Introduction to Plant
Plant, any member of the plant kingdom—a realm of nature made up largely of living organisms that make their own food. Plant life in general is called vegetation. Flora refers to the plant life of a particular locality or geologic age. The scientific study of plants is called botany. Traditionally, biologists have placed bacteria, fungi, slime molds, and algae in the plant kingdom; today, most biologists place them in other kingdoms.
Importance of Plants
Most plants are able to convert simple chemical substances into the complex carbon compounds required by all living things. Animals lack this ability, and must feed, directly or indirectly, on plants.
In the food-making process, plants supply the air with uncombined oxygen and take up carbon dioxide. If plants did not maintain a balance of these two gases in the air, animals would die of suffocation—either directly from lack of oxygen or indirectly from an oxygen-blocking accumulation of carbon dioxide.
Human beings, like all other animals, depend on the plant world for food—either by eating the plants themselves or by eating the flesh of animals that have fed on plants. Humans use plant materials in countless ways for shelter, clothing, medicines, and manufactured goods. Plants beautify our surroundings, are favorite subjects in folklore, and have profound symbolic meanings in religion. The cultivation of crops, beginning in prehistoric times, gave humans a settled way of life and started them on the road to civilization.
The World of Plants
Botanists have described and named more than 400,000 species of plants. These include matlike mosses, low-growing herbs, and towering trees. Some plants live for only a few months, while others are more than one thousand years old. Some trees, such as the sequoia, can be more than 350 feet (107 m) tall and more than 3,000 years old. Bristlecone pines can be more than 4,500 years old and the creosote bush can grow even older—specimens 11,700 years old have been found.
DistributionSome plants live in lakes, rivers, or oceans. Most plants, however, grow on land, where they are distributed from sea level to the tops of the highest mountains. Even the air above the peaks may contain plant life, in the form of reproductive spores and seeds carried aloft by the wind.
The combined influence of moisture, temperature, and sunlight is the principal factor that determines the general distribution and character of the vegetation that covers the earth's surface. The various species of plants in any one area make up distinct types of communities. Plant communities vary in different areas and are generally characterized by the abundance of one specific group of plants. For example, a plant community in which the predominant species are trees is called a forest. Other types of plant communities include wetlands, grassland, desert, scrub, and tundra. In transitional zones, the plant life is characteristic of two or more of the major types of plant communities.
AdaptationEvery species of plant is adapted, or adjusted in form and function, to a particular habitat, or natural environment. The following conditions determine plant habitats:
Climatic Conditions include moisture (chiefly in the form of rainfall), temperature, and sunlight. Most habitats are determined primarily by moisture. Botanists therefore classify plants, in relation to environment, as (1) hydrophytes, which live in water (aquatic plants) or in wet places; (2) xerophytes, which live in dry places; and (3) mesophytes, which are intermediate between hydrophytes and xerophytes. (The suffix-phyte, much used in botanical names, comes from a Greek word meaning “plant.”)
Edaphic Conditions are those relating to the soil. A plant may grow best in an acidic or an alkaline soil; a wet or a dry soil; and a heavy (clayey) or a light (sandy) soil.
Biotic Conditions have to do with the presence of other plants, and animals. For example, a habitat may contain hardy plants that will choke out less vigorous species. Many plants require a habitat that includes particular insects for pollination (a reproductive process). Many plant habitats are modified by the presence of humans, who import grazing animals, drain swamps, use pesticides, and otherwise may radically alter natural conditions.
The plant world includes many species having special adaptations. Included in this group are parasitic plants, such as mistletoe and dodder, which absorb food from the living bodies of other plants. Saprophytic plants, such as the snow plant, live only on dead organic matter. Predatory plants, such as the Venus's flytrap, capture and eat insects. Epiphytic plants (also called air plants), such as Spanish moss, grow attached to trees or other objects; they obtain water from moisture in the air and obtain nutrients from leaves and other organic debris that falls on them.
The study of the relationships of plants and other living organisms to each other, and of living organisms to their environment, is called ecology. These relationships, when stable, are called the balance of nature, or the ecological balance.
For the plant life of various regions, see articles on specific countries, and vegetation maps in the article United States and articles about continents.
Plant Characteristics
Basic Life ProcessesTo sustain life, plants carry out certain basic chemical and physical processes that are common to all living things.
Typical FeaturesAll plants are multicelled, tissue-forming, and eukaryotic—that is, all plants are composed of various tissues formed from many cells and each cell contains a nucleus that is bound by a membrane. Some of the other characteristics, although typical of the plant kingdom as a whole, are not necessarily possessed by all species.
Plants that make their own food use a process called photosynthesis. This process requires the presence of chlorophyll, a cell pigment that gives most plants their characteristic green color.
Other typical plant features—serving, in most cases, to distinguish plants from other organisms—are the following:
1. Lack of voluntary movement. Because a plant does not have to search for food, it can occupy a fixed position; some plants drift in water.
2. Branching form. A plant exposes as much surface as possible to sunlight, water, and air, from which it obtains the raw materials for photosynthesis.
3. Indefinite shape and size. A plant does not fulfill an exact body pattern or have an absolute limit of growth. Instead, its shape and size are partially determined by environment.
4. Localized, unequal growth. Certain parts of a plant continue to grow while other parts die, and similar parts may grow at different rates.
6. Lack of a nervous system and distinct sense organs. A plant responds to its environment by tropism—a generalized response to such stimuli as light, heat, and moisture. For example, phototropism (“light-turning”) causes a plant to turn or grow toward a source of light, usually the sun.
7. Lack of an excretory system. A plant does not produce solid or liquid wastes. Instead, it gives off gases (water vapor, carbon dioxide, and oxygen, at various times) through tiny pores in its surface.
Types of Plants
Plants are classified as either bryophytes (nonvascular plants) or tracheophytes (vascular plants). The bryophytes are simple, ground-hugging plants highly dependent on a moist environment. There are about 24,000 species, which include hornworts, liverworts, and mosses. Bryophytes lack structures, such as roots, for absorbing water; most also lack tissues for conducting water. Instead, water is absorbed directly into the outer cells and then spreads slowly throughout the rest of the plant.
Tracheophytes include more complex forms of plant life. They are generally well adapted to conditions on dry land. All tracheophytes have a vascular system—a system of specialized tissues, called the xylem and phloem, for conducting water, minerals, and nutrients throughout the plant.
Tracheophytes are frequently divided into two general subgroups: the lower and higher vascular plants. The lower vascular plants—including the whisk ferns, club mosses, horsetails, and ferns—have more primitive characteristics (characteristics that arose early in evolution) than the higher vascular plants.
The higher vascular plants have more advanced characteristics, such as the ability to produce seeds. Seed-producing plants are divided into two groups: the gymnosperms (cone-bearing plants and the ginkgo) and the angiosperms (flowering plants). Besides the ginkgo, the gymnosperms include the cycads, gnetophytes (such as the welwitschia), and conifers. The angiosperms are divided into two subgroups: the monocotyledons (or monocots) and the dicotyledons (or dicots).
A classification of the plant kingdomNonvascular plantsDivision BryophytaLiverworts, hornworts, and mosses make up this division of plants. These plants reproduce by means of spores and lack true leaves, stems, or roots. Most live in moist areas. However, some mosses can withstand severe temperatures and are found in Arctic or desert regions.Class Hepaticae: Liverworts make up this class. These small plants may be flat and ribbon-shaped or leafy. They grow close to the ground.Class Anthocerotae: Hornworts usually grow only 3/8 to 3/4 inch (1 to 2 centimeters) across. The gametophyte is ribbonlike. Spores are contained in tubular sporangia that grow continuously from the sporangium base.Class Musci: True mosses make up this class. Stems may be erect or horizontal and bear many leaflike growths. They seldom grow over 8 inches (20 centimeters) long.Vascular plantsDivision TracheophytaTracheophytes are the vascular plants. All have two kinds of special tissues-xylem and phloem. Xylem tissue cells carry water from the roots to the leaves. Phloem tissue cells carry sugars made during photosynthesis in the leaves to other parts of the plant. Tracheophytes occur in most parts of the world.Subdivision LycophytinaThese plants, which include club mosses, isoetopsids, and selaginellas, have leaves with a single central vein. Spores are produced in sporangia that grow in the stem-leaf nodes or on the leaves. The sporangia are distinctly kidney shaped. Plants in this subdivision tend to grow in moist, shady areas.Class Lycopodiopsida: Lycopodiopsids are the club mosses and ground pines. Many species have stems that produce needlelike leaves. Spores are often tightly clustered at the tips of branches. Lycopodiopsids commonly grow on forest floors.Class Selaginellopsida: Selaginellas produce separate male and female spores. They are leafy and usually delicate in construction. Though most selaginellas live in moist places, one kind, the "resurrection plant," occurs in desert environments.Class Isoetopsida: Isoetopsids produce two kinds of spores, one male and one female. These small plants have grasslike leaves and live in wet, sometimes flooded places. During the Carboniferous Period, about 360 million to 300 million years ago, some isoetopsids were large trees.Subdivision EuphyllophytinaEuphyllophytes include most of the vascular plants. Their leaves usually have many veins, and their reproductive organs and internal structures differ from those of the Lycophytina plants.Class Psilopsida: The plants in this class are called whisk ferns or fork ferns. They have many slender, highly branched aerial stems by which the plants spread. They reproduce from spores. These rare plants are found in tropical and subtropical regions.Class Sphenopsida: Living sphenopsids are known as horsetails or scouring rushes. They have small leaves that occur in whorls at the nodes on the stem. Stems are hollow except at the nodes, giving them a jointed appearance. They grow 2 to 3 feet (61 to 91 centimeters) tall and reproduce by means of spores. During the Carboniferous Period, sphenopsids made up a much larger group that included trees.Class Pteridopsida: This class includes the ferns. Most ferns have large leaves called fronds. They reproduce by means of spores. Ferns are commonly seen on forest floors, but tropical fern trees may grow more than 65 feet (20 meters) high. Some floating aquatic ferns have leaves only about 1 inch (2.5 centimeters) long. Next to the flowering plants, ferns are the most diverse group in the division Tracheophyta.Class Gymnospermopsida: Gymnosperms reproduce by means of naked, or uncovered, seeds. Many gymnosperms are evergreens with a wide variety of leaf structures. The class consists of conifers, cycads, ginkgoes, and gnetaleans.Order Coniferales: Most conifers are evergreen trees or shrubs with needlelike or scalelike leaves. Almost all conifers bear their seeds in woody cones.Order Cycadales: Cycads grow fernlike leaves. Their seeds are borne in large cones. Many species have unbranched, erect stems, while others have partially underground stems called tubers. Some cycads are trees.Order Ginkgoales: Ginkgoes are trees with fan-shaped leaves. They bear fleshy seeds at the end of short branches. The seeds are not in cones. Only one species of ginkgo exists today.Order Gnetales: Gnetaleans are closely related to flowering plants and share many characteristics with them. Gnetalean seeds are borne in complex cones. Living forms vary widely in appearance and inhabit deserts and tropical rain forests.Class Anthopsida: Flowering plants, or angiosperms, make up this class. All angiosperms reproduce by means of covered seeds. They bear their sexual organs in flowers. After fertilization, the ovary grows into a fruit that encloses the seeds. Angiosperms are the most diverse group of plants, growing from the tropics to the polar regions. They are divided into two subclasses-Monocotyledonae and Dicotyledonae.Subclass Monocotyledonae: Monocots have seeds with only one cotyledon (seed leaf). The main veins in the leaves of these plants usually run parallel to each other. Flower parts usually occur in multiples of three.Subclass Dicotyledonae: Dicots have seeds with two cotyledons. Their leaves have a complex system of veins. Dicot flower parts usually occur in multiples of four or five.Plant Improvement
In each plant species, certain individual plants have more desirable qualities than others, from the viewpoint of human needs. These qualities include size and flavor of edible parts and attractive foliage. Plants are able to pass these qualities on, through their seeds, to their descendants.
Humans learned long ago, by trial and error, to select the seeds of desirable plants and thus improve, year after year, the quality of the species they cultivated. Today there are thousands of varieties of agricultural and garden plants.
Since the late 19th century, when the laws of heredity were discovered, plant improvement has become a science. Plant breeders have produced varieties that resist diseases, such as rot, smut, and wilt. Experimenters also are able to develop new varieties to fulfill special requirements. Examples are double-blossomed petunias, seedless grape-fruit, and early-maturing wheat. Desirable traits can be introduced into the genes of crop plants using the techniques of genetic engineering. .)