The kingdom Plantae accounts for the largest proportion of the earth’s biomass with its approximately 250,000 species of mosses, liverworts, ferns, flowers, bushes, vines, trees, and other plants. Aquatic and terrestrial plants are the basis of all food webs. They contribute life-supporting oxygen to the atmosphere and provide humans with the fossil fuels, medicines, and other substances so important to our present existence.

Plant cells, like animal cells, show a high degree of organization with membrane-bound internal structures. The nuclear envelope forms a barrier between the chromatin (genetic material) and cytoplasm of the cell. Convoluted mitochondria convert nutrients into energy the plant can use. Unlike animal cells, however, plant cells also contain chloroplasts, organelles capable of synthesizing energy from sunlight. Further differences include the cell wall, which contains cellulose and is quite rigid, and the fluid-filled vacuole, single and quite large in plants.

The photosynthetic tissue of leaves consists of two types of thin-walled, flexible cells: the long, columnar palisade parenchyma, where most of the chemical reactions take place, and the irregularly-shaped spongy parenchyma. Both types of cells contain chloroplasts, photosynthetic organs that adjust their position within the cytoplasm for maximum exposure to the sun. Veins carry water and nutrients to the parenchyma cells. These inner structures, the parenchyma and veins, collectively called mesophyll, are sealed between layers of epidermal cells. Openings called stomata allow the entrance and exit of gasses. A transparent, waxy cuticle forms the outermost layer of the leaf.

An examination of leaves, stems, and other types of plant tissue reveals the presence of tiny green, spherical structures called chloroplasts, visible here in the cells of an onion root. Chloroplasts are essential to the process of photosynthesis, in which captured sunlight is combined with water and carbon dioxide in the presence of the chlorophyll molecule to produce oxygen and sugars that can be used by animals. Without the process of photosynthesis, the atmosphere would not contain enough oxygen to support animal life.

Stems differ between gymnosperms (conifers and related plants) and angiosperms (flowering plants) and between the two divisions of angiosperms—monocotyledons and dicotyledons. Common to all of them, though, are basic tissue types: vascular tissue (xylem and phloem), which conducts water and nutrients to the cells of the plant; ground tissue, called pith at the center of the stem, which surrounds the vascular tissue; and dermal tissue, a protective layer.

The stem of a plant provides pathways for the distribution of water and nutrients between the roots, leaves, and other parts of the plant. The herbaceous stem of the dandelion (top, center) lacks lignin, the stiffening material in rigid, supportive woody stems. For this reason, herbaceous plants are generally limited in their physical size. Spurges and cacti (bottom, left), their leaves reduced to needles to prevent evaporation in a dry climate, consist entirely of stem material. Tubers, such as potatoes (top, right), are swollen, food-storing, underground stems that nourish growing buds. The stems of some plants are adapted for protection, as in the hawthorn (bottom, left). Others actively compete for sunlight, using touch-sensitive, curling tendrils (top, left) or other structures to climb upwards.

As the trunk of a tree expands with secondary growth, new phloem forms on one side of the cambium tissue, and new xylem on the other. This secondary xylem is known as wood.

The main root of many plants divides as it grows downward. The branches, called lateral roots, further divide to form a network that anchors the plant in the ground. New growth takes place at the ends of the smallest roots. Tiny root hairs absorb water and nutrients from the soil, channeling them up to the stem and leaves of the plant through the xylem tissue at the center of the root.

Although similar in structure and function to the roots of plants living in soil, the roots of epiphytes, or air plants, are adapted for growth above the soil surface. Usually growing on the branches or trunks of trees and shrubs, where there is increased access to light, the plants develop aerial roots.

This lengthwise section through the tip of a plant root reveals the apical meristem, characterized by rapidly dividing cells that are responsible for primary growth. Apical meristem can also be found at the tips of stems.

The two subclasses of angiosperms, or flowering plants, differ in a number of ways. Dicotyledons, represented here by the dandelion, have floral organs (sepals, petals, stamens, pistils) in multiples of four or five. In contrast, the floral organs of the iris and other monocotyledons generally occur in multiples of three. The leaves of dicots have a netlike vein pattern, while those of monocots have parallel veination. The vascular tissue (xylem and phloem) inside the stem of a dicot is arranged in a ring. Inside a monocot stem, xylem and phloem are scattered. Dicot seeds have two seed leaves, or cotyledons, that nourish the growing seedling, while monocots have only one. The stem and root of dicots expand with secondary growth, adding vascular cambium and secondary xylem and phloem; monocots show no secondary growth. These differences reflect an early divergence in the evolutionary history of angiosperms. Monocots, the more advanced of the two groups, evolved from a primitive dicot.

A flower consists of up to four types of modified leaves. Sepals, closed over the bud before it blooms, are outermost. One step inward lie the petals. These serve to attract pollinators, both by coloration and by scent-producing glands. Inside the petals are one or two circles of pollen-producing stamens, the flower’s “male” reproductive organs. The carpels, composed of stigma, style, ovary, and ovule, are innermost. It is the carpel that receives pollen grains and, in the case of fertilization, swells to form fruit. The carpel is believed to have evolved for protection from ovule-eating insects and other harmful elements of the environment.

Many species of butterflies eat plant nectar. When these butterflies land on a series of flowers in search of food, they brush their bodies against both male and female floral organs, inadvertently transferring pollen from one flower to another.

Cypress, like all other coniferous trees, is wind pollinated. The tiny male “flowers” are located at the ends of the small branchlets, where the wind can easily pick up and distribute their pollen.

Strictly defined, the fruit of a flowering plant is its mature, swollen ovary. Pollen grains (the male gametes, carried from the anther of one flower to the stigma of another flower by a foraging insect) germinate on the stigma, growing down the style and into an ovule, where they may fertilize the egg within. If fertilized, the ovules develop into seeds, and the receptacle protecting the ovary enlarges to form what we recognize as the flesh of the fruit.

A seed has three main parts. The embryo consists of the cells that will develop into the structures of the adult plant (root, bud, stalk, and leaf). The cotyledons—one in monocots and gymnosperms and two in dicots—are organs of absorption, drawing food from the seed’s storage tissue. In monocots, this tissue is called the endosperm, and in gymnosperms, the megagametophyte. The cotyledons themselves serve as storage tissue in dicots. The seed coat protects all of these structures from predation, injury, and moisture loss.

Firs are coniferous, evergreen trees with needle-like leaves and woody fruit called cones. The modification of the leaves into needle-like structures is thought to increase the surface area for photosynthesis. The cones bear the seeds on their woody scales.

Cones are specialized seed-bearing structures unique to coniferous trees, such as firs, cedars, pines, cypress, and spruces. The seeds develop within the cones. In the pine tree the developmental period may take as long as three years. Shortly after the seeds mature, the protective scales of the cone open up, and the seeds are released.

Most seeds begin to germinate only with the warming days of spring, months after they have fallen to the ground. As the embryo inside expands, the seed cracks and a root emerges to provide the seedling with both stability and nutrients from the soil. While the root continues to grow and branch downward, the embryonic stem sprouts upward. Nourished from this point by the cotyledons, or seed leaves previously folded within the seed coat, the seedling will develop a shoot with adult leaves.

Since green plants are autotrophic, or able to manufacture their own food from water, carbon dioxide, sunlight, and inorganic molecules, they must grow in areas with available sunlight. In response to this need, green plants are phototropic, or able to grow toward a source of light.

Coniferous trees constitute a large majority of the forestlands in Canada. Coniferous trees maintain their leaves year-round, while deciduous trees turn color and lose their leaves annually. Both types of trees can inhabit the same regions and forest belts. In this park in Québec, the deciduous trees’ bright autumn hues contrast with the deep green of the conifers.

The green color in a normal holly leaf results from the uniform distribution of the green pigment chlorophyll. In a variegated leaf this important photosynthesizing pigment is reduced or lacking altogether in certain parts of the leaf, as indicated by the yellow color. Though the holly plant mounts a spiny defense against leaf-eating animals, its variegated leaves generally will not survive in nature due to this pigment deficiency.

In contrast to the rows of leaflets found on trees such as the walnut, a single, or simple, leaf arises from the buds of oak and maple trees. The netlike pattern of veins visible here is characteristic of dicotyledonous plants.

The arolla pine has needles that grow in bundles of five. Pine needles are actually highly modified leaves that are not shed annually but rather remain on the tree for long periods. Each needle has a tough outer layer called the cuticle, which in turn has a waxy coating that helps prevent water loss.

The eucalyptus tree has two totally different shapes of leaves on the same stem. The young leaves are small, circular, and completely encircle the branch, while older leaves are long, blade-like, and borne at the ends of short stalks.

The thick flesh of leaf succulent plants, native to marshes and semidesert conditions, swells in damp conditions to store as much fresh water as possible. Adaptations such as the white coloration and waxy, water-sealing coating of some leaves reduce evaporation. The leaves wrinkle as water is used.

The rhododendron is classed as an evergreen, that is, it does not replace its leaves each year. In order to survive the effects of wind, rain, sun, and insect predation, the leaves of the rhododendron have a tough, waxy upper surface. They also sometimes have a felt-like lower surface to help retain water and repel insects.

Compound leaves, although they appear to be a collection of many leaves, arise from a single bud. The leaflets fall as a group in the autumn. The leaf pictured here is from a Hercules’-club. It is pinnately compound (with paired, equally-sized leaflets arising from a central blade), and doubly so, with leaflets attached to matching side stalks. The leaflets of palmate compound leaves, such as those of the horse chestnut family, radiate from a single point.

Dorling Kindersley Compound Leaf," Microsoft® Encarta® 96 Encyclopedia. © 1993-1995 Microsoft Corporation.

Monocot Leaf

Leaves of monocotyledonous plants, such as the palm pictured here, usually have parallel leaf veins. Dicots show netlike venation. Palm leaves, native to windy environments with little rainfall, have tough leaves that resist drying out.

Dorling Kindersley Monocot Leaf," Microsoft® Encarta® 96 Encyclopedia. © 1993-1995 Microsoft Corporation.

Colors of Autumn Leaves

The brilliant autumn colors characteristic of the leaves of many plants is due to the presence of accessory leaf pigments that normally assist the plant in photosynthesis by capturing specific wavelengths of sunlight. These pigments, called carotenoids, become visible when the leaf dies in the fall.

Dorling Kindersley Colors of Autumn Leaves," Microsoft® Encarta® 96 Encyclopedia. © 1993-1995 Microsoft Corporation.

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