What is a plant?
Plants are the key to life on Earth. Without them many other living organisms would soon disappear. This is because higher life forms depend on plants, either directly or indirectly, for their food. Most plants however, are able to make their own food using sunlight. All plants fall into two basic categories. Flowering plants produce true flowers. The non-flowering plants include "primitive plants, such as mosses, ferns, horsetails, and liverworts, and the "gymnosperms", a group of plants which includes the conifers, like the wellingtonias. There are a about a quarter of a million species of flowering plant in the world today, and they grow almost everywhere from snowy mountain slopes to arid dessert.
The parts of a plant
Flowering plants are busy throughout the day and night. During the hours of daylight, the leaves collect the Sun's energy. The plant then uses this energy to create food, in the form of sugars. This second process is also carried out in the dark. As the food is produced, it has to be transported away from the leaves to the places where it is needed. At the same time, water and minerals , which the roots have absorbed from the soil, have to be carried in the opposite direction to the farthest stems and branches. Respiration, the way in which the plant breathes, occurs throughout the 24 hours, just as in animals. As the plant matures, it embarks on the complicated process of growing flowers, producing and receiving pollen, and eventually setting seed.
A plant is born.
A seed is a tiny life support package. Inside it is an embryo, which consists of the basic parts from which the seedling will develop, together with a supply of food. Th food is needed to keep the embryo alive and fuel the process of germination. It is either packed around the embryo, in an endosperm, or stored in special seed leaves, known as cotyledons. For weeks, months, or even years, the seed may remain inactive. But then when the conditions are right, it suddenly comes alive and begins to grow. During germination the seed absorbs water, the cells of the embryo start to divide, and eventually the seed case, or testa, breaks open. Firstly, the beginnings of the root system, or radicle, sprouts and grows downwards, followed rapidly by the shoot, or plumule, which will produce the stem and leaves.
A light diet
Unlike animals, most plants do not need to find food, because they can make it for themselves, The key to the way they do this lies in green pigment called chlorophyll, which gives them their characteristic green color. By means of chlorophyll, plants can convert energy from sunlight into chemical energy which can be stored, usually in the form of starch, and used to fuel the growth and development of the plant. The light energy is used to convert carbon dioxide and water into an energy-rich food compound called glucose. This process, known as photosynthesis, works rather like a bonfire in reverse. If you throw a log on to the fire, the carbon that is contained in compounds in the log is reconverted into carbon dioxide gas, and the stored energy is released in the form of heat and light.
How a plant is pollinated?
The fascinating shapes and brilliant colours of many flowers have evolved over millions of years to make sure that tiny grains of pollen are carried from one plant to another. Pollen grains have to travel from the anthers to the stigma for fertilization to occur and for seeds to be produced. Some plants are able to pollinate themselves(self-pollination), but most rely on receiving pollen from another plant of the same species(cross-pollination). Pollen may be dispersed by wind or by water, but the most important pollinators are insects. Plants entice insects to their flowers by their bright colors, and by food in the form of nectar. While the visiting insect feeds, pollen from the anthers is pressed on to its body, often at a particular place such as on the back, or on the head. The stigma of the flower that receives the pollen is in just the right place to collect it as the insect arrives. Some flowers are pollinated by wide range of insects such as honey-bees, bumblebees, hover-flies,
and butterflies. Others are more choosy and rely so heavily on a particular pollinator that no other insect species can do the job for them. Some species of yucca, for example are pollinated exclusively by a small moth, called the yucca moth. In return, the yucca provides the moth with food and a home.
Bursting Into bloom
Plant levers are often puzzled as to why, try as they might, they can not get their houseplants to flower. A plant may be covered in blooms when it is bought, but the following year it will often produce nothing but a mass of green leaves. The reason for this is that nurserymen treat plants in a particular way to make them flower. All plants have a special control mechanism which makes sure that their flowers develop and open at exactly the right time of year. The main factor that brings a plant into bloom is the length of the night. Some plants will only flower when the nights are long and the days are short(short day plants). The chrysanthemum, for example, will not flower at all if it is grown indoors in a position where it gets natural light by day and artificial light at night. Other plants, especially those that live far from the equator, only flower at the height of summer, when the days are long. Some flower in any day length. Once a flower has opened, other mechanisms come into play. Many flowers turn so that they are always facing the sun, and some close up every night, reopening in the morning.
Flowering plants have very different life-spans, ranging from months to centuries. A common poppy will germinate, flower, set seed, and die within a single year. Plants that live in this way are known as annuals. Other plants, such as the wild carrot take two years to complete the same process. They only flower in their second year-the first is spent growing and building up food reserves, which they store in a thick, fleshy root. These plants are known as biennials. Perennial plants are those that live for a number of years. They include species such as the dandelion.
A simple flower dissected
Flowers have become extraordinarily varied during the course of evolution. Nature has produced them in a tremendous wealth of shapes and colours, and to add to this profusion people have bred flowers that are even more brilliant or bizarre than the ones found in the wild. But behind baffling array of shapes and sizes there is a common pattern. For seed production, all flowers use the same underlying structures. The lily flower shown on these can all be clearly seen. They fall into three groups. The male parts(the stamens) produce the pollen, the female parts(the carpel) produce the ovules, which will eventually become the seeds. Around both the male and the female parts are sepals and petals which attract insects. When, as in this lily flower, the sepals and petals look the same, they are known as perianth segments or tepals.
A complex flower
Compared with the lily flower, the flowers of the Himalayan balsam flowers are much more complex and specialized. These flowers are pollinated by long-tongued insects, such as bees, and they are shaped to ensure that when the insect approaches and enters the flower, it picks up the grains of pollen from anthers. Bees are attracted to the flower by sugary nectar which is produced in a spur attached to a pouch at the back of the flower. To reach this nectar, a visiting bee first has to land on a platform made up of petals. It then has to climb right inside the flower and stretch out its long tongue. When the bee is in this position, its back touches the anthers. These give it a dusting of pollen which it them carries to the next flower it visits.
From flower to fruit
After a flower has been pollinated, it normally has to be fertilized before it will produce seeds and fruits. When a pollen grain lands on the stigma of a flower of the same species, it germinates to produce a pollen tube. This grows through the stigma and down the style to fertilize the ovule. One of the two male cells in the pollen grain fuses with the egg cell on the ovule. This fused cell then divides to form an embryo plant. A food reserve, or endosperm, is formed around the embryo plant by the second male cell, which fuses with two other cells in the ovule. The embryo plant together with its food store and protective coat, or testa, is known as the seed. The fruit is usually formed from the ovary, the protective structure around the seeds. However, in some cases, other parts of the flower protect both the seed and the ovary. In the rose, the "pips" inside the rose hip are technically the fruits, and the fleshy outer part is known as the receptacle. The fruit often helps in seed dispersal. The most obvious fruits are sweet, juicy, and brightly coloured, tempting animals to eat them and so disperse the seeds. However, the fruits of some plants include dry pods, which flick the seeds in all directions or fluffy plumes that help carry the seeds and fruits high on the breeze.
How seeds are spread?
As all gardeners know, a patch of bare soil never stays bare for long. Within days, seedlings start to spring up, and if the conditions are right, they eventually cover the ground. Even if the earth is sterilized by heating, so that all the seeds are killed, more somehow arrive and germinate. Plants have evolved some very effective ways of spreading their seeds. In certain plants, exploding seed pods fling the seeds into the air. Others have flying or floating seeds, or fruits, which are carried fare and wide by the wind and by water currents. Animals also play their part. Many plants have fruits with hooks that stick to fur. The seeds of some species develop inside tasty berries. Although the berries are eaten by animals and birds, the seeds pass through these creatures unharmed and fall to the ground where they germinate.
Borne on the wind
According to tradition, if you blow o a dandelion's seed head, the number of puffs needed to blow away all the seeds will tell you the time of day. whether or not this is true, it is a custom that certainly helps the plant to spread. The seeds of the dandelion are encased in tiny fruits and have their own special feathery parachutes to help them float through the air. If you blow on them, you may be starting the seeds on a journey that takes them high up and far
away. The dandelion's flower , like that of sunflower is actually a composite flower head made up of many tiny florets. Each of the florets produces a single fruit. Like the dandelion, many other composite plants, such as hawkweeds, ragworts, and thistles, rely on the wind to disperse their seeds. The fruits of some of these have parachutes; others have fine hairs that stick out in all directions to form feathery ball. Many of these plants are troublesome weeds because they quickly colonize bare soil in gardens and on farmland.
Spreading without seeds
Plants can reproduce in two quite different ways. As well as reproducing by means of seeds, they can sometimes also turn small pieces of themselves into new plants. This is known as vegetative reproduction. When a plant reproduces in this way, the young plantlets are genetically identical to the parent. This is quite different from reproduction with seeds, which produce seedlings that are all slightly different from their parents. Vegetative reproduction is very useful for farmers and gardeners. It means that they can multiply a plant that has attractive flowers or tasty fruit, knowing that each young plant will have exactly the characteristics they want. Some of the oldest plants in the world are perpetuated by vegetative reproduction , like creosote plant "clones" in California. Each clone is created when a single creosote plant begins to spread by producing young plants connected to it. The original creosote plant at the centre, which began life about 10,000 years ago, is no long dead, but its clones are still alive and spreading today.
Leaves are so varied that botanists have invented a whole new language to describe their shapes and the way they are fixed to plants. One reason for all this variety is that each species of plant has its own special problems in harvesting sunlight. A plant living on the gloomy floor of rain-forest, for example, may need large leaves to catch enough light. A plant growing on a cliff top has no shortage of light, but is lashed by strong winds. So it needs small, strong leaves if ti is to survive. Some plants have more than one type of leaf. This is most marked in plants which start their lives underwater but then flower above it. One example of this si the water crowfoot. Its submerged leaves are fine and feathery, to let water flow past without tearing them, while the upper leaves are flat and broad so that they float on the surface.
Plants cannot run way from their enemies in the same way as animals, so they have evolved special weapons and armour to protect themselves. The main enemies of most plants are the animals that feed on them. These range in size from tiny insects , which suck up, or chew their way through leaves, to large mammals, which eat entire plants. To keep the smallest enemies at bay, many plants have a mat of fine hairs on the surface of their leaves. Larger animals are deterred by means of special weaponry which includes spines, thorns, and stings. As a final defence, many plants have chemicals in their cells which make them unpleasant to eat. Once an animal has tasted the plant, it is unlikely to want to repeat the experience.
Creepers and climbers
Wherever there is moisture and warmth, plants struggle against each other for light. The tallest plant usually gets the greatest share, but it also has to spend the most energy in growing a strong stem, or a tree trunk, to hold up its leaves. But there are some plants-epithytes and climbers which take a short cut to the top. They take advantage of other plants and even buildings to get a place in the light with much less effort. Epiphytes may grow in the trunks or upper branches of trees and are lifted up with them as they grow. These plants do not have roots on the ground and are able to absorb all the water they need from the air and rainwater. Climbers need supports.Some twine themselves around a plant, while others put out touch-sensitive feelers, or tendrils, which curl around the support when they come into contact with it. A third group of climbers raise themselves by means of stiff side branches, prickles, roots, or hairs.
Although man-eating plants belong to the world of fiction, there are many plants that eat insects and other small animals. These meat-eating, or carnivorous, plants fall into two groups. Some species, such as the Venus flytrap, have active traps, with moving parts that catch their prey. Other species have inactive traps with no moving parts. They simply attract their victims with a scent reminiscent of food, and then catch them on a sticky surface or drown them in pool of fluid. The victims of carnivorous plants are mostly insects. Once an insect has been caught, it is slowly dissolved by digestive fluids produces by the plant. after many days, all that is left is the insect's exoskeleton-the hard outer casing of the body. The rest of the insect has been absorbed by the plant. Carnivorous plants can make food from sunlight like ordinary plants. the insects they catch are simply used as a dietary supplement-they are the plant's equivalent of vitamin tablets. Many plants need this extra source of food because they grow in waterlogged ground where the soil is deficient in nitrates and other essential nutrients.
Caught in a trap
To an unwary insect, the unusual leaf tips of the Venus flytrap appear most inviting. Not only is the insect attracted by what looks like a safe landing place, it is also tempted by the promise of food in the form of nectar. But it is all a trick. As soon as the insect settles, the leaf tips spring to life with lightning speed. Within a second, the hapless insect finds itself trapped, as the two halves of the leaf snap shut. There is a second, slower phase of closure after the plant has tested what is has caught using sensory glands on the surface of its lobes. If the prey contains protein, the trap closes fully, and digestion begins. The traps of the Venus flytrap are formed by two kidney-shaped lobes at the tip of the leaf with a hinge formed by the midrib. The whole of the leaf is green and therefore able to photosynthesize.
Large bristles on the upper surface of the trap work like triggers with a clever device. If just one bristle is touched, by a raindrop for example, the trap stays open. But if two of more bristles are touched in quick succession, it quickly shuts to catch its victim.
Parasitic plants are cheats. Rather than making their own food using the energy from sunlight, they have developed a means of stealing the food made by other plants, known as host plants. Because they do not need sunlight, many parasitic plants spend most of their lives hidden from sight. They attach themselves to the stems or roots of their host plants by means of suckers, known as haustoria. The haustoria penetrate the host's food channels and absorb the sugars and minerals which the parasitic plant needs to live. The world of parasitic plants is a complicated one. Some plants, such as mistletoe and the eyebrights, are only partly parasitic and are known as 'hemiparasites'. These plants have green leaves and so they can use the sun's energy to make some food themselves.
Not all plants that live on others are parasites. In fact, many more of them are simply passengers that grow on larger plants, such as trees, without causing them any harm. such plants are described as 'epiphytic' and many of them can get all the water they need simply by absorbing it from the air, or by collecting it in structers fromed for the purpose. They collect minerals by extracting them from tricking rainwater and plant debris. Being an epiphyte gives a small plant a chance to collect a lot of light without the need of tall stems. So successful is they way of life that few trees are without their passengers. In cool parts of the world, epiphytes are usually small, simple plants, such as algae, lichens, and mosses. But in moist regions close to the equator, they are much larger. As well as the plants that spend their entire lives up in the trees, there are others that start or end their lives in this way. Some creeping plants, known as stranglers, germinate on trees and then become rooted in the soil. Others climb up on to plants but then their roots wither away, leaving them perched high up near the light.
Adapting to water
The first plants on earth evolved in water. Today, water still teems with microscopic plants that have changed little from those distant ancestors. But aquatic flowering plants have a different history. their ancestors originally left water and evolved on land, but as time has gone by they have returned to the watery habitat. Only a few flowering plants, such as the eel-grasses, live in the sea. Far more plant species live in ponds, lakes, and rivers. Most of them are rooted to the bottom, but some have no roots and receive all the nutrients they need from the water instead of the soil. Some water plants are not often noticed because they spend all their lives underwater. Species like the water lilies, are much more obvious because their leaves float on the surface. Plants such as reeds and rushed form a group known emergent plants. they grow up out of the water and often form thick beds at the water's edge.
Surviving above the snowline
The higher the altitude at which a plant grows, the colder the temperatures it has to endure. Very low temperatures create specific problems for plant life. Thin mountain air holds little heat, and on exposed mountainsides high winds create a chill factor which makes the cold even more penetrating. In addition, low rainfall and thin, frozen soils, mean that water is scarce. However, many plants manage to survive despite the inhospitable conditions. In the Himalayas, flowering plants have been found at over 6,000 m, sheltering in hollows in the frost-shattered rock. These plants, known as alpine plants, are generally small and compact
so they can survive on the high mountain peaks, or in the frozen polar regions. Alpine plants often grow in dense cushions or flattered mats, giving them protection against the cold, drying wind. Upright, spreading branches would quickly be battered by the wind, and large leaves would lose valuable heat and water.
Living without water
No plants can live entirely without water, but in very dry regions, where water is scarce, some plants called succulents, are able to survive for a number of years between rainstorms. In the world's driest places, rain often comes in irregular but heavy bursts, so the plants that live there have evolved ways of collecting as much water as possible during downpours. The water is then stored in preparation for the next drought. Many succulents have very long roots, most of which grow near the surface, so that when it rains, they can collect water from a wide area. Once the waters in inside the plant, it is kept there by a number of special adaptations. Plants normally lose water from stomata, tiny pores in the leaf surface. The plant can control these pores and keeps them closed if it begins to lose too much water. Many succulents open the pores only at night, when the air is cool and less water can evaporate. Some of these plants have got around the problem of water loss by losing their leaves altogether.
Food from plants
Plants have been cultivated as food crops for thousands of years. The earliest humans lived in nomadic groups, roaming the countryside in search of food. Eventually, these peoples settled down and instead of collecting plant foods from the wild they began to cultivate them. When the time came to gather seeds to produce crops for the following year, they tended to take seeds from the healthiest plants . As they did this year after year, they began to produce better crop plants. Later, more deliberate efforts were made to improve crops by selecting and cultivating the strongest plants, and this process is continuing today. As farming settlements were established independently in different parts of the world,, so different crops were cultivated in each place. this meant that when early travellers first visited distant continents, they found many new and exciting foods to bring home. The crops we eat today come from many different parts of the world.
The story of wheat
Wheat has been cultivated by humans as a valuable source of food for at least 9,000 years. Grains of wheat have been found preserved in ancient Egyptian tombs, and it is known that it was the chief cereal of the ancient Greeks and Romans. The cultivation of wheat originated in the region known as the Fertile Crescent, which includes part of Israel, Turkey, Iraq. Once a rich farming area, today much of it is desert. Wheat is now grown in most parts of the world and the quality has improved greatly. The early, primitive species, such as einkorn and emmer had long thin stalks which were easily broken in bad weather. The small grains meant that a large number of plants only produced a relatively low yield of grain. Today, as a result of extensive breeding programmes, better varieties have been found which have higher yields, resist drought, and withstand disease.
Potions and poisons
In ancient times, plants were the main source of medicines. By trail and error, it was discovered that particular species could cure certain diseases. These plants were often grown in special gardens, and their details noted in herbals. Today, many plants are still used by the pharmaceutical industry. The chemicals they produce may be poisonous in large quantities, but small amounts can prove very useful in the treatment of some illnesses. The search for new medicines continues today and every year pharmacologists examine thousands of plants from all over the world.
The plant collectors
Many of the plants that have become common in gardens all over the world are, in fact, very far from home. Most fuchsias, for example, come originally from South America, wisteria from China and Japan, many azaleas from the Himalayas, and tulips from western and central Asia. These are just some of the thousands of plants that have been carried across the world by plant collectors. Plant collecting had its heyday in the 19th and early 20th centuries, as intrepid botanists travelled farther and farther afield in search of unknown plants. Some plant collectors experienced great hardships on their voyages to distant places-being shot at, caught in earthquakes, and attacked by wild animals. But despite all such adversities, the lure of making new discoveries spurred them on to explore some of the world's most remote and dangerous places.
Looking at plants
Plant collections are of two kinds-living plants and preserved specimens. The preserved specimens are mostly pressed, and are kept in a herbarium, where they can be examined by botanists. Collections of living plants are equally important, and sometimes ensure that rare plant species do not die out. Making your own collection of flowers and pressing them is a good way to learn about plants. However, you should not pick flowers that are growing wild in the countryside, as this prevents them from producing seeds. All wild flower species are protected by law, and you must not uproot them without receiving permission from the person who owns the land. If you want to try to grow your own plants, you can collect small amounts of seed, or you can buy wild flower seeds, produced by plants that have been raised in nurseries. Growing your own plants gives you a chance to study them without harming plants in the wild.