Botany is the scientific study of plants. Many people consider everything from bacteria to the giant sequoia trees to be plants. That would include algae, fungi, lichens, mosses, ferns, conifers, and flowering plants. Today, scientists believe that bacteria, algae, and fungi have their own distinct kingdoms. The kingdom Plantae includes 250,000 species of mosses, liverworts, ferns, flowers, bushes, vines, trees, and other plants. Aquatic (water) and terrestrial (land) plants are the basis of all food webs. They emit into the atmosphere the oxygen that is needed by animals for survival.
Categorizing Plants Plants are categorized according to the structure by which the plant absorbs water. Plants are either vascular or nonvascular. Vascular plants transport water from the roots to the stems and to the leaves by means of tubelike structures. Nonvascular plants absorb water only through their surfaces.
Another way to categorize plants is according to how they reproduce. Some plants reproduce by producing seeds. Others produce spores. Yet another way to categorize plants is by whether they produce flowers (angiosperms, such as roses or apple trees) or don't produce flowers (gymnosperms, such as pine trees). Seeds store food for the growing plant; the part of the seed that stores the food is called the cotyledon. Some seeds have one cotyledon and are called monocots. Others have two and are called dicots.
Plants can also be classified by their life cycle. Annuals go through their entire life cycle, from germination through seed production to death, in one growing season. Corn, zinnias, beans, marigolds, and mums all have to be planted each year. Biennials have a two-year growing cycle. In year one, the seed germinates, produces leaves and roots, and forms a compact stem for food storage. In year two, the plant forms an elongated stem, produces flowers and fruits, and then ends with seed production. After seed production, the plant dies. Examples are onions, raspberries, hollyhocks, and carrots. Perennials live for many years. Parts of the plant may die back during the winter, but the plant will grow back in the spring.
Plants can also be categorized by their behavior in winter. Deciduous plants, including shrubs and trees, lose their leaves in the winter. Maple and oak trees are examples. Evergreen plants keep their leaves or needles throughout the year, sometimes shedding only old leaves or needles that are more than two years old. Pine and spruce trees are examples of evergreen trees.
Major Parts of a Plant Plants are made up of a number of different parts, including roots, stems, and leaves.
Roots Roots have several functions: absorbing nutrients and water, anchoring the plant into the soil, holding up the stem and leaves, and storing food. They are usually below ground and do not have chloroplasts (see the section on plant cells).
There are basically two types of root systems, a taproot system and a fibrous root system. A taproot system has one fat or sturdy main root, with just a few branching roots. Carrots, radishes, and parsnips are good examples of a taproot. A fibrous root system has many branched roots. Examples of plants with fibrous roots are most grasses.
Roots have tiny root hairs. Roots can have a very large number of root hairs, increasing the capability of the root to absorb water and food.
Stems The stem is the main trunk of a plant. Leaves, flowers, and fruits get support from the stem. Stems also carry nutrients and water. Some stems store food. Stems are generally above ground and vertical. Some stems grow below ground (bulbs) or fasten themselves to the ground. The strawberry plant is an example of a plant with a stem that hugs the ground.
Nodes are places on the stem where buds form. Spaces between the nodes are called internodes.
One of the major functions of the stem is to move water, nutrients, and food through the plant. The system that does this is called the vascular system and is similar to the circulatory system in humans. Phloem tubes move food from the roots through the stem to the leaves. Xylem tubes move minerals and water. These tubes are surrounded by the main tissue in the stem, the cambium.
Leaves Leaves grow out from the stem. The leaves' major job is to make food for the plant. For the most part, leaves are flat, broad, and green. The flat surface maximizes their ability to absorb light and transform it into food. There is a protective layer on leaves called the cuticle that reduces the evaporation of water from the plant and helps to protect the plant from disease-causing organisms. Leaves have tiny openings called stomata that enable the plant to take in carbon dioxide and release oxygen into the atmosphere. Guard cells cover the stomata openings and regulate the exchange of water vapor, oxygen, and carbon dioxide into and out of the stoma.
Flowers The main job of flowers is sexual reproduction or seed production. Various parts of the flower are involved in sexual reproduction. The pistil is the female portion of the flower. It includes the stigma, the surface that captures and holds pollen, and the style, the area between the stigma and the ovaries. The stamen is the male portion of the flower. It includes the filament that holds the anther. The anther is where the pollen is formed and released.
Some plants have flowers with both male and female parts, and others make flowers with only male or only female parts. Corn is an example of a plant that has both male and female flowers on one plant. The tassel is the male flower, and the ear is the female flower. The holly is an example of a plant that has either a male or a female flower. Plants of both sexes need to be planted close together so that the seeds (berries) can form on the female plant. Other parts of a flower are sepals, which enclose the various flower parts, and petals, the bright-colored parts like the petals of a rose or daisy.
Any edible part of a plant that is not a flower is considered to be a vegetable.
Fruits A fruit is a ripened ovary or group of ovaries containing the seeds. When the ovary is fertilized, the seeds develop and the ovary enlarges, forming the fruit. Examples of fruits are peanuts, sunflower seeds, barley, walnuts, tomatoes, grapes, oranges, apples, raspberries, cucumbers, squash, corn, eggplants, and strawberries.
How Plants Make Food The process is called photosynthesis. Photosynthesis is a process that converts the energy in sunlight into chemical forms of energy that can be used by biological systems. Green plants are the only organisms that make their own food. Photosynthesis begins when light strikes a plant's leaves. Cells in the leaves, called chloroplasts, contain a green pigment called chlorophyll. Chlorophyll interacts with sunlight to split the water in the plant into its basic components. Carbon dioxide enters the leaf through the stomata and combines with the stored energy in the chloroplasts through a chemical reaction to produce a simple sugar. This sugar is then transported through tubes in the leaf to the roots, stems, and fruits of the plants. Some of the sugar is used immediately by the plant for energy; some is stored as starch; and some is built into a more complex substance, like plant tissue or cellulose. Some of the sugar is stored for future use or for use by us when we eat the plants. During the process, oxygen is released into the atmosphere.
Plant Responses Plants respond either positively or negatively to various stimuli. These responses are called tropisms. For example, the roots of plants respond positively to gravity, while the stem responds negatively (moving against gravity). Response to gravity is called gravitropism. Also, plants respond to light, growing toward it. This is called phototropism. Plants also respond to touch.
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