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There are at least 400,000 known species of plants in the world. Vast numbers of plants have existed in earlier geological eras, a great number of which are now extinct. Of the plants that exist today, most belong to a group of “higher plants” that have roots, stems, and leaves, and that produce seeds in order to reproduce. Plants range in size from a single cell to giant sequoia trees, which can grow up to 240 feet (80 meters) tall.
Biologists have classified all living organisms into two kingdoms-the animal kingdom and the plant (Plantae) kingdom. Plants are distinguished from animals because most animals can achieve locomotion at will, while plants are rooted and cannot move to a new location unless carried by water or wind. Plants compensate for this lack of mobility by giving movement to their seeds. Many plants produce fruits that are eaten by birds and then deposited elsewhere. Some plants simply drop their seeds to the ground where they are moved by water, insects, or animals; others, such as cottonwood and maple trees, use the winds to broadcast their seeds.
Plants also differ from animals in their growth cycles. While animals stop growing at maturation, plants continue to grow until they die. They grow thicker in their stems, their roots deepen, and new growth is added. Some plants have short life spans while others live for centuries. Some trees live for thousands of years; there are trees that are estimated by biologists to be 5,000 years old.
Plant Food Production
Another difference between plants and animals is that plants can produce their own food, while animals eat either plants or other animals. The roots, stems, and leaves of plants are used to manufacture food. Roots take in water and minerals like calcium, iron, nitrogen, phosphorous, and potassium, then use these and other chemicals to make food. In order to do so, the water and nutrient chemicals have to reach the leaves. Because the nutrients travel through the stem of the plant the process can be a long journey in tall trees. In most cases the stem has vascular tissue through which the nutrients travel. As the water moves from one cell to another it eventually reaches xylem, which is a complex plant tissue. Xylem is formed into long tubes or vessels through which the water carries dissolved salts of the nutrient chemicals.
Eventually the fresh nutrient supply reaches the leaves of the plant. At the same time, food from the leaves of the plant, often in the form of some type of sugar, moves down the xylem in a liquid watery solution in tube-like cells called sieve cells. These cells have little holes in the wall of the tube that act like a sieve, allowing food to pass to the plant. The sieve cells are supported by phloem cells. These are plant tissues that are in parallel with the xylem tissues. A third type of plant tissue called cambium separates them. This tissue provides support and also keeps the nutrient solution separate from the food solution.
Plants also have a variety of stems-the trunks of trees, for example, are large stems. Some are underground stems, such as rhizomes, which send roots deeper into the soil while forcing branches to the surface, where leaves can grow. Some stems are climbing stems and use tendrils for support, like ivy plants; other plants, such as tulips and daffodils, have bulbs as stems. Plants are the only organisms that can convert inert matter into living matter. Plants absorb nutrients from the earth, from water in which they grow, or even from the air. An example of a plant that absorbs nutrients from the air is spanish moss. Some plants, such as fungi, do not manufacture their own food. Some of these are parasitic, such as the dodder plant, which begins life rooted in the ground, but then grows like a vine around its host plant, sucking water and nutrients from the host. Meanwhile, its own root dies and it becomes permanently attached to its host.
The leaves of plants convert carbon dioxide, water, and nutrient chemicals into plant food via a process called photosynthesis. Without this process there would be little or no life on the planet. Most leaves have two parts: The lamina is the leaf’s blade; the petiole is its stalk. There are plants that have leaves that grow directly from the stem-these plants do not have a petiole. The petiole extends to the tip of the leaf and branches out to form its veins. Water and chemical nutrients flow through the veins of the leaf where they will be used in the making of the plant’s food.
Leaves vary widely in shape and placement upon the stem. A cross section of a leaf would show that it has layers of different kinds of tissues. The epidermis is the leaf’s surface. Below the epidermis surface layer are the palisade layers, which have long, foodproducing cells containing chloroplasts. There are also palisade cells and spongy cells. Thousands of holes called stoma allow air to circulate between cells. Leaves usually have a glossy or waxy side that faces the sun, and many plants have leaves that can move to face the sun. Sunflowers have a large flowering head that moves with the sun during the day. Leaves absorb sunlight through the lamina, which also takes in carbon dioxide directly from the air.
Respiration takes place continually in a plant as the cells use energy, in the form of glucose, to make food. It happens in a nucleus where a jelly-like substance, cytoplasm, is surrounded by a cell wall. The nucleus uses its coded information to make a new plant, and it also can control energy production in the cytoplasm. Aerobic respiration in plants uses oxygen for the air, as do animals, but plants produce more oxygen than they use. The surplus oxygen is used by other organisms. While some plants use respiration, others do not. There are plants that breathe with anaerobic respiration. This process breaks down carbohydrates to form other chemicals; among the chemical by-products are alcohol and lactic acid.
The food making process of photosynthesis is a complex chemical process that can only take place in daylight. The leaves of the plant trap sunlight with chlorophyll. The green pigment chlorophyll is located in the chloroplasts, where the photosynthesis process takes place. Plants have pigments in their leaves other than chlorophyll, but the green chlorophyll so dominates the leaf that other pigments, such as the reds, browns, and yellows of fall foliage, are not seen until the chlorophyll is destroyed.
The structure of a chloroplast begins with a membrane on the outside. It has a stroma, which is a layer covering the thylakoid and the grana. The thylakoid is composed in sheets, as is the grana, which contains the chlorophyll. Through the photosynthesis process the plant makes carbohydrates and oxygen. The oxygen is expelled through the stoma air holes and the carbohydrates are dissolved in watery solutions that are transported to other parts of the plant to be used as sugars or starches.
Food and Water Storage
Sugars or starches may be stored in the plant as pigments or fats. For example, the avocado plant produces fruits that are fatty in content. Others, such as dates, beets, or sugarcane, are high in sugar content. Plants store food in roots, leaves, and seeds. An example of a plant that stores its food in roots is the potato plant. The mature tubers are very high in starch content, having grown as the plant stored food from the photosynthesis process as carbohydrates in the tubers.
After the plant has converted nutrients, water, and carbon dioxide into food through photosynthesis, it also uses a process called transpiration. The water that the plant has drawn to its leaves is used to make food and also to deliver it to the plant’s cells. However, a great deal of the water is breathed out as water vapor through the plant’s leaves. A tree can expel or breathe out hundreds of quarts (or liters) of water a day.
Some of the water that a plant transpires is lost through its stem, and some is lost through flowers. However, the bulk is lost through stomata. The water that is lost evaporates, cooling the leaves of the plant and regulating the plant’s temperature. The stomata, through which respiration and transpiration take place, are flanked by guard cells that control their closings and openings. Opening allows air to enter into a sub-stomal chamber. The leaves of plants from moist or humid climates have flat surfaces that allow water vapor to easily pass through the stomata opening and into or out of the leaf.
In order to control their use of water, plants have adapted many mechanisms and leaf structures. In the winter deciduous trees lose their leaves and go dormant. The process of shedding leaves annually is called abscission. In temperate and polar seasons the leaves are shed during the autumn in preparation for winter; in wet-dry climates the abscission takes place in the dry season.
Evergreen trees and plants are sometimes called semi-deciduous. In winter, conifers keep their needle-like leaves, which are adapted to continue respiration and transpiration even in winter. In desert climates cacti function well without leaves, storing water in their stems. The waxy surface of the plant reflects the sun and regulates the temperature of the plant, allowing little water to escape.
Gymnosperm plants do not use ovaries to enclose their seeds, while angiosperms have flowers that are pollinated to produce seeds enclosed in an ovary. Conifers such as pine trees and fir trees are the best known of the gymnosperms. They enclose their seeds in woody cones, which are male or female. Ovules form on the scales of female conifer cones, while pollen forms on the scales of the male cones. Wind carries the pollen from the male cone to the female cone.
There are two groups of angiosperms. The dicotyledons have two seed leaves; monocotyledons have only one seed leaf. Wheat seeds are monocotyledon because upon germination only one leaf appears at first. However, beans are dicotyledons because as the seed embryo matures it has two seed leaves. For example, roasted peanuts when split reveal what looks like the head of a man with a bow tie-the bow tie is actually two cotyledons, which means the peanut seeds will germinate as dicotyledons.
The flower is the main reproductive part of the angiosperm. When the flower is mature it opens anthers in order to let out pollen grains. Many plants use insects to assist with pollination; others allow their pollen to be carried by the wind. Western junipers and southern pine trees will emit so much pollen that cars and other surfaces will be coated in yellow pollen dust for days during pollination. Some plants are capable of self-pollination, but others need a pollinator. Varieties of plums or pears are good at pollinating, while other fruit trees will not produce unless pollinated. Some trees such as holly have males that provide pollination for the females, which have the characteristic red berries.
A flower usually has a male and a female part. The stamen is the male part and has delicate filaments that hold grains of pollen that contain the active sex cells of the plant. The pistil is the female part of the flower. It has at least one carpel; the carpel has an ovary, inside of which are ovules that contain sex cells. At the far end of the pistil is the sticky surface of the stigma, which receives the pollen grains with the sex cells of the male portion of the flower. Protecting the stamens and the carpels are the sepals and the petals, which together form the corolla. When fertilization takes place, the pollen is transported from the flower’s stamen to its stigma. After the pollen falls on the stigma, it forms a long tube that goes down though a structure called a style until it reaches the flower’s ovule, where fertilization takes place. Once the ovule is fertilized, the flower will shed its petals.
The ovary takes food from the plant’s leaves, and as it does, the seed or seeds develop rapidly. The seed is covered in a tough protective coating and contains an embryo plant made up of a root, a shoot, and one or more shoot leaves (cotyledons).
There are many ways in which the ovaries of plants develop after fertilization. Some dry out. Others become fleshy or develop filaments that are so delicate they can be blown about on the winds-dandelions, for example, use the wind to spread their seeds. Others plants, such as wisteria, use an explosive rupturing of the seed pod to propel seeds some distance from the parent. Plants may also depend on animals such as birds. Birds eat the seed, but only the fleshy outer portion is digested; the seed with its thick protective coating is passed from the bird to a new location where it puts down roots after germinating. Some seeds stick to the coats of animals-burrs and other “riders” may be carried miles before they are dropped.
After the seed reaches a moist spot it germinates. Water softens the coating and allows it to take up the water and begin growing by putting down its tap root and sending up shoots. Sometimes fruit flesh serves as a built-in fertilizer. The fleshy part is covered with a skin. A drupe is a fleshy fruit with a stone-like pit or seed that is covered with a fleshy fruit. The skin is called the epicarp and covers the fleshy mesocarp, which in turn covers the hard shell that contains the seed. The seed is an endocarp and it contains the embryo plant.
A few plants do not produce seeds but instead reproduce using spores. Lichen, which is a form of algae living in a symbiotic relationship with fungi, reproduces with spores. Lichens grow on a number of surfaces including on rocks and tree trunks. Other plants such as ferns or mosses have spore capsules on the underside of the leaves or fronds. When a moss spore capsule is mature it drops to the ground and germinates; each spore then develops a thin branch or protonema on which a bud grows.
From mountaintops to seas, from deserts to swamps in the tropics, plants have found ways to adapt to the enormous climatic variations of the earth. They have also adapted to soil and moisture conditions. For a vast number of plants, conditions in temperate zones are nearly ideal, except for the presence of other plants that compete for soil, moisture, and sunlight. In other environments, such as jungles, the hot, moist growing conditions are too favorable. Enormous numbers of plants crowd together to capture every bit of space for rooting, for water, and for sunlight. Taller plants will capture the sunlight and shade out small growing plants. To meet this challenge, some plants grow as vines to reach the sun.
Orchids are epiphytic plants that grow in the heights of trees. Some grow on branches and others on tree trunks in order to reach the sunlight. These types of plants have no roots for obtaining moisture so they get it through their leaves or through finehaired roots that hang in the moist air. Their aerial roots also capture minerals, especially when the plant is near oceans that cast water vapor into the atmosphere containing salts that the plant can use. Others plants use leaf structure to capture waterthe bromeliad has a leaf that forms a basin shape in which water is retained for the plant’s use.
Some plants, such as mangrove trees and willows, have adapted to living in or next to water. Others, such as the herb water crest, live in water with their leaves on the surface. Water lilies are rooted in the muck of ponds but have a stem that supports a leaf that floats on the pond’s surface. Other plants live completely under water-many seaweeds never reach the surface. In order to gather nutrients in a nutrient-poor environment, plants have adapted a number of strategies. Insectivorous plants feed on insects. The pitcher plant uses a scent to attract insects, then traps them in a watery solution where they drown and are dissolved for their nutrients. Others use traps that spring shut or have sticky hairs that trap the insect much like a spider’s web.
Agriculture is the human activity of growing plants for use. Because of the usefulness of plants, people have been cultivating them for at least 12,000 years. Cultivation of plants depends upon the character of the plant. For example, viticulture-the cultivation of grapes-involves more that just planting grape vines. The vines have to be pruned each year in order to stimulate growth of grapes. Otherwise all of the growing energy would be put into new vines and leaves. Plants such as coffee or tea are grown for their stimulating effects; others are grown for the intoxication value. Beer is brewed from wheat and a great many other plants such as rice, coconut, chocolate, and fruit juices can be used to produce alcohol. Plants such as marijuana and poppies can produce addictive drugs that can also be used medicinally.
Plants are uniquely important in the ecology of the earth because they supply life-giving materials to other creatures. They make their own food by combining water, carbon dioxide, and mineral salts with sunshine. Green leafy plants are the beginning of the food chain. Some of these exist in the oceans or other bodies of water. Herbivores are animals that eat only plants; they include deer, buffalo, cows, rabbits, and many others. Carnivores, in turn, eat the flesh of herbivores. The waste materials of both are returned to the soil as droppings that can be used later by plants.
Plants also provide humans with food, fiber, and shelter; people have been using plants for the entire history of the human race. The earliest humans gathered nuts and berries and used vegetation for shelter. This is still the case with millions of people living close to nature even now.
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