most of the thickness of the leaf blade is the spongy layer. The spongy layer consists of roundish cells that are packed loosely together and have numerous air spaces between them. In most plants the spongy layer extends down to the lower epidermis. However, in certain grasses, irises, and other plants whose leaves grow straight up and down, the spongy layer is wedged between two palisade layers of mesophyll. Like the guard cells, all the cells of the mesophyll contain chloroplasts.

E Veins Running through the middle of the mesophyll and branching out to all of its cells are veins. The veins extend into the petiole and connect with other veins in the stem of the plant. A major function of the veins is to help support the leaf blade. Each type of plant has a characteristic pattern of veins forming lines and ridges in the blade.

Leaf [ Botany ] Dictionary of botanic terminology - index of names

A leaf is an outgrowth from a node on a plant's stem and come in many shapes and sizes, they are often green and flattened to maximise their surface area for the capture of energy from sunlight and exchange gases, that are used for photosynthesis by means of the chlorophyll pigment contained in chloroplasts.

Leaves vary greatly from plant to plant and are useful in classification and identification. Some leaves also store food and water, provide support, or form new plants. Where high light intensities are harmful, leaves may reduce the effects of the light by orientating themselves vertically; by becoming thickened or covered with hairs or wax or by having a highly reflective surface. Leaf Parts: Any of these parts may be lacking, modificate or reduced. Lamina: The expanded portion or blade of a leaf. Leaf apex: The tip of a leaf opposite to the petiole Μ idrib The the central vein of a leaf it is usually continuous with the petiole. Vein: The vascular structures on a leaf which arrangement is called venation. Petiole: The stalk or stem that connects the leaf to the plant, frequently with a basal enlargement called pulvinus (petiole are absent in sessile leaves) Axil: The angle between the upper side of the stem and a leaf or petiole Stem (also called the axis) from which the leaves arise. Stipule: The pair of small, appendages one on each side at the base of the petiole. The forms of leaves lamina vary greatly from plant to plant and are useful in classification and identification.

Leaves

Function and structure The principal function of leaves is to absorb sunlight to manufacture plant sugars through a process called photosynthesis. Leaf surfaces are flattened to present a large area for efficient light absorption. The blade is the expanded thin structure on either side of the midrib and usually is the largest, most conspicuous part of a leaf (Figure 11). A leaf is held away from its stem by a stem-like appendage called a petiole, and the base of the petiole is attached to the stem at a node. Petioles vary in length or may be lacking entirely, in which case the leaf blade is described as sessile or stalkless. The node where a petiole meets a stem is called a leaf axil. The axil contains single buds or bud clusters, referred to as axillary buds. They may be either active or dormant; under the right conditions, they will develop into stems or leaves.

A leaf blade is composed of several layers (Figure 12a and Figure 12b: click on images to display larger versions.). On the top and bottom is a layer of thick, tough cells called the epidermis. Its primary function is to protect the other layers of leaf tissue. The arrangement of epidermal cells determines the leaf's surface texture. Some leaves, such as those of African violet, have hairs (pubescence), which are extensions of epidermal cells that make the leaves feel like velvet. The cuticle is part of the epidermis. It produces a waxy layer called cutin, which protects the leaf from dehydration and disease. The amount of cutin on a leaf increases with increasing light intensity. For this reason, when moving plants from shade into full sunlight, do so gradually over a period of a few weeks. This gradual exposure to sunlight allows the cutin layer to build up and protect the leaves from rapid water loss or sunscald.

The waxy cutin also repels water. For this reason, many pesticides contain a spray additive to help the product adhere to, or penetrate, the cutin layer. Special epidermal cells called guard cells open and close in response to environmental stimuli, such as changes in weather and light. They regulate the passage of water, oxygen, and carbon dioxide into and out of the leaf through tiny openings called stomata. In most species, the majority of the stomata are located on the underside of leaves. Conditions that would cause plants to lose a lot of water (high temperature, low humidity) stimulate guard cells to close. In mild weather, they remain open. Guard cells also close in the absence of light. Located between the upper and lower epidermis is the mesophyll. It is divided into a dense upper layer (palisade mesophyll) and a lower layer that contains lots of air space (spongy mesophyll). Located within the mesophyll cells are chloroplasts, where photosynthesis takes place.

Internal Structure of the Stem of a Typical Dicotyledonous Plant

The illustration above shows an herbaceous stem of a dicotyledonous plant. The white stripe between xylem & phloem is the cambium layer. Note: The diagrams that students will draw should show zones or general regions. Individual cells need not be illustrated.

Internal structures to know of herbaceous stems: Pith: Large central area for storage & support. Cambium: Found as a circle around inner stem & outer surface. Forms woody secondary tissue for support. Cortex: Storage area between cambium and epidermis. Epidermis: Thin layer of skin cells. Protection. Xylem: Water conduction up. Phloem: Sap (organic molecules) conduction, usually down to roots.

Note:The diagrams that students will draw should show zones or general regions. Individual cells need not be illustrated.

Internal Anatomy of Woody Stems: Pith: Original stem at very center of stem. Xylem: Water conduction tubes connect leaf to roots. Inner most xylem dies & forms wood. Phloem: Outer tubes just inside bark to carry food from leaves to roots. Heartwood: Dead wood (xylem) in center of stem. It is either dry or filled with tars. Frequently darker than live wood. Sapwood: Live outer wood conducting water and sap. Cambium: Special cells that make new wood (xylem) & new phloem & bark to make tree trunk thicker. Found between xylem & phloem. Bark: Outer protective (from insects, fire, and injury) and waterproof layer of stem. Made by cambium. Outer barks is dead, inner is alive. Springwood: Light colored rings of xylem in wood made when growing season is good (spring & early summer). Summerwood: Darker colored tree rings made when growing season is poor (late summer, fall, & winter).

The upper diagram shows a young woody dicot stem before it has started to grow in width. The lower diagram shows the stem after several layers of wood have been layed down.

Flower Structure and Function

Flowering plants are the dominant type of plants on the earth today (there are about 250000 species). Flowers are therefore the most common plant organs for sexual reproduction. Flowers produce gametes (sex cells). Flowers play a key role in pollination. Pollination is the transfer of pollen (containing the male gametes), from the anther of a flower, to the stigma (receptive surface of the female part of the flower) of the same or a different flower. Parts of the Flower:

Flower Part

Form and Function

Peduncle

Flower stalk.

Receptacle

Part of flower stalk bearing the floral organs, at base of flower.

Sepal

Leaf-like structures at flower base, protects young flower bud.

Calyx

All the sepals together form the calyx.

Petal

Located in and above the sepals, often large and colourful, sometimes scented, sometimes producing nectar. Often serve to attract pollinators to the plant.

Corolla

All the petals together form the corolla.

Stamen

Male part of the flower, consisting of the anther and filament, makes pollen grains.

Filament

The stalk of the stamen which bears the anther.

Anther

The pollen bearing portion of a stamen.

Pollen

Grains containing the male gametes. Immature male gametophyte with a protective outer covering.

Carpel\Pistil

Female part of the flower. Consisting of the stigma, style and ovary.

Stigma

Often sticky top of carpel, serves as a receptive surface for pollen grains.

Style

The stalk of a carpel, between the stigma and the ovary, through which the pollen tube grows.

Ovary

Enlarged base of the carpel containing the ovule or ovules. The ovary matures to become a fruit.

Ovule

Located in the ovaries. Carries female gametes. Ovules become seeds on fertilization.

The sex of a flower can be described in three ways:

1. Staminate flowers: Flowers bearing only male sex parts. These are sometime referred to as "male flowers". 2. Carpellate\Pistillate Flowers: Flowers bearing only female sex parts. These are sometimes referred to as "female flowers".

3. Hermaphhrodite\Complete flowers: Flowers bearing both male and female sex parts.

In many cases flowers are borne as a group on a common stalk, called an inflorescence. They are many different types of floral inflorescences. The type of inflorescence present is sometimes used to aid in classifying flowering plants. Below are a number of common floral inflorescences.