|
|
The significance of plants is all-pervasive. The energy obtained from food is first converted from sunlight to usable, transferable energy by green plants. The oxygen supply in the Earth's atmosphere is a result of PHOTOSYNTHESIS by green plants. Fossil fuels come from plant material. Plants also create and modify local environmental conditions on which many species of animals and other plants depend.
Historically, plants have been considered one of the two kingdoms of living things, the other being animals. Plants, kingdom Plantae, are broadly distinguished from animals, kingdom Animalia, by being stationary, by manufacturing their own food, by having a continuous type of growth that is readily modified by the environment, and in possessing a less definite form when mature. Possibly 400,000 kinds of plants exist, however, and many do not fit well into either kingdom. The recent trends are to recognize five different kingdoms based on evolutionary origins and relationships: Monera, the bacteria and blue-green algae; Protista, including all other algae and the protozoans; Fungi, such as mushrooms and molds; Plantae, or plants, containing the mosses, ferns, seed plants, and several minor groups; and Animalia, or animals.
CLASSIFICATION OF THE PLANT KINGDOM
In the five-kingdom system, a plant is defined as being multicellular and eukaryotic, or with a membrane around the nucleus of each cell. It has a life cycle consisting of alternating sexual and asexual generations. A plant also contains special types of light-absorbing molecules called chlorophyll a and chlorophyll b as well as a number of carotenoid pigments; it stores food in the form of starch. A plant has cell walls composed mostly of cellulose and develops a separation layer (cell plate) during cell division; it lacks centrioles, which are structures involved in cell division in animals.
Under the traditional two-kingdom system, organisms having only several, or even just one, of the foregoing characteristics are considered plants. The two-kingdom system includes the procaryotic bacteria, the single-cell algae, and the fungi--which lack chlorophyll--in addition to those defined as plants according to the five-kingdom system. Although the two-kingdom system is broader and more inclusive, the five-kingdom system will be followed here.
In the five-kingdom system the thallobionts, or thallophytes (thallus plants), are classified in three different kingdoms and not as part of the plant kingdom. Thallophytes have a simple plant body (thallus) without roots, stems, or leaves. They may be unicellular, filamentous, or of simple structure and live in or near water.
The green (photosynthetic) thallophytes are the
algae, and the nongreen thallophytes are the fungi and bacteria (although
some bacteria are photosynthetic). When these plants reproduce sexually,
the zygote (fertilized egg) develops directly into a new plant or into
spores and does not go through an embryonic stage as do the zygotes of
plants in the subkingdom Embryobionta. The gametes, or sex cells, are produced
within single cells. For the most part the plants are haploid; that is,
they have a single set of chromosomes.
Embryophytes
The embryophytes (embryo plants), or true plants,
all develop from an embryo and have multicellular reproductive structures
(gametangia). Chlorophyll a is the primary photosynthetic pigment, and
chlorophyll b and carotenoids are the principal accessory ones. All divisions
after the mosses and other bryophytes have specialized fluid-conducting,
or vascular, tissues containing elongated, hollowed tracheid cells and
thus are known as tracheophytes. The more primitive vascular plants lack
seeds and instead utilize spores in their dispersal.
The phylum Bryophyta includes the mosses, liverworts, and hornworts. These plants are fairly small, multicellular plants that usually occur in moist, shaded places. They may have stemlike and leaflike structures but actually lack true roots, stems, and leaves. They also may have an elementary water-conducting system of simple cells but not the advanced xylem and phloem tissues of the higher, vascular plants.
The phylum Tracheophyta comprises all vascular plants. The subphylum Rhyniophytina is an extinct group of primitive plants without roots or true leaves. They are of disputed classification but represent the earliest known vascular plants, dating from the Upper Silurian Period, more than 400 million years ago. They are the apparent ancestors of the other vascular plant groups.
The subphylum Psilophytina is a primitive plant group that includes the living "whisk fern." They lack roots and leaves and have a forked shoot with spore sacs, which form along the sides. Rootlike hairs anchor the plant and absorb nutrients. The lycopods (subphylum Lycophytina), or club mosses, possess simple, primitive leaves known as microphylls. They are another ancient group. Also primitive are the horsetails (subphylum Sphenophytina), which are characterized by jointed stems and the presence of silica in the walls of the outer cells, giving the plants a rough texture. Small, fused leaves encircle the nodes of the stem.
The subphylum Pterophytina includes true ferns, gymnosperms, and flowering plants. The ferns (class Filicineae) possess large, much-divided leaves (fronds). The spore sacs, or sporangia, are clustered on the underside of the leaf in various patterns. The leaves arise from an underground stem (rhizome). The conifers (class Coniferinae), cycads (class Cycadinae), and ginkgoes (class Ginkgoinae) are gymnosperms: their seeds lie exposed, typically at the base of scales (which are actually modified leaves) in a cone. The gymnosperms have no flowers; they lack the specialized water-conducting vessels of flowering plants but have tracheids, tubular cells that serve both as support and for water transport. The cycads are an ancient group of tropical plants with a short, stout trunk that is unbranched and also with palmlike leaves. The sperm of cycads are flagellated and motile.
The class Angiospermae are the flowering plants, in which the seeds are enclosed in a dry or fleshy fruit that develops from the ovary of the flower. Angiosperms are the most diverse and successful of plant groups, with well-developed vessels in the xylem and other adaptations to a variety of land habitats. Angiosperms are divided into two subclasses: Dicotyledonae and Monocotyledonae. The dicotyledons have two seed leaves (cotyledons) in the embryo; typically netted, or branched, leaf veins; flower parts in multiples of four or five (such as four stamens or ten petals); and a cambium (the cell-producing growth layer), which often develops secondary growth. The monocotyledons have one seed leaf, parallel leaf veins, flower parts in threes, and no cambium.
The earliest known forms of life date back to about 3.5 billion years ago. These organisms were apparently BACTERIA and BLUE-GREEN ALGAE. The green algae are believed to have appeared about 1 billion years ago, as did the FUNGI. The photosynthetic ALGAE began putting oxygen into the atmosphere, and by about 600 million years ago the atmospheric oxygen, although but a fraction of the amount in today's air, was sufficient to support life on land and, almost as important, to shield this life from direct exposure to damaging ultraviolet radiation from the Sun.
The earliest land plants occurred a little more than 400 million years ago, during the Late Silurian Period, and were similar to the "whisk fern," Psilotum, of today. The first seed plants appeared about 350 million years ago, during the Late Devonian Period, and are referred to as seed ferns because of the large fernlike leaves. The flowering plants date back to about 120 million years ago, or earlier, in the Early Cretaceous Period, and today total about two-thirds of all plants.
A green plant's physiological processes are functions adapted to fulfilling its needs for energy, nutrients, water, reproduction, and dispersal. It accomplishes these by the processes of photosynthesis (the manufacture of carbohydrates from carbon dioxide and water using light as an energy source), assimilation, respiration, and growth. A plant's structural features, although markedly diverse in the various groups, are each especially adapted for carrying out these functions.
The algae, living in water, directly absorb the water and nutrients into the cells. Various devices, such as gas bubbles, oil droplets, and cell-wall extensions, keep the free-living individuals close to the surface and near the light. Reproduction is accomplished by division and by motile gametes and spores. In the semiterrestrial mosses and other bryophytes, water and nutrients are absorbed from the soil through rootlike hairs (rhizoids), which also serve to anchor the plant. Materials move directly from cell to cell. Photosynthesis is concentrated in the thin leaflike structures, which, without efficient means of preventing water loss, merely curl up when conditions are dry. Reproduction and dispersal involve motile gametes and small wind-borne spores. The vascular plants on land have more complex structures allowing them to survive away from a permanently wet habitat. The typical flowering plant consists of roots, stems, leaves, and flowers, each with its particular structure and function.
The functions of the root are to anchor the plant, to absorb and transport nutrients and water, and sometimes to store food and serve in asexual reproduction. Adventitious roots, which do not arise either from the primary seed roots or as branches of later-developed roots, often appear at the nodes, or junctions of the stem.
Root systems may consist of one major root (taproot) or of a profuse mass of similar-sized branches. Penetration into the soil is accomplished by cell division and, largely, by the elongation of cells just behind the tip. A protective cap covers the tip. Just behind the region of elongation are the root hairs, which are small projections of the epidermal cells. The tremendous combined surface area of the myriad root hairs is responsible for absorption. Once absorbed, water and minerals pass through the cortex, or root wall, into the center of the root, called the stele or vascular cylinder; here these substances are conducted upward through the tracheids and vessels of the xylem.
Functions of the stem are to produce and support new leaves, branches, and flowers; to place them in positions where they can function most efficiently; and to transport materials to and from the roots. Frequently, stems serve to store food, carry on photosynthesis, and reproduce new plants.
Support is provided by various thick-walled cells found in the xylem or in strands outside the xylem. In herbaceous stems, turgor, or internal water pressure, is also important, as evidenced by the limp shape of a wilted plant.
Water and minerals are transported in the xylem and manufactured food in the phloem. In monocot stems the conducting tissues occur in separated, usually scattered, bundles, whereas in dicot stems the vascular tissues are arranged in a ring, with the primary xylem on the inside, the primary phloem on the outside, and a layer of dividing cells, called the vascular cambium, between them. The term wood in its commercial sense refers to secondary xylem. Secondary xylem is produced by the vascular cambium inward toward the center of the stem between itself and the primary xylem, increasing the thickness of the stem. The yearly production of secondary xylem usually forms a ring around that of the previous year, and these rings can be used to determine the age of the tree. In a similar manner secondary phloem is produced by the vascular cambium outward toward the surface of the stem between itself and the primary phloem; this, too, contributes to the thickness of the stem.
The leaves intercept light, exchange gases, and provide a site for photosynthesis. Some leaves also store food and water, provide support, or form new plants.
A flat, broad, thin structure gives more surface area for light interception and penetration. 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 by having a highly reflective surface.
Intake of carbon dioxide and release of oxygen occurs through small pores (stomata) in the leaf surface. The stomata are mostly on the lower surface and are able to close at midday. The cells within the leaf may be formed into two layers, the upper, tightly packed with elongated palisade cells, and the lower, loosely packed with spongy tissue. Photosynthesis occurs mostly in the palisade cells.
The flower is the sexual reproduction unit that functions to produce and house gametes (sex cells) and to attract pollinators. The stalk portion (pedicel) of the flower ends in an enlargement, called the receptacle, from which arise the sepals, petals, stamens (male units), and carpels (female units). Sepals are often green, being the least evolutionarily modified from leaves. The petals are usually larger and colorful, serving to attract the pollinator. Stamens produce and house the pollen, which contains the male gametes. Carpels produce and enclose the ovules, which contain the female gametes. A carpel may develop into a simple pistil, or several carpels may fuse to form a compound pistil. Whether simple or compound, a pistil typically consists of an enlarged ovary, from which arises an elongated style topped by a pollen-receiving stigma. Plants utilize many agents for transporting pollen from one flower to another, including wind, insects, birds, and bats.
Upon fertilization, the ovary begins developing into a fruit and the ovules into seeds. The function of the fruit is to aid in the dispersal of the seeds. Some fruits develop a fleshy, edible wall that attracts fruit eaters, which will disperse the seeds in their droppings; others develop a dry, hard wall that splits open, allowing the seeds to be shaken out by some disturbance. Some, such as the dandelion, develop light, feathery structures that are carried by the wind. Still others have hooks or barbs that stick to passing animals.
As the fertilized egg within the ovule develops into an embryo, the ovule walls develop into a seed coat, forming the ovule into a seed. The seed serves as the unit of dispersal for the new plant. It also provides some protection from injury and drying and some nourishment for the young plant until it can make its own food.
Plants may reproduce either sexually or asexually, with many utilizing both modes. Sexual reproduction involves two important processes: the fusion of reproductive cells--sperm and egg, collectively referred to as gametes--and a reduction division (meiosis) to halve the number of chromosomes present in each gamete. The zygote that results from the fusion of the sperm and egg, therefore, contains twice the chromosome number of either gamete.
Asexual reproduction involves no change in chromosome number. Vegetative cells or organs may become separated from the parent plant and continue to grow and develop into a new plant. Only mitosis, or cell division that retains the full number of chromosomes, is involved. The new plants have a genetic structure identical to that of the parent. In some plants embryos and seeds are formed asexually from an unfertilized egg cell (parthenogenesis or apomixis). Asexual reproduction has advantages in being more rapid, often with large parts of the plant already formed, and also in not requiring the presence of another plant. Species that are often invaders of new habitats, such as weeds, often reproduce asexually. Use is made of a plant's ability to reproduce vegetatively in propagation by means of cuttings, divisions, and grafting.
The life cycle of plants that reproduce sexually involves an alternation of two phases, or generations: the gamete-producing phase (the gametophyte plant) and the spore-producing phase (the sporophyte plant). Special cells of the sporophyte plant undergo reduction division (meiosis) to produce haploid spores, that is, spores with a single set, or haploid number (n), of chromosomes. The spores each develop into a multicellular, haploid, gametophyte plant. On it are formed special organs (gametangia) that produce and house the gametes (sex cells). The fusion of gametes from these plants results in a diploid zygote, that is, a fertilized egg with a double set, or diploid number (2n), of chromosomes. The zygote develops into the sporophyte. The cycle is then repeated, the sporophyte producing sporangia containing cells that undergo meiosis to form haploid spores.
This generalized cycle characterizes all embryophytes. The thallophytes (fungi and algae) for the most part lack a multicellular sporophyte. In the embryophytes the tendency toward a reduction of the gametophyte seems to be evolving in the progression from the mosses and other bryophytes to the flowering plants. In the bryophytes the gametophyte is the dominant generation, being the green structure readily visible. The sporophyte is embedded in the gametophyte and consists of a small capsule at the tip of a stalk.
In the vascular plants the sporophyte is the larger, dominant generation. The spores of the lower vascular plants are dispersed and develop into small, inconspicuous gametophytes, living away from the sporophyte, usually in the soil. The lower vascular plants also require some moisture to provide a transport medium for the motile, flagellated sperm. The gametes, which are usually produced on the same plant, mature at different times to ensure cross-fertilization between two different plants. There is usually a chemical produced by the female structure that attracts and guides the sperm.
The spores of seed plants are not dispersed; thus the gametophyte develops in place within the flower or cone of the sporophyte. During pollination, the entire male gametophyte (pollen grain) is transported to an area near the female gametophyte (within the ovule). The pollen that settles on the stigma of a flower develops a long tube that grows down through the style to the female gametophyte. Sperm nuclei migrate down the tube into the gametophyte, where fertilization occurs.
Once the egg is fertilized, it begins dividing and develops into an embryo. The embryo is nourished, in the flowering plants at least, by endosperm, a special tissue that is begun when one of the pollen grain's two sperm nuclei fuses with the two nuclei of the endosperm mother cell of the female gametophyte. The germinating seed first sends down a root (radicle) to establish itself in the ground. Until it can produce its own food, it may use food stored in the endosperm or in the cotyledons. Once established, it can continue to grow and develop organs.
One of the general characteristics of plants, compared to animals, is that they tend to grow continuously throughout their lives. Growth serves not only to increase a plant's size but also to provide the plant with a limited means of movement and orientation for placing itself in a more favorable position with regard to light, nutrients, reproduction, and dispersal. The growth of plants involves both the production of new cells and their subsequent enlargement. Following enlargement, a cell undergoes differentiation to become a part of a specific tissue.
There are two aspects of plant growth: primary and secondary. Primary growth takes place in young, herbaceous organs, resulting in an increase in length of shoots and roots. Secondary growth follows primary growth in some plants and results in an increased girth as layers of woody tissue are laid down. Monocots and herbaceous dicots typically exhibit only primary growth.
The formation of new cells takes place in regions known as meristems. At the tip, or apex, of each stem and root is an apical meristem, where cells are actively dividing. Each apical meristem produces three other meristems (protoderm, ground meristem, and procambium) called primary meristems. Tissues derived from the primary meristems are called primary tissues and include epidermis, cortex, pith, and primary xylem and phloem (vascular tissues). The elongation of cells produced by the primary meristems accounts for most of the increased length of stems and roots.
The important factors affecting plant growth and development include heredity, hormones, nutrition, and environment.
Hereditary, or genetic, factors control the general species characteristics of the individual and set limits on size and rate of growth. The genetic structure, through DNA and RNA patterns, acts by regulating protein synthesis, especially the manufacture of enzymes, as well as cell division, cell enlargement, the incorporation of substances into the cell walls, and the production and activity of the hormones. The gene action, in turn, is controlled by various growth regulators, particularly hormones and nutrients.
So-called plant hormones are organic chemicals produced in small amounts at one place in the plant that cause some physiological action in another. The several classes of plant hormones include the AUXINS, CYTOKININS, GIBBERELLINS, ABSCISIC ACID, and ethylene. Cytokinins are especially important in cell division; elongation is promoted by auxins and gibberellins. The bending of stems toward light is caused by auxins in higher concentration on the dark side, inducing more cell elongation.
Cell and organ differentiation are usually regulated by the interaction of several hormones. The initiation of roots by auxins and of buds by cytokinins depends on the presence of opposing hormones in the proper amounts. Other growth-related activities regulated by hormones include seed germination, flower and fruit development, and leaf enlargement.
Plants require all the essential ingredients of photosynthesis to construct the necessary compounds and structures. Water is especially important, because cell enlargement is a result of internal water pressure (turgor) extending the walls. In periods of drought plants tend to have smaller leaves. Calcium interacts with auxins and cytokinins in regulating cell divisions and elongation. Nitrogen is involved in the structure of chlorophyll, proteins, auxins, and cytokinins.
The intake and use of nutrients and the activities of hormones and other regulators are affected greatly by the external environment, particularly temperature and light. Certain wavelengths of light affect the activity of a pigment called phytochrome, which in turn interacts with hormones in regulating flowering, leaf expansion, stem elongation, sleep movement of leaves, and seed germination.
All physiological activities are directly related to temperature, with warmer temperatures favorable to more growth. Cold temperatures are required for some seeds to germinate, some buds to begin growing, and some plants to flower.
Green plants, because of their photosynthetic processes, form the base of the food chain and thus the beginning of the energy flow through an ecosystem. They are also the only important organisms able to assimilate inorganic elements and incorporate them into organic compounds in living tissues, and therefore form a vital link in the cycling of nutrients. Bacteria and fungi serve as the other major link in the cycling process because they decompose organic tissues and release the elements to the soil or water. The energy that is not used directly by the plant in carrying out its life processes goes into the production of new tissues, or biomass, which can then be used by other organisms as a food source.
Edible, concentrated portions of various plants--such as seeds, fruits, and tubers--are used as a food source not only for the human populations of the world but also for livestock feed. Since prehistoric times desirable types of plants have been selected and bred in order to obtain types that produce, for instance, greater yields or larger seeds, fruits, or roots.
The most important food plants are the GRAINS of the grass family, particularly wheat, rice, corn, sorghum, and barley. In many tropical countries the higher-protein cereal grains do not grow well, and the basic foods are the more starchy root crops, such as yams, sweet potatoes, and manioc (cassava). Plant foods contribute about 88 percent of the world's calories and about 80 percent of the proteins; the more developed the country, however, the less of its diet is from plant foods.
Certain plants also provide the major beverages of the worlds, including coffee, tea, mate, and fruit juice. Beer is usually brewed from fermented barley and hops; wines, from grapes; and such spirits as whiskey and vodka, from grains or potatoes. Many textile fibers are derived from plants, including cotton, flax, and hemp. The wood of trees is used to make tools, furniture, and houses; such chemicals as acetic acid, methanol, and turpentine are obtained from trees.
Certain plants are useful in medicine because they contain chemicals that have a desired physiological effect on the human system. Some of these chemicals and their plant sources are the antimalarial quinine from cinchona bark, the heart-stimulant digitalis from foxglove leaves, the antispasmodic atropine from belladonna (nightshade), and rauwolfia tranquilizers from the genus Rauvolfia. Most of the chemicals used, under different concentrations and uncontrolled use, are extremely poisonous. It is thought that many of these have been developed in the plant as a defense against predators.
Scarcely a product exists in which plants have not played some important role, either as a component, an implement in its construction, or at least as the energy source (fossil fuels) of its production. Some of the more important plant products include wood, fibers, oils, and rubber. Fibers are elongated cells abundant in the bark and leaves of some plants. Cotton, which comes from the long hairs on the cotton seed, is the world's most important fiber. Others are flax, jute, and kapok. Oils are stored as food reserves in the seeds and fruits of many plants. Most are used as human food, but some are used in industry. The most significant oil plant is the olive, but also important are the coconut, oil palm, soybean, and corn.
Plants can also be harmful to humans, especially bacteria and fungi. Parasitic plants such as dodder and broom rape can seriously damage plants valued by humans.
GEOGRAPHICAL DISTRIBUTION AND PLANT COMMUNITIES
Few places on the Earth can be found where at least some of the 400,000 species of plants are not adapted to live. Only the polar zones, the highest mountains, the deepest oceans, and the driest deserts are devoid of plants (other than bacteria). A given plant species has a limited distribution, however, depending on its own particular requirements. Some species are broadly distributed, being tolerant of a wide range of conditions. Narrowly restricted species have limited tolerance to a specific factor, such as soil type.
Climate is the major factor affecting the distribution of plants and determining their structural adaptations. The greatest number of species are found near the equatorial regions in tropical climates, where moisture and temperature are seldom limiting. The number of species per area decreases toward the poles.
Plants assume various adaptations as they occupy drier or colder areas away from the tropics. In drier climates plants develop features, known as xeromorphic ("dry form") characteristics, such as smaller, thicker leaves, spines, dense hairiness, and water-storage organs. In colder zones plants become lower in stature, with the growing points protected at or just beneath the ground. Because of similar adaptations, plants of a given climatic zone form a characteristic vegetation type. A large area occupying a given climatic zone and with a characteristic vegetation and associated groups of animal species is called a BIOME. The major biomes include the tropical rain forest, desert, and tundra.
Much of the emphasis in research is on the development and use of new techniques and equipment. In the field of classification especially, the use of new chemical methods and computers has become important in elucidating relationships and in handling data. The electron microscope, and in recent years the scanning electron microscope (SEM), are important tools of the plant anatomist in determining the structure and function of plants at the subcellular level. The SEM is used in particular in studying the detail of leaf surfaces and pollen grains.
Studies of the structure and function of membranes, where much of the plant's activities take place, are widely pursued. Recent research in molecular botany has been in the synthesis of chlorophyll and the interrelationships of nucleic acids and hormonal functions. The role of hormones and their interactions with phytochromes in affecting flowering continues to be an intensively studied area in plant physiology. Nearly all photosynthetic plants utilize the carbon from carbon dioxide to manufacture sugar molecules by employing one specific set of chemical reactions to fix, or transfer, the carbon atoms. This series of chemical reactions is called the C = 3 (or Calvin-Benson) cycle because the three-carbon compound, phosphoglyceric acid, is formed during its operation. An area of recent interest has been the discovery of an alternative carbon-fixing pathway in a number of other plants. This is called the C = 4 (or Hatch-Slack) cycle because four-carbon compounds are produced during this process.
In agriculture, the main emphasis is still on increased food production, with breeding programs to develop high-yield strains, especially those yielding more protein. Computers are also used to simulate the growth of several food crops and study the factors involved.
Rodney G. Myatt
To the top