Plants

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Plants

A plant is a multicelled organism that makes its own food by photosynthesis. Although plants show a variety of form, function, and activity, all belong to the kingdom Plantae and generally are characterized by being immobile, or anchored, in soil, having strong woody tissues for support, and by being green and carrying on photosynthesis. Plants are essential to life on earth, especially human life, since they are at the beginning of the food chain and take in carbon dioxide (an atmospheric gas) and give off oxygen. Plants are also a source of medicine and useful materials. Botanists (people who study plants) have identified about 500,000 species of plants, although there are many undiscovered species yet to be classified.

The plant kingdom is one of the five main groupings of organisms; the four others being the monerans, protists, fungi, and animals. Although algae were long considered to be part of the plant kingdom, they are now regarded as being part of either the Moneran or Protista kingdom. Plants are found in virtually all land and water habitats and can range in size from tiny mosses to giant sequoia trees more than 300 feet (10.94 kilometers) tall. Whatever their size or habitat, all plants have the following characteristics: they are multicellular at some point in their life; they are eukaryotic (their cells have nuclei); they reproduce sexually (through the union of sperm and egg); they have chloroplasts (are the energy-converting structures) for photosynthesis; they have cell walls; they develop organs; and they have life cycles.

TYPES OF PLANTS

Although plants have all these things in common, scientists distinguish among the many different types of plants and classify plants as they do every other living thing. Therefore, in the kingdom Plantae, there are ten phyla or divisions of plants, each of which represents a number of classes, or more specific types. Most of these ten divisions can be grouped into five major types: seed plants, ferns, lycophytes, horsetails, and bryophytes.

Seed Plants. Seed plants are exactly what they sound like—plants that use seeds to reproduce. Plants that produce seeds that are enclosed in a protective case are called angiosperms. These include most of well-known plants like trees, wildflowers, and fruits and vegetables. Plants that produce seeds without any covering are called gymnosperms. Most gymnosperms produce their seeds in cones. Evergreens like firs and pine trees are a good example of gymnosperms.

Ferns. Ferns often vary greatly in size, but almost all grow in moist areas. Only their leaves, called fronds, grow above ground. The rest of the plant spreads out in stems that grow horizontally underground.

Lycophytes, Horsetails, and Bryophytes. Lycophytes are mostly mosses and have a single, central stem. Horsetails have tiny leaves and hollow, jointed stems that are scratchy. Bryophytes grow in shady areas and are a type of moss, but they do not have any vascular tissue or tubing that carries water throughout the plant. Sphagnum or peat moss is a good example of bryophytes.

PLANT ANATOMY

Despite the many types and divisions of plants, most of the common plants reproduce in one of two ways, have the same basic parts, and make their food the same way. Since the seed plants make up the largest single group of plants (around 250,000 species) and is the one most familiar, they will serve as a good example for plant anatomy. Nearly all these plants have three major body parts: roots, stems, and leaves.

Roots. A plant's roots anchor it in the soil and provide the plant with what it needs to grow by absorbing water and minerals. This underground root system also places a major restriction on plants since they are unable to move about and must cope with changing conditions instead of moving away as an animal would. Some plants use their roots to store food for the aboveground part of the plant to use, such as radishes and carrots. Others, like potatoes, are examples of plants with tubers, or a swollen underground stem, in which food is stored. As a plant grows in size, its root system must expand not only to feed it more, but also to simply hold it upright.

There are two main types of root systems: taproots and fibrous systems. A taproot is a large main root that grows straight down and has smaller, lateral roots growing off it. Carrots and dandelions have taproots, as do trees which sometimes send down an anchor as far as 15 feet below ground. Fibrous roots are all the same size and spread out horizontally not very far beneath the surface. Grass, wheat, and corn have fibrous roots. As roots grow, the tip of each slender branch is protected by a root cap as it pushed through the soil. Behind the cap are threadlike tubes, or root hairs, that spread out and increase a plant's ability to absorb what it needs from the soil. Some water plants have roots that float, while other plants like orchids have roots that attach themselves to the branches of another plant or tree.

Stems. The stem is that part of a plant that supports the plant's buds, leaves, and flowers. Although stems vary greatly in size and type, they all connect the roots to the leaves by a network of pipelines, and they also hold up the part of the plant that needs to reach for sunlight. Some stems are short and green, like those of lettuce that appear to have no stem at all, while others are woody and large like the trunk of a tree. Almost all plants grow by putting out buds from different parts of their stems. Terminal buds grow near the end or apex of a stem, while lateral buds grow where a leaf joins a stem. Buds are specialized and may grow into new branches, leaves, or flowers. Stems are made of vascular tissue that serve as the plant's pipeline, or tubing system, that performs two functions: one system (made of vascular tissue called xylem) is used mainly for transporting material from the roots to the leaves, and the other (made of tissue called phloem) is used for moving material from the leaves to the roots or other parts of the plant. The stringy strands of celery that get caught between our teeth are a good examples of a plant's vascular tissue.

Leaves. The leaves of a plant are where the really important and amazing work is done—making food. This food-making process in which green leaves change inorganic raw materials into organic nutrients is called photosynthesis. Photosynthesis involves a leaf's ability to trap light energy and convert it into chemical energy. As a solar collector, a leaf consists of a thin, flat blade that is attached to a stalk called a petiole. The blade is where photosynthesis takes place. The blade, also called the lamina, is made up of two layers of cells—the epidermis on the outer surface and the mesophyll on the inside—and is strengthened by a network of veins that also transport materials to and from the blade. The underside of a leaf has microscopic openings or pores called stomata (singular, stoma) that can open and close and allow oxygen to flow out and carbon dioxide to flow in. These pores also regulate the amount of water that a plant will lose.

Leaves are usually green because their cellular structures called chloroplasts contain the green pigment chlorophyll. It is chlorophyll that traps the Sun's energy and begins a four-step biochemical process of using carbon dioxide and water to produce a sugar called glucose and to release oxygen. The plant uses some of this food as fuel for itself, some to grow and repair, and some it stores. When a primary consumer eats a plant, it obtains the original light energy that was captured by the plant.

Leaves vary greatly in size and usually are arranged in definite patterns to make sure that each receives the most sunlight it can and does not shade its neighbor. Leaves also die during a process known as abscission (separation). As the days grow shorter in autumn, a layer of cells grows across the base of the petiole and stops the flow of food to it. This trapping of sugar in the leaf produces a bright red pigment called anthocyanin or a yellow pigment called carotene. In deciduous plants, all the leaves fall off at the same time, but in evergreen plants they are shed and replaced regularly, so the plant is never without leaves.

SEXUAL REPRODUCTION IN PLANTS

Plants reproduce either by sexual reproduction, in which a male sperm cell unites with a female egg cell to produce a unique individual plant, or by asexual reproduction, in which the plant divides itself up to produce an identical replica. A flower is the reproductive part of many plants, and it may contain the male or female reproductive structures or even both.

Flowers. Flowers have four main parts: the calyx, the corolla, the stamens, and the pistils. The calyx protects the petals (corolla), inside which are the stamens (the male reproductive part) and pistils (the female reproductive part). The purpose of a flower is to bring about pollination, which is the transfer of pollen (male sex cells) to the female parts. Insects, birds, bees, and even the wind play a role in pollination. When a pollen grain lands on a receptive pistil, fertilization takes place and an entirely new cell is formed that is the start of a seed. A seed containing an embryo of the new plant is often enclosed in something called a fruit.

Fruit. Whether fruit are hard and dry (like a walnut) or soft and juicy (like a raspberry), it is the plant's way of scattering its seeds as far as possible. Some fruit have burrs that cling to an animal's fur, while some fruit are capable of floating on water, and others can resist being digested as they pass through an animal's gut to be deposited somewhere else. By producing fruit that animals want to eat, plants are using animals to distribute their seeds and to make sure that they end up in a place where they may germinate or begin to grow. When conditions are right, the seed uses the water it receives and the food it has stored to send a root, or radicle, through its seed coat and into the soil, while a shoot (containing the beginnings of a stem, buds, leaves, and flowers) grows aboveground and toward the light. This begins a new plant.

ASEXUAL REPRODUCTION IN PLANTS

Plants can also spread asexually, usually by sending up new aboveground plants from an existing root system. Grass and strawberries grow this way, as do tulips. Many plants with tubers, like a dandelion or potato, will regenerate into a new plant if only a piece is left in the ground.

PLANTS ARE ESSENTIAL TO LIFE

Plants are basic to life on Earth. A world without plants would be a world humans could not recognize. Besides missing the beauty and pleasure that plants give, the world would be without any of the food the people and animals know and need. People would also be lacking the many medicines, shelter, and useful products that are based on plants. By capturing the energy of the Sun, plants make all other life on Earth possible.