Leaves

Leaves

Leaves are often the most conspicuous part of any plant. Leaves vary tremendously in shape and in size: from the tiny leaves (less than 1 millimeter across) of the floating aquatic plant duckweed to the giant leaves (more than 10 meters in length) of the raffia palm. Nevertheless, all leaves share certain features of construction and development and carry out the same basic function: photosynthesis.

Leaf Types

Leaves are designed to optimize the capture of light for photosynthesis. In dicots, leaves typically have a broad, flattened blade attached to a stalk or petiole . The flat shape of the blade facilitates the penetration of light into the photosynthetic tissues within, while the petiole positions the blade so that it is shaded as little as possible by neighboring leaves. Leaf blades are referred to as simple when they are undivided and as compound when they are subdivided into individual leaflets. Compound leaves are either pinnately compound (like a rose leaf) or palmately compound (like a horse chestnut leaf). Simple leaves may also have complex shapes: In plants such as the maple or oak, leaves are highly lobed. The lobing of simple leaves and dissection of compound leaves are thought to serve the same function: The leaf maintains a large photosynthetic surface, but the complex outline allows the leaf to radiate heat energy to the surrounding atmosphere, thus maintaining photosynthetic tissues at optimum temperatures.

The leaves of monocots are designed along a different ground plan. The base of the leaf typically surrounds the stem, forming a leaf sheath. The leaf blade is borne at the tip of the sheath. In grasses, sedges, lilies, and orchids, the leaf blade is simple, long, and strap-shaped. In other monocots, such as palms, the blade is typically compound, and, like compound-leaved dicots, leaves may be pinnately compound (like a date palm) or palmately compound (like a fan palm). In palms and some other monocots, the junction of the sheath and blade forms a petiole-like structure.

Plant species are often recognized by their distinctive leaf shapes. Some species, however, are distinguished by producing more than one leaf shape on the same plant, a phenomenon known as heterophylly. Heteroblasty is the most common subtype of heterophylly and typifies plants such as ivy or eucalyptus, which produce one leaf shape early during the juvenile phase and another leaf shape later during the adult or reproductive stage. Another type of heteroblasty is environmentally induced heterophylly, in which specific environmental cues cause an immature leaf to develop along one of two or more alternate pathways. This type of heterophylly commonly results in the formation of sun and shade leaves on the same plant: Leaves that develop on the exposed edge of the canopy are narrow and thick, while those produced in the shaded interior are broad and thin.

Anatomy of Leaves

Despite tremendous variation in size and shape, leaves generally possess the same cell types and arrangement of internal tissues. Leaf veins form a transport system that extends throughout the leaf. Major veins are the large veins that can be seen with the naked eye. The xylem of major veins functions to import water and dissolved mineral nutrients from the rest of the plant to the leaf, while the phloem of major veins exports carbohydrates produced by leaf photosynthesis. The vascular tissues of major veins are associated with collenchyma and sclerenchyma tissues and so contribute to the support of the leaf. Smaller veins are called minor veins. They lack associated supporting tissue and are embedded in the ground photosynthetic tissue. Minor veins form a network that acts as a distribution system: They supply leaf cells with water and solutes from the xylem and load photo-synthetic products into the phloem. Whether the arrangement of minor veins forms a netlike reticulate pattern (typical of dicots) or a gridlike pattern (typical of monocots), adjacent veins are usually no more than 200 micrometers apart. Thus water and solutes rarely have to diffuse more than 100 micrometers between vascular tissues and photosynthetic cells.

The photosynthetic tissue of the leaf is called mesophyll. Mesophyll tissue contains chloroplast -packed cells of two distinct shapes: palisade parenchyma cells that are elongated and spongy parenchyma cells that are spherical or lobed. In leaves with a horizontal orientation, palisade cells form one or two layers toward the upper side of the leaf. Palisade parenchyma cells have dense chloroplasts and, in fact, capture most of the light energy penetrating the leaf. Up to 90 percent of total leaf photosynthesis may occur within palisade parenchyma cells. Spongy parenchyma cells are arrayed in several layers below the palisade. They are exposed to more diffused light and tend to have fewer chloroplasts. Both palisade and spongy parenchyma cells have a relatively high surface-to-volume ratio: this gives a large surface area for the diffusion of carbon dioxide from the intercellular air space of the leaf into the cell where photosynthesis takes place.

Cells of the leaf epidermis typically are shaped like jigsaw puzzle pieces, which is thought to lend structural support to the leaf blade. Stomata usually occur on both the upper and lower surfaces of the leaf. The thinness of most leaf blades ensures that carbon dioxide diffusing inward through the stomatal pores will rapidly reach the mesophyll cells. While leaves are designed to maximize the uptake of CO₂ through the stomatal pores, they lose water vapor through those same pores while the stomates are open. Some plant species reduce such water loss by restricting the stomates to the lower, shaded side of the leaf blade where temperatures are lower and the diffusive loss of water vapor is slower.

Development of Leaves

Leaves are formed on the flanks of the shoot apical meristem . Leaf formation involves four overlapping stages: leaf initiation, morphogenesis, histogenesis, and expansion. Initiation occurs when an alteration of growth pattern within the shoot apical meristem results in a definite protuberance on the surface of the meristem, the leaf primordium. The leaf primordium is produced in a precise location on the meristem according to the phyllotaxis (leaf arrangement) of that particular species. In most dicots, leaf arrangement is helical, and each new leaf primordium is produced in the location that will continue the helix, 137.5 degrees from the last formed leaf. In most monocots, leaf arrangement is distichous, meaning each new leaf primordium is produced at 180 degrees from the previous leaf.

Morphogenesis is the development of the leaf's shape. In dicots, the primordium grows perpendicular to the meristem to form a fingerlike projection. Once the projection is formed, the primordium alters its growth direction to form a ledge around the margin of the protuberance. This ledge becomes the leaf blade, while the thicker original protuberance forms the petiole-midrib axis. At this stage of development, the distribution of growth is diffused, with the whole blade and petiole-midrib axis growing at an even rate. In species with a complex leaf shape, such as a lobed or compound blade, the distribution of growth becomes uneven: growth is enhanced where a lobe or a leaflet will be formed and suppressed between the lobes or leaflets. These events occur very early, so that a leaf often displays its mature shape when it is less than 1 millimeter in length.

In monocots, the original leaf primordium is formed in the same way, but its pattern of growth differs almost from the start. The zone of leaf initiation extends around the flanks of the shoot apical meristem, giving a crescent-shaped primordium. The crescent-shaped primordium then grows vertically. The "wrap-around" base becomes the leaf sheath, and the apical end becomes the strap-shaped blade. Monocots with more complex leaf shapes, such as palms, have a highly specialized pattern of morphogenesis.

Histogenesis is the process of tissue development. While the leaf is expanding and acquiring its final shape, precursor cells of all the tissue systems are undergoing cell proliferation. Cell proliferation is at first distributed throughout the leaf, but as expansion continues, cell division gradually ceases beginning near the tip of the leaf until it finally becomes restricted to the leaf base. In most dicots, this period is brief: the full complement of leaf cells may be already present when the leaf is only 10 percent of its final size.

In many monocots, cells near the base of the leaf continue to divide throughout the life of the leaf, forming an intercalary meristem. When you cut the grass of your lawn, cells in the intercalary meristem are induced to divide, producing more leaf tissue toward the leaf tip.

As leaf cells cease dividing, they first enlarge and then complete differentiation, acquiring the distinctive characteristics of specialized cell types. As with cell proliferation, cell differentiation occurs in a tip-to-base, or basipetal, direction.

Leaf expansion overlaps the morphogenesis and histogenesis stages. Usually all parts of the leaf expand the same amount so that the shape of the young leaf is preserved at maturity; this pattern is called isometric growth. In some species, however, different parts of the leaf expand at different rates, called allometric growth. Allometric growth can either enhance or minimize the degree of lobing in a leaf: if the lobes grow more than the interlobe region (the sinus), they will become more pronounced. In contrast, a leaf such as that of the nasturtium actually starts out with a lobed shape but becomes smooth and round in outline through increased growth of the sinus.

Leaf Modifications

Although leaves tend to share the same ground plan, species that have adapted to extreme environmental conditions often have highly modified leaves. Two well-known examples are the leaves of xerophytes, plants adapted to arid environments, and leaves of hydrophytes, plants adapted to wet environments. Xerophytes are desert plants that must carry out photosynthesis and conserve water at the same time. Xerophytes reduce water loss by having small, but thick, leaves, thus reducing the surface area for evaporative water loss. Light intensity is usually high in the desert, so sufficient light penetrates to all photosynthetic mesophyll cells, even in a thick leaf. Xerophytes have a thick cuticle and waxes on the leaf surface, further reducing water loss. Their leaves often have a thick covering of trichomes that both trap a layer of moister air next to the leaf and reflect heat energy away from photosynthetic tissues. Some xerophytes, such as the oleander, have their stomates restricted to pits called stomatal crypts that further reduce evaporation of water vapor. A few specialized desert plants such as the clock plant hold their leaf blades parallel to the sun's rays throughout the day, using a specialized region of the leaf petiole called a pulvinus. The leaf photosynthetic tissue is exposed to sufficient light but absorbs less heat energy, thus keeping internal tissue temperatures cooler.

Hydrophytes face the opposite challenge to xerophytes. Their leaves are submerged, so there is no shortage of water, but they must photosynthesize under conditions of low light and low availability of carbon dioxide. Hydrophyte leaves are typically very thin, both to absorb the low, diffused light available underwater and to allow for the diffusion of dissolved carbon dioxide and minerals into leaf tissue. Hydrophytes lack stomata and have only a thin cuticle. They also have reduced vascular tissue. (Xylem is missing altogether in the leaves of some hydrophytes.) As hydrophyte leaves are buoyed by water, there is little need for supporting sclerenchyma tissue.

Many other examples of highly modified leaves occur as specialized adaptations among the flowering plants. Insectivorous plants have leaves that serve as traps for their insect prey. Cacti and many other desert plants have leaves that are modified as spines that serve to protect the plant from herbivores while the stems carry out photosynthesis. Some monocots have leaves modified for storage: the leaf sheaths of an onion bulb are thickened, and the mesophyll parenchyma cells are filled with stored sugars.

Evolution of Leaves

The fossil record shows that the first land plants lacked leaves—or rather the stem functioned in both photosynthesis and support. Only toward the end of the Devonian period, about 350 million years ago, did plants begin to bear distinct leaves borne on stems. Leaves of some of these early land plants were huge. Tree club mosses and primitive conifers called Cordaites had meter-long strap-shaped leaves where their modern relatives have highly reduced scale or needle leaves. Fossils of some of the earliest flowering plants from the beginning of the Cretaceous period, about 125 million years ago, show that leaves were of medium size and simple in shape. During the evolutionary diversification of the flowering plants, some groups have developed large, highly elaborate leaves, while others form small, reduced leaves. The early evolutionary divergence of the dicot and monocot lines is reflected in the different basic construction and mode of development of leaves in these two groups.

see also Anatomy of Plants; Aquatic Plants; Cacti; Carnivorous Plants; Photosynthesis, Carbon Fixation and; Photosynthesis, Light Reactions and; Phyllotaxis; Tissues; Trichomes.

Nancy G. Dengler

Bibliography

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Gifford, Ernest M., and Adriance Foster. Morphology and Evolution of Vascular Plants, 3rd ed. New York: W. H. Freeman, 1988.

Prance, Ghillean T. P. Leaves. New York: Crown, 1985.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Taiz, L., and E. Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, 1998.