The Role of Roots in Plant Growth

Root (botany), organ of higher plants, usually subterranean and having several functions, including the absorption and conduction of water and dissolved minerals, food storage, and anchorage of the plant in the soil. The root is distinguished from the stem by its structure, by the manner in which it is formed, and by the lack of such appendages as buds and leaves. The first root of the plant, known as the radicle, elongates during germination of the seed and forms the primary root. Roots that branch from the primary root are called secondary roots. In many plants the primary root is known as a taproot because it is much larger than secondary roots and penetrates deeper into the soil. Beets and carrots are examples of plants with very large taproots. Some plants having taproots cannot be transplanted easily, for breaking the taproot may result in the loss of most of the root system and cause the death of the plant.

Roots arising from the stem are known as adventitious roots. Such roots may be seen near the base of a corn stem. Adventitious roots formed high up on a stem are termed aerial roots or prop roots. Such roots aid in supporting the stem, as in the banyan, the mangrove, and certain orchids.

COMPOSITION

The root is composed of three types of tissue: the epidermis, or surface layer; the ground tissue, or cortex; and the vascular core, situated at the center of the root. Certain cells of the epidermis are modified for an absorptive function. Long, tubelike projections, called root hairs, grow from these cells into the absorptive surface of the root and anchor the root to soil particles. Water absorbed by root hairs is transferred across the cortex, the region of water and food storage, and into the vascular core, which carries it up into the stem. Organization of the vascular core in a root is markedly different from that in a stem. In the stem the vascular tissues xylem and phloem are grouped together in vascular bundles. In the root a central core of xylem has radial bands that extend outward toward the cortex, and between these bands are strands of phloem. In aerial roots the xylem core, which is usually solid in subterranean roots, often has a central zone of pith.

GROWTH

Under normal conditions the growth of roots is influenced chiefly by gravity and by the presence of water. Roots tend to grow downward into soil, unless water is more readily available at the surface. In addition to the primary growth in length occurring at the apex of the root, a secondary growth occurs that adds xylem, or wood, to the inside of the root and phloem toward the outside. Phloem produced in this manner becomes involved in the formation of bark, which covers old roots as well as old stems. Old roots often are virtually identical therefore with old stems.

Because in many plants roots can be formed from a cut end of a stem, cuttings may be used for plant propagation. Some plants, such as the willow or geranium, root quite easily, whereas others, such as the conifer, rarely root without special treatment. Root formation can be stimulated on cuttings of many plants by the application of the so-called root hormones, substances found naturally in the plant when new roots are formed. Most commercial preparations of root hormones contain indoleacetic acid, one of the most common root-stimulating substances. Occasionally roots may be formed from leaves, as in the African violet, which may be propagated by rooting the cut end of a leaf base in water. In some plants roots may give rise to shoots. For example, the stems that are formed at various distances from the base of a Lombardy poplar arise from roots.

USE

Roots of many plants are edible and contain considerable quantities of food materials, particularly starch. Root crops important in agriculture include the sweet potato, beet, turnip, carrot, parsnip, and cassava. The wild forms of these plants have much smaller roots than the cultivated forms because continued development by agricultural peoples has improved the size, texture, food value, and flavor of the roots in cultivated varieties.

All About Plant

I INTRODUCTION

Plant, any member of the plant kingdom, comprising about 260,000 known species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees, and various other forms that mantle the Earth and are also found in its waters. Plants range in size and complexity from small, nonvascular mosses, which depend on direct contact with surface water, to giant sequoia trees, which can draw water and minerals through their vascular systems to elevations of more than 100 m (330 ft).

Only a tiny percentage of plant species are directly used by humans for food, shelter, fiber, and drugs. At the head of the list are rice, wheat, corn, legumes, cotton, conifers, and tobacco, on which whole economies and nations depend. Of even greater importance to humans are the indirect benefits reaped from the entire plant kingdom and its more than 1 billion years of carrying out photosynthesis. Plants have laid down the fossil fuels that provide power for industrial society, and throughout their long history plants have supplied sufficient oxygen to the atmosphere to support the evolution of higher animals. Today the world's biomass is composed overwhelmingly of plants, which not only underpin almost all food webs, but also modify climates and create and hold down soil, making what would otherwise be stony, sandy masses habitable for life.

II DIFFERENTIATION FROM OTHER KINGDOMS

Plants are multicellular eukaryotes—that is, their cells contain membrane-bound structures called organelles. Plants differ from other eukaryotes because their cells are enclosed by more or less rigid cell walls composed primarily of cellulose. The most important characteristic of plants is their ability to photosynthesize. During photosynthesis, plants make their own food by converting light energy into chemical energy—a process carried out in the green cellular organelles called chloroplasts (see Chlorophyll; Chloroplast). A few plants have lost their chlorophyll and have become saprophytes or parasites—that is, they absorb their food from dead organic matter or living organic matter, respectively—but details of their structure show that they are evolved plant forms.

Fungi, also eukaryotic and long considered members of the plant kingdom, have now been placed in a separate kingdom because they lack chlorophyll and plastids and because their rigid cell walls contain chitin rather than cellulose. Unlike the majority of plants, fungi do not manufacture their own food; instead they are saprophytic, absorbing their food from either dead or living organic matter.

The various groups of algae were also formerly placed in the plant kingdom because many are eukaryotic and because most have rigid cell walls and carry out photosynthesis. Nonetheless, because of the variety of pigment types, cell wall types, and physical attributes found in the algae, they are now recognized as part of two separate kingdoms, containing a diversity of plantlike and other organisms that are not necessarily closely related. One of the phyla of algae, the green algae, is believed to have given rise to the plant kingdom, because its chlorophylls, cell walls, and other details of cellular structure are similar to those of plants.

The animal kingdom is also multicellular and eukaryotic, but its members differ from the plants in deriving nutrition from other organic matter; by ingesting food rather than absorbing it, as in the fungi; by lacking rigid cell walls; and, usually, by having sensory capabilities and being motile, at least at some stage. See Classification.

III PLANT PHYLA

The many species of organisms in the plant kingdom are divided into several phyla, or divisions, totaling about 260,000 species. The bryophytes are a diverse assemblage of three phyla of nonvascular plants, with about 16,000 species, that includes the mosses, liverworts, and hornworts. Bryophytes lack a well-developed vascular system for the internal conduction of water and nutrients and have been called nonvascular plants. It takes two generations to complete the plant life cycle (Alternation of Generations). The familiar leafy plant of bryophytes is the sexual, or gamete-producing, generation of the life cycle of these organisms. Because of the lack of a vascular system and because the gametes require a film of water for dispersal, bryophytes are generally small plants that tend to occur in moist conditions, although some attain large size under favorable circumstances and others (usually very small) are adapted to desert life.

The other phyla are collectively termed vascular plants, or tracheophytes. Vascular tissue is internal conducting tissue for the movement of water, minerals, and food. There are two types of vascular tissue: xylem, which conducts water and minerals from the ground to stems and leaves, and phloem, which conducts food produced in the leaves to the stems, roots, and storage and reproductive organs. Besides the presence of vascular tissue, tracheophytes contrast with bryophytes in that tracheophyte leafy plants are the asexual, or spore-producing, generation of their life cycle. In the evolution of tracheophytes, the spore-producing generation became much larger and more complex, whereas the gamete-producing generation became reduced and merely contained in the sporophyte tissue. This ability to evolve into larger and more diverse sporophytes, together with the ability of the vascular system to elevate water, freed tracheophytes from direct dependence on surface water. They were thus able to dominate all the terrestrial habitats of the Earth, except the higher Arctic zones, and to provide food and shelter for its diverse animal inhabitants.

IV CELL STRUCTURE AND FUNCTION

The tremendous variety of plant species is, in part, a reflection of the many distinct cell types that make up individual plants. Fundamental similarities exist among all these cell types, however, and these similarities indicate the common origin and the interrelationships of the different plant species. Each individual plant cell is at least partly self-sufficient, being isolated from its neighbors by a cell membrane, or plasma membrane, and a cell wall. The membrane and wall allow the individual cell to carry out its functions; at the same time, communication with surrounding cells is made possible through cytoplasmic connections called plasmodesmata.

A Cell Wall

The most important feature distinguishing the cells of plants from those of animals is the cell wall. In plants this wall protects the cellular contents and limits cell size. It also has important structural and physiological roles in the life of the plant, being involved in transport, absorption, and secretion.

A plant's cell wall is composed of several chemicals, of which cellulose (made up of molecules of the sugar glucose) is the most important. Cellulose molecules are united into fibrils, which form the structural framework of the wall. Other important constituents of many cell walls are lignins, which add rigidity, and waxes, such as cutin and suberin, which reduce water loss from cells. Many plant cells produce both a primary cell wall, while the cell is growing, and a secondary cell wall, laid down inside the primary wall after growth has ceased. Plasmodesmata penetrate both primary and secondary cell walls, providing pathways for transporting substances.

B Protoplast

Within the cell wall are the living contents of the cell, called the protoplast. These contents are bounded by a cell membrane composed of a phospholipid bi-layer. The protoplast contains the cytoplasm, which in turn contains various membrane-bound organelles and vacuoles and the nucleus, which is the hereditary unit of the cell.

B1 Vacuoles

Vacuoles are membrane-bound cavities filled with cell sap, which is made up mostly of water containing various dissolved sugars, salts, and other chemicals.

B2 Plastids

Plastids are types of organelles, structures that carry out specialized functions in the cell. Three kinds of plastids are important here. Chloroplasts contain chlorophylls and carotenoid pigments; they are the site of photosynthesis, the process in which light energy from the sun is fixed as chemical energy in the bonds of various carbon compounds. Leucoplasts, which contain no pigments, are involved in the synthesis of starch, oils, and proteins. Chromoplasts manufacture carotenoids.

B3 Mitochondria

Whereas plastids are involved in various ways in storing energy, another class of organelles, the mitochondria, are the sites of cellular respiration. This process involves the transfer of chemical energy from carbon-containing compounds to adenosine triphosphate, or ATP, the chief energy source for cells. The transfer takes place in three stages: glycolysis (in which acids are produced from carbohydrates); the Krebs cycle, also called the citric acid cycle; and electron transfer. Like plastids, mitochondria are bounded by two membranes, of which the inner one is extensively folded; the folds serve as the surfaces on which the respiratory reactions take place.

B4 Ribosomes, Golgi Apparatus, and Endoplasmic Reticulum

Two other important cellular contents are the ribosomes, the sites at which amino acids are linked together to form proteins, and the Golgi apparatus, which plays a role in the secretion of materials from cells. In addition, a complex membrane system called the endoplasmic reticulum runs through much of the cytoplasm and appears to function as a communication system; various kinds of cellular substances are channeled through it from place to place. Ribosomes are often connected to the endoplasmic reticulum, which is continuous with the double membrane surrounding the nucleus of the cell.

B5 Nucleus

The nucleus controls the ongoing functions of the cell by specifying which proteins are produced. It also stores and passes on genetic information to future generations of cells during cell division. See Cell.

V TISSUE SYSTEMS

There are many variants of the generalized plant cell and its parts. Similar kinds of cells are organized into structural and functional units, or tissues, which make up the plant as a whole, and new cells (and tissues) are formed at growing points of actively dividing cells. These growing points, called meristems, are located either at the stem and root tips (apical meristems), where they are responsible for the primary growth of plants, or laterally in stems and roots (lateral meristems), where they are responsible for secondary plant growth. Three tissue systems are recognized in vascular plants: dermal, vascular, and ground (or fundamental).

A Dermal System

The dermal system consists of the epidermis, or outermost layer, of the plant body. It forms the skin of the plant, covering the leaves, flowers, roots, fruits, and seeds. Epidermal cells vary greatly in function and structure.

The epidermis may contain stomata, openings through which gases are exchanged with the atmosphere. These openings are surrounded by specialized cells called guard cells, which, through changes in their size and shape, alter the size of the stomatal openings and thus regulate the gas exchange. The epidermis is covered with a waxy coating called the cuticle, which functions as a waterproofing layer and thus reduces water loss from the plant surface through evaporation. If the plant undergoes secondary growth—growth that increases the diameter of roots and stems through the activity of lateral meristems—the epidermis is replaced by a peridermis made up of heavily waterproofed cells (mainly cork tissue) that are dead at maturity.

B Vascular System

The vascular tissue system consists of two kinds of conducting tissues: the xylem, responsible for conduction of water and dissolved mineral nutrients, and the phloem, responsible for conduction of food. The xylem also stores food and helps support the plant.

B1 Xylem

The xylem consists of two types of conducting cells: tracheids and vessels. Elongated cells, with tapered ends and secondary walls, both types lack cytoplasm and are dead at maturity. The walls have pits—areas in which secondary thickening does not occur—through which water moves from cell to cell. Vessels usually are shorter and broader than tracheids, and in addition to pits they have perforation—areas of the cell wall that lack both primary and secondary thickenings and through which water and dissolved nutrients may freely pass.

B2 Phloem

The phloem, or food-conducting tissue, consists of cells that are living at maturity. The principal cells of phloem, the sieve elements, are so called because of the clusters of pores in their walls through which the protoplasts of adjoining cells are connected. Two types of sieve elements occur: sieve cells, with narrow pores in rather uniform clusters on the cell walls, and sieve-tube members, with larger pores on some walls of the cell than on others. Although the sieve elements contain cytoplasm at maturity, the nucleus and other organelles are lacking. Associated with the sieve elements are companion cells that do contain nuclei and that are responsible for manufacturing and secreting substances into the sieve elements and removing waste products from them.

C Ground System

The ground, or fundamental, tissue systems of plants consist of three types of tissue. The first, called parenchyma, is found throughout the plant and is living and capable of cell division at maturity. Usually only primary walls are present, and these are uniformly thickened. The cells of parenchyma tissue carry out many specialized physiological functions—for example, photosynthesis, storage, secretion, and wound healing. They also occur in the xylem and phloem tissues.

Collenchyma, the second type of ground tissue, is also living at maturity and is made up of cells with unevenly thickened primary cell walls. Collenchyma tissue is pliable and functions as support tissue in young, growing portions of plants.

Sclerenchyma tissue, the third type, consists of cells that lack protoplasts at maturity and that have thick secondary walls usually containing lignin. Sclerenchyma tissue is important in supporting and strengthening those portions of plants that have finished growing.

VI PLANT ORGANS

The body of a vascular plant is organized into three general kinds of organs: roots, stems, and leaves. These organs all contain the three kinds of tissue systems mentioned above, but they differ in the way the cells are specialized to carry out different functions.

A Roots

The function of roots is to anchor the plant to its substrate and to absorb water and minerals. Thus, roots are generally found underground and grow downward, or in the direction of gravity. Unlike stems, they have no leaves or nodes. The epidermis is just behind the growing tip of roots and is covered with root hairs, which are outgrowths of the epidermal cells. The root hairs increase the surface area of the roots and serve as the surface through which water and nutrients are absorbed.

Internally, roots consist largely of xylem and phloem, although many are highly modified to carry out specialized functions. Thus, some roots are important food and storage organs—for example, beets, carrots, and radishes. Such roots have an abundance of parenchyma tissue. Many tropical trees have aerial prop roots that serve to hold the stem in an upright position. Epiphytes have roots modified for quick absorption of rainwater that flows over the bark of the host plants.

Roots increase in length through the activity of apical meristems and in diameter through the activity of lateral meristems. Branch roots originate internally at some distance behind the growing tip, when certain cells become meristematic.

B Stems

Stems usually are above ground, grow upward, and bear leaves, which are attached in a regular pattern at nodes along the stem. The portions of the stem between nodes are called internodes. Stems increase in length through the activity of an apical meristem at the stem tip. This growing point also gives rise to new leaves, which surround and protect the stem tip, or apical bud, before they expand. Apical buds of deciduous trees, which lose their leaves during part of the year, are usually protected by modified leaves called bud scales.

Stems are more variable in external appearance and internal structure than are roots, but they also consist of the three tissue systems and have several features in common. Vascular tissue is present in bundles that run the length of the stem, forming a continuous network with the vascular tissue in the leaves and the roots. The vascular tissue of herbaceous plants is surrounded by parenchyma tissue, whereas the stems of woody plants consist mostly of hard xylem tissue. Stems increase in diameter through the activity of lateral meristems, which produce the bark and wood in woody plants. The bark, which also contains the phloem, serves as a protective outer covering, preventing damage and water loss.

Within the plant kingdom are many modifications of the basic stem, such as the thorns of hawthorns. Climbing stems, such as the tendrils of grapes and Boston ivy, have special modifications that allow them to grow up and attach to their substrate. Many plants, such as cacti, have reduced leaves or no leaves at all, and their stems act as the photosynthetic surface. Some stems, including those of many grasses, creep along the surface of the ground and create new plants through a process called vegetative reproduction. Other stems are borne underground and serve as food-storage organs, often allowing the plant to survive through the winter; the so-called bulbs of the tulip and the crocus are examples.

C Leaves

The leaf is the primary photosynthetic organ of most plants. Leaves are usually flattened blades that consist, internally, mostly of parenchyma tissue called the mesophyll, which is made up of loosely arranged cells with spaces between them. The spaces are filled with air, from which the cells absorb carbon dioxide and into which they expel oxygen. The mesophyll is bounded by the upper and lower surface of the leaf blade, which is covered by epidermal tissue. A vascular network runs through the mesophyll, providing the cell walls with water and removing the food products of photosynthesis to other parts of the plants.

The leaf blade is connected to the stem through a narrowed portion called the petiole, or stalk, which consists mostly of vascular tissue. Appendages called stipules are often present at the base of the petiole.

Many specialized forms of leaves occur. Some are modified as spines, which help protect plants from predators. Insectivorous plants possess highly modified leaves that trap and digest insects to obtain needed nutrients. Some leaves are brightly colored and petal-like, serving to attract pollinators to otherwise small, unattractive flowers. Perhaps the most highly modified leaves are flowers themselves. The individual parts of flowers—carpels, stamens, petals, and sepals—are all modified leaves that have taken on reproductive functions.

VII GROWTH AND DIFFERENTIATION

The growth and differentiation of the various plant tissue and organ systems are controlled by various internal and external factors.

A Hormones

Plant hormones, specialized chemical substances produced by plants, are the main internal factors controlling growth and development. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects. Thus, auxin, one of the most important plant hormones, is produced by growing stem tips and transported to other areas where it may either promote growth or inhibit it. In stems, for example, auxin promotes cell elongation and the differentiation of vascular tissue, whereas in roots it inhibits growth in the main system but promotes the formation of adventitious roots. It also retards the abscission (dropping off) of flowers, fruits, and leaves.

Gibberellins are other important plant-growth hormones; more than 50 kinds are known. They control the elongation of stems, and they cause the germination of some grass seeds by initiating the production of enzymes that break down starch into sugars to nourish the plant embryo. Cytokinins promote the growth of lateral buds, acting in opposition to auxin; they also promote bud formation. In addition, plants produce the gas ethylene through the partial decomposition of certain hydrocarbons, and ethylene in turn regulates fruit maturation and abscission.

B Tropisms

Various external factors, often acting together with hormones, are also important in plant growth and development. One important class of responses to external stimuli is that of the tropisms—responses that cause a change in the direction of a plant's growth. Examples are phototropism, the bending of a stem toward light, and geotropism, the response of a stem or root to gravity. Stems are negatively geotropic, growing away from gravity, whereas roots are positively geotropic. Photoperiodism, the response to 24-hour cycles of dark and light, is particularly important in the initiation of flowering. Some plants are short-day, flowering only when periods of light are less than a certain length (see Biological Clocks). Other variables—both internal, such as the age of the plant, and external, such as temperature—are also involved with the complex beginnings of flowering.

VIII ECOLOGY

Rooted as they are in the ground, plants are commonly thought of as leading sedentary, vegetative, passive lives. However, a look at the ingeniously developed interactions that plants have with the other organisms in their ecosystems quickly corrects this notion.

A Cooperation and Competition

Many plant species exist as separate male and female plants, and pollen from male flowers must reach the female flowers in order for pollination and seed development to take place. The agent of pollination is sometimes the wind (a part of the physical environment), but in many cases it is an insect, bat, or bird (members of the biological environment). Plants may also rely on agents for dispersing their seed. Thus, after pollination, cherry trees develop cherries that attract birds, which ingest the fruit and excrete the cherry stones in more distant terrains.

Plants have evolved many other mutually beneficial relationships, such as the nitrogen-fixing bacteria that occur in the nodules on the roots of legumes (see Nitrogen Fixation). Many prairie grasses and other plants that flourish on open land depend on various herbivores to keep forests from closing in and shading them.

In the competition among plants for light, many species have evolved such mechanisms as leaf shape, crown shape, and increased height in order to intercept the sun's rays. In addition, many plants produce chemical substances that inhibit the germination or establishment of seeds of other species near them, thus excluding competing species from mineral resources as well as light. Walnut species, for example, use such an allelopathy, or chemical inhibition.

B The Food Web

Because plants are autotrophs—organisms that are able to manufacture their own food—they lie at the very foundation of the food web. Heterotrophs—organisms that cannot manufacture their own food—usually lead less sedentary lives than plants, but they ultimately depend on autotrophs as sources of food. Plants are first fed upon by primary consumers, or herbivores, which in turn are fed upon by secondary consumers, or carnivores. Decomposers act upon all levels of the food web. A large portion of energy is lost at each step in the food web; only about 10 percent of the energy in one level is stored by the next. Thus, most food webs contain only a few steps.

C Plants and Humans

From the prehistoric beginnings of agriculture until recent times, only a few of the total plant species have been taken from the wild and refined to become primary sources of food, fiber, shelter, and drugs. This process of plant cultivation and breeding began largely by accident, possibly as the seeds of wild fruits and vegetables, gathered near human habitations, sprouted and were crudely cultivated. Plants such as wheat, which possibly originated in the eastern Mediterranean region more than 9,000 years ago, were selected and replanted year after year for their superior food value; today many domesticated plants can scarcely be traced back to their wild ancestors or to the original plant communities in which they originated. This selective process took place with no prior knowledge of plant breeding but, rather, through the constant and close familiarity that preindustrial humans had with plants.

Today, however, the human relationship with plants is nearly reversed: An increasing majority of people have little or no contact with plant cultivation, and the farmers that do have such contact are becoming more and more specialized in single crops. The breeding process, on the other hand, has been greatly accelerated, largely through advances in genetics. Plant geneticists are now able to develop, in only a few years, such plant strains as wind-resistant corn, thus greatly increasing crop yields.

At the same time, humans have accelerated the demand for food and energy to the extent that entire species and ecosystems of plants are being destroyed before scientists can develop an understanding of which plant species have the potential to benefit humanity. Most species remain little known; those that seem to offer the greatest hope for providing new sources of food, drugs, and other useful products exist in tropical rain forests and other areas where rapidly growing human populations can quickly reduce the land to arid, sandy wastes. According to the World Conservation Union, about 34,000 species of plants are at risk of becoming extinct. This amounts to about one of every eight known species of ferns, flowering plants, and conifers and related plants. Increased knowledge of plants and attention to their survival are needed to solve many of the problems confronting the human world today. (World Food Supply.)

See also Dicots; Diseases of Plants; Fruit; Monocots; Nut; Plant Distribution; Plant Propagation; and Poisonous Plants.

Leaf morphology (Introduction)

I INTRODUCTION

Leaf, part of a plant that serves primarily as the plant's food-making organ in a process called photosynthesis. Leaves take part in other plant functions as well, including transpiration and guttation, both of which remove excess water from the plant, and respiration, the process by which a plant obtains oxygen and energy. Leaves also may store food and water and provide structural support.

A leaf is an extension of a plant's stem. Although most leaves are flat, broad, or bladelike, they also may be many other shapes, including round, oval, or feathery. In general, the leaves of trees such as hardwoods tend to be broad and relatively large, and the leaves of conifers, or cone-bearing trees, are usually small and needlelike in shape. In size, leaves range from only several millimeters (a fraction of an inch) long, as in the water plant Elodea, to 15 to 18 m (15 to 60 ft) long, as in some palm trees.

Green leaves derive their color from a green pigment called chlorophyll. The presence of additional pigments causes other leaf colors such as red in coleus and purple in cabbage. In temperate regions of the world, the leaves of some plants change color in autumn. Leaves of most garden plants turn yellow in the autumn, but those of many trees take on brilliant orange or red colors.

Most plants whose leaves change color also lose their leaves in the autumn. Such plants are called deciduous. In other plants, such as laurels and pines, the leaves do not change color and do not fall off in autumn. Such plants are called evergreens.

II THE PARTS 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.

The veins of a leaf are made up of two specialized tissues, xylem and phloem. Xylem usually forms the upper half of the vein. It consists of tubular open-ended cells that are arranged end to end. The walls of the cells are thick and rigid. Xylem conducts water and dissolved minerals to the leaf blade from the rest of the plant.

Phloem lies on the underside of the vein. It is made up of thin-walled tubular cells with tiny openings at their ends, somewhat like a sieve. These cells are also arranged end to end. Phloem carries food manufactured in the blade to the rest of the plant.

III LEAF GROWTH AND LEAF FALL

A leaf has a limited life span, usually living for only a single growing season in most deciduous plants and seldom more than a few years in evergreen plants. In temperate regions, leaves develop and grow during spring and early summer. In autumn they grow old, change color, and die. In nonwoody plants (low in xylem) the leaves wither and fall away because of decay and various external conditions. Woody plants (rich in xylem) lose their leaves as a result of characteristic changes in the base of the leaf. In tropical regions that have distinct wet and dry seasons, the formation and fall of leaves depend on moisture conditions rather than temperature. Contrary to popular belief, evergreen plants also shed their leaves. However, evergreens are never bare because the old leaves are pushed out only as new leaves develop.

A Budding

All leaves develop from buds, which are located at the nodes, or joints, of a plant stem and at the end of a plant stem. Contained in the buds are areas of rapidly growing tissue, called the meristem. The meristem gives rise to the first recognizable signs of the leaf. In the spring the buds shed their outer covering and open, exposing the leaves.

As leaves develop, they are arranged on the stem in one of three ways: alternate, opposite, or whorled. The arrangement provides an equal distribution of leaf weight on the stem. It also prevents overlapping so that each leaf can receive adequate sunlight.

B Color Changes

In addition to chlorophyll, leaf cells also may contain other pigments. These pigments account for the color of autumn leaves. Among the pigments found in leaves are yellow xanthophylls, yellowish-orange carotenes, and red and purple anthocyanins. Leaves also may contain tannins, which give them a golden-yellow color in autumn.

Like chlorophyll, xanthophylls and carotenes are contained in tiny granules in some leaf cells. Although these pigments are present throughout the leaf's lifetime, their colors are usually masked by the green of chlorophyll. In the autumn, however, chlorophyll production decreases, and the yellow and orange pigments become visible in the leaves. Eventually all pigment production stops, and the leaves turn brown.

Unlike xanthophylls and carotenes, anthocyanins are not contained in granules but are dissolved in the liquid part of leaf cells. In some plants, such as coleus and red cabbage, anthocyanins are always present, giving the leaves a reddish or purplish color. In other plants, anthocyanins are not present throughout the life of the leaf, but are produced only under certain conditions. In oak and maple leaves, for example, sugar accumulates in autumn. This accumulation is believed to result in the formation of anthocyanins and the production of vivid colors in the leaves.

C Leaf Fall

The leaves of evergreens continue to function and manufacture food throughout the year. In deciduous plants, however, the leaves stop functioning in the autumn and drop off. Leaves may be killed by frost, but changes due to age and growing conditions occur well before then. Decreased day length, reduced light intensity, lower temperatures, lack of water, and decrease of growth-promoting substances in the plant all contribute to the decline of the leaves. The changes start in the weakest part of the petiole, at the base. During autumn the cells in the base of the petiole begin to disintegrate and die. As a result, the leaf blade is supported only by the veins in the petiole. Soon the vascular bundles become plugged, decreasing the flow of water, food, and minerals to and from the leaf blade. When the blade is disturbed, as by wind, it breaks off the plant at the base of the petiole.

IV IMPORTANCE

Unlike leaf-bearing plants, animals cannot manufacture their own food. For this reason, animals must receive their nourishment, either directly or indirectly, from leaf-bearing plants. For example, cattle, sheep, and horses eat the leaves of grasses and other plants. These animals, in turn, are consumed by various carnivorous animals, including humans. Humans also eat many kinds of leaves directly, including artichoke, cabbage, lettuce, and spinach.

In addition to being a source of food, leaves provide many useful products. For example, the leaves of tea plants are made into a beverage, and the leaves of thyme, sage, and parsley are used for seasoning foods. Tobacco leaves may be smoked or chewed. Drugs are obtained from the leaves of foxglove, witch hazel, senna, and many other plants. Oils extracted from the leaves of geranium and citronella plants are used in manufacturing perfumes and soaps, and oils from mint and wintergreen leaves are made into flavoring extracts. Tannins, chemical substances used in preparing leather, are derived from sumac leaves, and dyes are made from indigo and henna leaves. The leaves of many plants may be used as fertilizer.