Leaves

1. The most active and conspicuous organs of plants.

2. The most important function is absorbing sunlight for photosynthesis. To do this, they expose large amounts of surface area to the environment.

Phyllotaxis (from the Greek: phyllon - leaf; taxis - arrangement). The arrangement of leaves on a stem. It is determined at the shoot apex and is species-specific.

1. Spiral or alternate phyllotaxis - most plants have this arrangement of leaves. One leaf per node. Birch (Fagus), oak (Quercus), mulberry (Morus) have alternate phyllotaxy. If leaves form 2 parallel ranks along the stem, the phyllotaxis is said to be distichous, as in ginger, pea (Pisum sativum), and saltgrass.

2. Opposite phyllotaxis - have 2 leaves per node, as in maple (Acer), ash (Fraxinus), and olive (Olea). When pairs of leaves at successive nodes are at right angles to each other, the phyllotaxis is decussate, as in coleus.

3. Whorled phyllotaxis - have 3 or more (and as many as 25) leaves per node. Oleander (Nerium oleander), Peperomia, and horsetail (Equisetum) have whorled phyllotaxy.

Phyllotaxis is independent of leaf shape.

Anatomy and function

1. Leaves are adapted to perform certain important functions: photosynthesis, respiration, and transpiration.
2. A typical foliage leaf consists of a large, flat leaf blade (lamina), a petiole (leaf stalk), and a leaf base with which the leaf is attached to the stem.
3. Veins (vascular tissue strands) are often clearly visible on the leaf blade.
4. Leaf blades of some plants show indentations or clefts in the leaf margin. If these indentations reach all the way to the midrib so that the leaf blade is divided into a number of smaller pinnae (leaflets), the leaf is called a compound leaf. If the leaf blade is not divided into leaflets, the leaf is termed a simple leaf. Most monocots have simple leafs, while dicots have either simple or compound leaves.
5. Epidermis:
1. The cuticle prevents water loss.
2. The epidermis protects the internal tissues from injury.
3. The stomata allow for gas exchange for photosynthesis and respiration.
4. Since the epidermis is translucent it allows light to reach the mesophyll tissue for photosynthesis.
6. Mesophyll: this tissue forms the bulk of the leaf. It makes up the green tissue of the leaf and consists of chloroplasts.
1. Differentiated into palidade parenchyma and spongy parenchyma.
2. Palisade parenchyma – thin-walled cells with large numbers of chloroplasts. Cylindrical in shape. Responsible for photosynthesis.
3. Spongy mesophyll – ball-shaped cells with large intercellular spaces, but contains fewer chloroplasts than palisade cells. Allows for gas exchange.
7. Veins: a vein contains the vascular tissue which contains xylem and phloem. The lignified xylem cells are situated towards the upper epidermis and the phloem towards the lower epidermis
1. Veins strengthen the lamina.
2. Xylem conducts water and dissolved ions (minerals) to mesophyll tissue.
3. Phloem conducts organic food such as glucose from mesophyll to other parts of the plant.

Why are there leaves of so many different shapes?

1. The shape of a leaf is a response to a plant species’ long term ecological and evolutionary histories.
2. An ecosystem’s limiting factors may also modify the finished form and shape of a plant’s leaves.
3. Understanding the “logic” behind the varied forms of leaves is facilitated by a grasp of the functions a leaf must accomplish.
1. A leaf must capture sunlight for photosynthesis (as it does this it may absorb a great deal of heat).
2. A leaf must take in carbon dioxide from the surrounding air via pores (stomata) for photosynthesis. CO2 in, water out, when stomata are opened. Leaf must perform a balancing act: absorb enough sunlight and CO2 to power photosynthesis; but not to absorb too much heat or lose too much water in the process.

Structure of leaves

1. Simple leaves - have a flat, undivided blade, not separated into leaflets, that is supported by a stalk, called a petiole. Leaves of plants, such as Zinnia, that lack petioles are called sessile leaves. Redbud, elm, and maple have simple leaves.

2. Compound leaves - have blades that are divided into leaflets that form in one plane. Leaflets lack axillary buds, but each compound leaf has a single bud at the base of its petiole. There are two kinds of compound leaves: pinnately compound leaves and palmately compound leaves.

a. Pinnately compound leaves - form in pairs along a central, stalk-like rachis. Ash, walnut, and rose.

Bipinnately compound leaf – pinnately compound leaf with the leaflets divided pinnately again.

b. Palmately compound leaves - attach at the same point, mush as the fingers are attached to your palm. Horse chestnut (Aesculus), marijuana (Cannabis), clover (Trifolium) and lupine (Lupinus).

3. How to tell a single leaflet of a compound leaf from a simple leaf?

a. Axillary buds are present only in the axils of primary petioles and absent from the axils of leaflets. The position of the axillary bud it can be used to determine whether a leaf is simple or compound.

b. Another diagnostic hint: orientation. All leaflets of a compound leaf are oriented in the same plane, whereas if each leaflet were to be a simple leaf instead, they would be oriented in different planes.

Venation – the arrangement of veins. Used in plant identification.

1. Reticulate veined (net-like veined) – in the form of a network. Dicots

2. Parallel veined – with main veins parallel to the leaf axis or tow each other. Monocots. Grasses.

3. Pinnately-netted – blade bisected along long axis by large midvein. Smaller branch veins are produced from the midrib, each giving rise to successively smaller branches that ultimately form a network.

4. Palmately-netted – several veins of equal size diverging from the tip of the petiole instead of just a single midrib.

Internal structure

1. Epidermis - in most leaves, epidermis is transparent and non-photosynthetic. It contains numerous stomata (2 guard cells and the pore between them).

2. Vascular tissue - xylem and phloem in leaves form in strands called veins. Xylem forms on the upper side of a vein (on a vertical leaf, on the side next to the stem), and phloem on the lower side (side away from the stem).

a. Veins are like a leaf's fingerprint and can be used to identify plants.

b. Most dicots have netted (reticular) venation, meaning they have one or a few prominent mid-veins from which smaller minor veins branch into a meshed network.

c. Leaves of most monocots have parallel venation, meaning that several prominent and parallel veins interconnect with smaller, inconspicuous veins.

d. Each vein ending services a small neighborhood of cells and is where most water and solutes are exchanged within cells of the leaf.

3. Ground tissue - the ground tissue of leaves is called the mesophyll. It contains several types of cells, including sclerenchyma (sclereids), storage parenchyma, and chlorenchyma (chloroplast-containing parenchyma cells specialized for photosynthesis).

Environmental control of leaves

The morphological differences that distinguish plants growing in different habitats is most striking in the leaves. Leaf development is affected by several environmental factors, the most influential of which are light and moisture.

Light - day length, light intensity, and presence or absence of light strongly affect leaf development.

1. Presence or absence of light - leaves of most dicots require light to expand and produce chlorophyll. The light-controlled expansion of leaves is important because it ensures that a plant will not form leaves unless light is present for photosynthesis.

2. Daylength - (amount of light per day). During short days, plants such as Kalanchoe produce small, succulent leaves that are sessile and not lobed. During long days, Kalanchoe produces large thin, lobed leaves with petioles.

3. Light intensity - leaves of many plants respond to differing intensities of light. Consider plants growing in a dense rain forest. Leaves atop the plant canopy are bathed in intense light and are called sun leaves. These leaves have significantly different structures than do shade leaves, which grow in the dim light on the forest floor.

a. Sun leaves are smaller and thicker than shade leaves. Leaves of Plectanthrus (Coleus) grown in intense light are 3 x thicker than leaves grown in dim light.

b. Leaves high in a tree canopy receive a great deal of sunlight. These leaves tend to be smaller (reduced light absorptive surface) and tend to have more complex edges and lobes (enabling the dispersal of absorbed heat quickly).

c. Leaves in lower canopy are more shaded and, therefore, larger (more light absorptive surface), and have a reduced expression of lobes and complex edges.

d. In intense light, sun leaves fix carbon faster than do shade leaves.

e. Sun leaves have smaller and more numerous chloroplasts than do shade leaves

f. Epidermal cells of shade leaves often contain chloroplasts.

Moisture - leaves we have been considering so far have, for the most part, been mesophytes, plants that grow best in moist, but not wet, environments

a. Xerophyte - plant that grows in a habitat characterized by seasonal or persistent drought, such as bright deserts. Availability of light seldom limits the growth of xerophytes. Rather, their growth is usually limited by how efficiently they use water.

1. Xerophytes have small, thick leaves with well-developed spongy and palisade layers. The leaves are often modified for water and typically contain little intercellular spaces.

2. Xerophytes, such as sagebrush (Artemisia spp.) that grow in seasonally dry habitats produce relatively large leaves during wet season, which are replaced by smaller leaves during the dry season.

3. Some plants forgo leaves altogether during drought, and have green and succulent stems for photosynthesis and storage of water.

4. Xerophytes are covered by an epidermis having thick cell walls, numerous stomata, and a thick cuticle. Mesquites growing in dry areas have considerably thicker cuticles than those growing in wet areas. The stomata or xerophytes are often sunken and overlaid with trichomes.

5. Xerophytes have large amounts of supporting tissues in their leaves. Turgor pressure supports leaves of plants growing in most environments. Since turgor cannot always be maintained in xerophytes, their leaves usually contain large amounts of sclerenchyma for support.

6. Some of these modifications (adaptations) may not necessarily be a response to conserving water, but just as likely as adaptations to intense light, or limited nitrogen

Needle leaves

1. These leaves of conifers (and others) have a low light absorptive surface area. Each needle is not able to capture much sunlight energy for photosynthesis.

2. Needles also have a thick, outer cuticle coating designed to prevent excessive water loss. Suited to dry climates.

3. Needles last for 4 or 5 years (or more or less), while many broad leaves live for only a season. Needle leaves have an advantage in that the metabolic cost of the leaf’s synthesis can be recovered by photosynthesis over several growing seasons. Continuous presence of the needles enable the leaves to exploit moderate environmental or climatic conditions (very early spring, very late “Indian summer”), by photosynthesizing and gathering energy for the plant to use or store.

a. A German study showed that a broad-leafed beech tree photosynthesized for 176 days in a year, while a nearby Norway spruce conifer photosynthesized for 260 days a year.

b. The spruce was 58% more productive than the beech.

Modified leaves

1. Tendrils - leaves modified for support, as in sweet pea (Lathyrus odoratus) and trumpet flower (Bignonia capreolata). Leaf or leaflet may be converted to a tendril. Photosynthesis in these plants is delegated to leaflike structures called stipules, at the base of each leaf. In the potato vine (Solanum jasminoides) and garden nasturtium (Tropaeolum majus), only the petiole is modified into a tendril. Tendrils may grow to 30 m long.

2. Stipules - small, leaflike structures at the base of petioles. They have a variety of functions. Stipules of woodruff (Asperula) and sweet pea are photosynthetic; those of black locust (Robinia pseudoacacia) and spurge (Euphorbia) form protective spines. Stipules protect buds in oak and beech, and in plants like Smilax they become tendrils that coil around objects that they touch. (These tendrils are very sensitive - they'll coil around a wire weighing only 1.23 mg, about 1/50^th the weight of a paper clip.

3. Spines - spines of plants such as ocotillo (Fouquieria splendens) and cacti are leaves modified for protection.

4. Bud scales - tough, overlapping, waterproof leaves that protect buds from frost, dessication, and pathogens. Bud scales form before the onset of unfavorable growing seasons such as winter.

5. Bracts - floral leaves that form at the base of a flower or flower stalk. usually small and scalelike, and protect developing flowers. some plants have colorful bracts, like poinsettias, Indian paintbrush (Castilleja), bird-of-paradise (Strelitzia), Bougainvillea. In these plants, bracts replace petals and attract pollinators (the petals of these plants characteristically have small and inconspicuous flowers).

6. Storage leaves - storage leaves of onion (Allium sativa) and lily are fleshy, concentric leaves modified to store food. Onions have tubular leaves, the white bases of which form the bulb. The leaves of most bulbs store starch and sugar.

7. Cotyledons - embryonic leaves.

a. Monocots usually have only 1 cotyledon.

b. Dicots have two cotyledons.

c. Cotyledons have several functions:

1. in beans they absorb the endosperm and therefore store energy used for germination. Because the embryonic plant is nonphotosynthetic, it must have its own nutrients during germination until it becomes self-sufficient.

2. The cotyledons of many plants function as storage organs and become large and thick as they absorb the food reserves initially produced as endosperm. These seeds have little or no endosperm at maturity. Examples of seeds with thick and fleshy cotyledons - peas, beans, squash, sunflower, and peanut. Wheat and corn have very thin cotyledons that function primarily to help the young plant digest and absorb food stored in the endosperm. Storage products in cotyledons are usually carbohydrates, but they may be oils, as in peanuts (Arachis hypogaea).