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The Importance of Plants
Cereals are grasses that contain grains. Grains are the edible, dry fruits of a cereal, such as
rice, wheat, corn, oats, sorghum, rye, and millet. Root crops are roots of underground stems that are
rich in carbohydrates. Root crops include potatoes, beets, carrots, and radishes. Legumes are
members of the pea family and bear protein-rich seeds in pods. Soybeans are the most important
legume crop because they are produced in the largest amount and have many important uses. A fruit
is the part of the flowering plant that usually contains seeds. Food derived from the leaves, stems,
seeds, and roots of soft plants are often called vegetables. Most nuts have a hard outer layer and
contain a dry, one-seed fruit. Nuts include almonds, walnuts, pecans, and hazelnuts.
The study of the interactions between plants and the environment is called plant ecology.
The most important interaction involves the ability of plants to capture solar energy through
photosynthesis. In photosynthesis, plants absorb carbon dioxide from the air, produce sugar and
break apart water, releasing oxygen into the air. Consumers use oxygen in aerobic respiration and
produce carbon dioxide and water. Organic compounds from plants provide consumers with energy,
building blocks, and essential molecules like vitamins and fibers.
Plant Evolution and Classification
Plants can be divided into two groups based on the presence of vascular tissue. Nonvascular
plants have neither true vascular tissue, nor true roots, stems, or leaves. Most members of the
vascular plant group have vascular tissue and true roots, stems, and leaves. Vascular plants can further
be divided into two groups—seedless plants and seed plants. Seedless plants include the phylum of ferns
and three phyla made up of plants closely associated with ferns. Seed plants—plants that produce
seeds for reproduction—include four phyla of gymnosperms and one phylum of angiosperms.
Gymnosperms, which include pine trees, produce seeds that are not enclosed in fruits. Angiosperms, also known as
flowering plants, produce seeds in a protective fruit. Examples are apple and orange trees.
Bryophytes are mostly terrestrial, or found on land. They are seedless, produce spores, and are usually
very small because they don’t have vascular tissue—usually less than 2 cm in height. Mosses, one kind of
bryophyte, are called pioneer plants because they are often the first species to inhabit a barren area. This
is an important environmental function because mosses gradually accumulate inorganic and organic matter
on the surface of rocks, creating a layer of soil in which other plants can grow. In areas devastated by
fire, volcanic action, or human activity, pioneering mosses can help trigger the development of new
biological communities. They also help prevent soil erosion by covering the soil surface and absorbing water.
Comparing Monocots and Dicots
Plant Type
Embryos
Leaves
Stems
Flower Parts
Examples
One cotyledon
Parallel
venation
Scattered vascular
bundles
Usually occur in
threes
lilies, irises,
orchids, palms,
tulips, bananas,
pineapples, onions,
bamboo, coconut,
wheat, corn, rice,
oats, barley,
sugarcane
Net venation
Radially arranged
vascular bundles
Usually occur in
fours or fives
beans,
lettuce, oaks,
maples, elms,
roses, carnations,
cactuses, most broadleaved forest trees
Monocots
Dicots
Two
cotyledons
Plant Structure and Function
Leaves come in a wide variety of shapes and sizes and are an important feature used
for plant identification. The broad, flat portion of a leaf, called the blade, is the site of most
photosynthesis. The blade is usually attached to the stem by a stalk-like petiole. The maple leaf is a
simple leaf; it has a single blade. In compound leaves such as a white clover, the blade is divided into leaflets.
Leaves consist of three tissue systems. The dermal tissue system is represented by the epidermis. In
most leaves the epidermis is a single layer of cells coated with a nearly impermeable cuticle. Water, oxygen, and
carbon dioxide enter and exit the leaf through stomata in the epidermis. Epidermal hairs are often present and
usually function to protect the leaf from insects and intense light.
Sexual Reproduction in Flowering Plants
Flower parts are usually found in four concentric whorls, or rings. Sepals make up the outermost whorl of
flower parts. They surround and protect the other parts of a developing flower before it opens. Petals
make up the next whorl. The petals and sepals of wind-pollinated plants are usually small or absent.
The two innermost whorls of flower parts contain the reproductive structures. The male reproductive
structures are stamens, each of which consist of an anther and a filament. An anther produces the
pollen. A stalk-like filament supports the anther. The innermost whorl contains the female
reproductive structures, which are called the capel. One or more carpels fused together are called a pistil. A
style, which is also stalk-like, rises from the ovary. The tip of the style is called the stigma. Generally, a stigma is
sticky or has hairs, enabling it to trap pollen grains. Most species of flowering plants have flowers with both
stamens and carpels. However, some species have flowers with only stamens or carpels.
Seasonal Responses
Some trees leaves are noted for their spectacular fall color. The changing fall colors are
caused mainly by a photoperiodic response but also by a temperature response. As night becomes
longer in the fall, leaves stop producing chlorophyll. As the chlorophyll degrades, it is not replaced.
Other leaf pigments, the carotenoids become visible as the green chlorophyll degrades. Carotenoids
include the orange carotenes and the yellow xanthophylls. The carotenoids were always in the leaf;
they were just hidden by the more abundant chlorophyll. Another group of pigments found in leaves,
the anthocyanins, are produced in cool, sunny weather. Anthocyanins produce beautiful red and purplish-red
flowers.
Plants as Medicines
For thousands of years, people throughout the world have used plants to treat illness. Ethnobotanists—
scientists who study the plant knowledge of native cultures—have made many discoveries that led to the
development of important medicines we use today. In fact, one fourth of all prescription drugs in the US have
plant chemicals as an active ingredient.
In the early 1600s, a European monk saw Native Americans in the Andes Mountains grind the roots, bark
and branches of the cinchonas trees. They used the finely ground powder they called “fever bark” to treat high
body temperatures. The fever-reducing ingredient in the plant is called quinine, from the Spanish word quinaquina.
Today, an artificial form of quinine is used in controlling the recurrence of a disease called malaria.
The commonly used drug, aspirin, came from a plant called queen of the meadow. Many years ago, the roots
of the plant were boiled. People drank the “tea” containing salicylic acid as a treatment for fever and pain. Today,
we use a synthetic form of aspirin with the same characteristics.
Some cancer medicines were discovered because Native Americans used the rosy periwinkle plant in the
treatment of diabetes. When scientists studied the plant to find out whether it was effective against diabetes,
they discovered that two of the plant’s ingredients kill leukemia cells. Their findings resulted in treatments for
leukemia and Hodgkin’s disease.
In the mid-1990s, ethnobotanists began looking at a plant used by native healers to treat diseases
caused by viruses. The native healers, or shamans, traditionally have been the one trained to know which
plants are good for healing. Ethnobotanists found that the plant used by the shamans contains a chemical
called prostratin. Prostratin might stop the growth of the virus that causes AIDS. Some
ethnobotanists think shamans could be an important link in the process of turning plants into prescriptions.