What living thing is essential to your life on earth? It's not a hamburger! Plants are the key living organisms that keep us alive. Plants include grasses, trees, flowers, bushes, food crops, shrubs, mosses, and more! Plants provide us with oxygen, food, energy, and medicines. They create habitat for animals, enrich the soil, and provide shade and beauty for human beings. We have a lot to thank plants for!
is the study of plants, and botanists are scientists who learn about, classify, and protect plants. The more we know about botany, the more we understand about life on earth. Let's find out more about botany.
What Is a Plant?
Plants are living things. All living things share these characteristics:
made of cells
grow, develop, and die
use nutrients, energy, air and water
respond and adapt to the environment
Plants differ from animals in that they are rooted to one spot and can't move to another place on their own. Also, plants are the only living organisms that can make their own food. Animals and humans must find their food outside their own bodies.
There are over 400,000 different kinds of plants on earth. There are many ways to classify, or organize, plants. Vascular plants have parts that are able to move materials such as water through the plants. Nonvascular plants, such as mosses, use osmosis to move material through the plant. Most common plants such as trees, bushes, or flowers, are in the vascular group. Vascular plants can be further divided into flowering and non-flowering plants. Another way to divide plants is between herbaceous and woody species. Herbaceous plants, such as most vegetables and houseplants, have soft, green, bendable stems, while trees and many shrubs have hard, rigid stems with woody material.
Like all living things, plants have different "body parts" that do different things. The six basic parts of most vascular plants are roots, stems, leaves, flowers, fruits, and seeds.
Vascular plants have a special system that transports water, minerals, and nutrients throughout the plant, much like our own circulatory system. The underground roots anchor the plant in the ground and absorb water and nutrients needed for growth from the soil. Food for the plant may also be stored in the roots. The stem carries water and nutrients to the leaves where food is produced and then moved to other parts of the plant. The cells that do this work are called xylem cells, which move water, and phloem cells, which move food. The stem also supports the plant, allowing the leaves to reach sunlight and capture its energy. Leaves have openings called stomata to allow air and water to come and go, and veins to carry water and nutrients within the leaves. The outer surface is coated with a waxy cuticle which protects the leaf.
Flowers are an important part of many plants' systems of reproduction. They contain both male and female parts, including eggs called ovules that develop into fruit. The fruit can be fleshy like an apple, or hard like a nut. The fruit's job is to contain and protect the seeds, which are tiny new plants, waiting for the right conditions to grow.
We are used to eating fruit such as apples and cherries, but we also eat other parts of many plants. We may eat the roots (carrots), the stem (celery), the leaves (lettuce), the flower (broccoli), and the seeds (peas.)
When you get hungry, you usually eat food that was purchased from a store. Your food gives you energy so you can live and grow. Just like you, plants are living things and they too must eat. But unlike you, they don't spend time searching for, shopping for, or cooking their food. They make their own food through a process called photosynthesis.
Leaves are the food-making factories in plants. Cells inside leaves contain tiny structures called chloroplasts, which have a chemical called chlorophyll that gives leaves their green color. But chlorophyll also has another job: it absorbs energy from the sun. Yes - leaves can "eat" sunlight! At the same time the chlorophyll is soaking up sun-energy, leaves are pulling water up from the plant's roots, and taking carbon dioxide from the air through their stomata. Those three things -- sunlight, water, and carbon dioxide --combine to form glucose, a sugar which is food for the plant. Glucose provides energy for the growth of all parts of the plant. Some glucose is stored for the plant to use later.
When a plant makes glucose, there is something left over: oxygen! A plant gives off oxygen through its leaves, stems, or flowers. This is the very same oxygen we need to breathe.
The chemical formula for photosynthesis looks like this:
Can you see why photosynthesis is the most important chemical process of all? Without it, plants, animals, and people could not exist! Through photosynthesis, plants do two very important things:
They make the food they need to live, which in turn allows animals and people to live, since plants are the basis of all food chains. All of our energy for growing, moving, and thinking comes from eating food either from plants or from animals who eat plants.
They release oxygen into the air, which animals and plants need to breathe. A green plant helped make the oxygen you are breathing today. Some large tropical forests are known as "the lungs of the world." Plants also help reduce the amount of carbon dioxide in the air.
All living things experience life cycles that include birth, growth and development, reproduction, and death. For plants, the life cycle includes a series of stages between the germination of the seed until the plant produces its own seeds and dies. Some plants go through their life cycles in just a few weeks, while some tree species live for thousands of years. To complete their life cycles, plants need air, water, nutrients, space, light, the right temperature, and time.
Plants are classified by the number of growing seasons required to complete a life cycle. Annuals pass through their entire life cycle, from germination to seed production, in one growing season and then die. Many common flowers, such as petunias and marigolds, are annuals.
Biennials complete their life cycle in two seasons. Beets and carrots are biennial plants. Perennials live for many years, and usually produce seeds every year. Perennials include woody plants like shrubs and trees, and also herbaceous plants like daisies and lilies, whose tops die back to the ground each winter and grow again in the spring.
Almost all plants begin life as a seed. Every seed contains a tiny plant (embryo), protected by an outer seed coat. The seed also contains the endosperm, a short-term food supply that gives the embryo the energy it needs to sprout.
How do seeds get started? Some seeds are planted by people for growing flowers, fruits, grains, and vegetables. Other seeds fall to the ground when a plant or fruit withers and dies, or are blown by the wind, carried by water, or scattered by animals in their fur or droppings. Not all seeds will grow into plants. If the seed is buried too deeply in the soil, or the temperature is too cold, or there is too little or too much water, nothing will happen. But when the conditions of warmth, water, soil and oxygen are right, the embryo inside the seed breaks out of the seed coat. The seed germinates, or begins to grow.
The first roots stretch downward into the soil in search of water, and the first shoot, or stem, begins to stretch upward in search of light. The baby plant is called a seedling. At this point, the energy in the endosperm has been used up, and most of the plant's nutrients and energy are within the cotyledon, or "seed leaves." These are the first to emerge and are used to feed the baby plant before it can start the process of photosynthesis. Plants with one cotyledon (such as corn) are called monocots. Plants with two cotyledons (such as beans) are called dicots. Watch a video of the process of germination.
Before long, however, the plant needs to create its own food, and to do this the plant must grow leaves. In the leaves, the process of photosynthesis takes place and the plant makes food to power its further growth and development. The stems grow longer, roots grow deeper, branches may develop, and leaves become abundant.
Plant survival depends on many things, including the conditions of the environment, drought, cold, disease, predators, and competition from other plants. Many do not survive extreme changes in temperature or infestations of insects. But when conditions are right, we see thriving, healthy plants that continue to grow throughout their lives.
Most plants can survive with a combination of roots, stems, and leaves, but for reproduction, they must create seeds that can grow into new plants. About 80% of green plants on earth are angiosperms, or flowering plants. Flowers are loved by people because of their beauty and sweet smell, but their main purpose to help the plant reproduce, or have young.
You have probably noticed that flowers have many parts. Each part has a specific job to assist in reproduction. All parts of the flowers are attached to the stem at the receptacle. The outside of the flower that we easily see consists of petals, the colored part, and sepals, the green structures at the base of the flower that protect the bud. If you look closely inside the flower, you will see the male and female parts. The female part is the pistil, which looks like a vase with a long neck. It is located in the center of the flower and is made up of three parts: the stigma, style, and ovary. The style is the long tube and the stigma is the sticky knob at the top of it. The style leads down to the ovary that contains the egg cells called ovules. The male part of the flower is called the stamen. It usually surrounds the pistil and is made up of two parts: the anther and the filament. The anther produces the yellow dust we call pollen, and the filament holds the anther up.
In pollination, pollen must be moved from the stamen to the stigma. Pollen can be dispersed by water and wind, but most plants depends on bees, butterflies, hummingbirds, bats, or moths to move pollen from place to place. These pollinators are attracted to flowers by the colorful petals, the nice smell, or the nectar produced in the flower. When they brush up against the pollen, it clings to their bodies. Then the pollen is spread around the flower, or to other flowers of the same species that they visit. Find out more about the pollination process and about pollinators.
When pollen lands on the sticky stigma, it travels down the style to the ovary. In the ovary, these male cells join with the ovules in what is called fertilization. The fertilized ovule becomes the seed, capable of growing into a new plant. The ovary around the seed ripens and becomes the fruit, which is often tasty food for humans and animals.
Since fruit contains the seeds, many things that we call vegetables are actually fruits. Did you know that tomatoes, cucumbers, beans, and zucchini are really fruits?
The amazing system of pollination and fertilization in flowering plants is not only essential to reproduction, but very important to humans. Throughout the world, animals pollinate more than three-fourths of the food crops that people eat. Scientists estimate that one out of every three bites of food we take is the result of a successful animal-plant pollination!
Non-flowering plants reproduce in different ways. Gymnosperms produce seeds, but not in flowers. One class of gymnosperms is conifers, such as pine trees, firs, and junipers. They have needle-like leaves and produce seeds in cones. The male cones release pollen, which is carried by the wind to female cones, where the seeds are produced and protected. When released, these "winged" seeds float on the wind until they reach the ground, where they can germinate and grow.
Ferns and mosses don't produce seeds at all. They make large numbers of spores, which are tiny, dust-like organisms that are carried by wind or water to new locations, and eventually grow into new plants. All spore-producing plants need lots of moisture to reproduce, so they usually live in damp, shady areas.
Plants have many predators, from insects that chew on leaves to large animals that eat whole plants. While it is true that plants can't run away from creatures who want to eat them, they do have defenses to help them survive.
To keep small predators away, some plants have a mat of thick hairs on the surface of their leaves. Many bushes and shrubs have thick bark on their branches to keep insects out. Some plants, such as roses and cacti, have thorns or spines to keep them from being eaten by larger animals. Other plants produce chemicals that are poisonous to animals who try to eat them. Predators learn to avoid plants that cause pain, such as poison ivy, or that sting them, such as nettles. Another chemical defense is bitter taste, as in tomato leaves. Animals learn to stay away from plants with a bad taste, and move on to something better!
Just like all living things, plants can get sick and die. To help guard against diseases, all plant cells have rigid cell walls to keep out pathogens and bacteria, and they have a waxy cuticle on their leaves for protection. Cotton produces chemicals that fight fungus and bacteria. Plants may even use chemicals to defend against other plants taking over their area. Some sunflowers produce toxic substances that kill other plants around them.
Some plants have the amazing ability to send out chemicals that call other predators to come feast on the insects that are chewing on their leaves. Learn more about this and other plant defenses in this fascinating video from PBS Learning Media.
Do plants really 'behave?' They do! Plants are actively engaged in their own survival. Just as animals and humans sense changes in their surroundings and respond to those changes, plants also perceive and react to their environment. Plants move in response to changes in temperature, light exposure, water levels, bacterial infection, physical disturbance, and touch.
The growth of a plant toward or away from stimuli is known as tropism. Positive tropism is growth toward a stimulus, while negative tropism is growth away from a stimulus. Phototropism refers to plants growing toward the direction of light. You have probably seen plants that sometimes grow in odd ways in order to reach sunlight. Gravitropism refers to the ability of the seedling to sense and respond to the direction of gravity. Regardless of how the seed is planted, the roots always grow downward and the shoots grow upward.
Another example of plant behavior is the winding, climbing movement of vines. By flailing its tendrils in a whirling motion, the vine seeks out an object to coil around. When the tendrils touch the trellis or tree, they attach to it and produce a special hormone within the plant to encourage fast growth.
Plants-in-Motion, a PBS interactive activity adapted from Indiana University, features amazing time-lapse videos that reveal the movement and growth of plants in their pursuit of light, water, and support. Some botanists have even observed foraging behavior in plants, similar to the way animals hunt for food, then stop and eat when they find it. Other plants have the ability to track the path of the sun during the day by the movements of the flowers or their leaves.
Although plants have no choice about where they are rooted, they do respond to environmental cues in ways that enable them to survive. Learn more about plant behavior in this video from PBS Learning Media.
There is a lot of diversity in plant life, which makes botany a fascinating area of science. Let's look at some examples of unusual plants.
Some plants are actually meat-eaters! There are over 600 different species of carnivorous plants that kill their prey (mostly insects and spiders), then digest the bodies for nutrients. They do this with special leaves that act as traps. For example, Venus Flytraps have thick, padded leaves covered with tiny hair. When a fly touches the hairs, the leaves snap shut like powerful jaws. Then the plant releases chemicals that break down and digest the fly's body. Pitcher Plants are shaped like a narrow drinking glass. The pitcher is covered with sweet nectar, and when insects come to take a sip, they fall into the pitcher. Some pitcher plants are so large, they can catch and eat rats or frogs! You can watch these carnivores in action in this video from PBS Digital Media.
Most plants are in bloom during the daytime hours to absorb sunlight and attract pollinators, but there are some species that bloom only at night. Nocturnal plants are pollinated by bats and moths that sleep during the day. Four O-Clocks are flowers that open in late afternoon (around 4:00 p.m.,) bloom throughout the night, and then close for good in the morning. A few plants, such as the night-blooming cereus, blossom as a group, only one night each year!
Succulent plants, such as cactus, thrive in hot, dry climates where water is scarce by storing water in thick, fleshy leaves and stems.
Sensitive plants, sometimes called shy plants, fold up their leaves when touched. They will re-open a few minutes later, but if touched or shaken their leaves fold inward and droop again.
Plants and Humans
Plant and humans are completely connected. Without plants, human life could not exist.
Plants provide almost all food we eat. Humans eat plants such as fruits, vegetables, and grains, and products such as meat, milk, and cheese that come from plant-eating animals. Of all the things we eat, only water and minerals, such as salt, do not originate with plants.
Plants remove carbon dioxide from the air and supply oxygen that we breathe to keep our cells and bodies alive. All of the oxygen available for living organisms comes from plants. Also, the moisture that evaporates from the leaves of plants accounts for 10% of the water in our atmosphere.
Energy sources such as natural gas, coal, and oil (fossil fuels) were made millions of years ago from dead plants, and from dead animals which got their energy from photosynthesis. Yes - one more reason to thank a plant!
Many medicines that humans use come from plants. Some plants have healing properties that have been used for thousands of years. The first type of aspirin (which reduces pain and fever) came from the bark of a tree. Some of the drugs we use today are made from chemicals that plants produce to protect themselves.
Plants alter the effects of climate in several ways. They provide shade that cools the air temperature. In some tropical forests, plants change rainfall patterns over large areas. They protect structures, farms, and animals from the wind.
The roots of plants help hold the soil together, which reduces erosion. When plants die, their remains decompose to create compost. Decaying plants help make the soil rich with nutrients.
Plants provide habitats for animals who live in, on, or under plants. They provide shelter and safety for animals, who then benefit humans by dispersing seeds, providing food, and keeping ecosystems healthy.
Plants provide useful products for people, such as fiber for clothing, wood for building, and pesticides to help control bugs.
Plants make our world beautiful and reduce noise pollution. Humans are happier and less stressed when they have plants around them!
Many humans, such as farmers and gardeners, make their living from working with plants. Plants present a branch of scientific study called botany. Botanists use chemistry, biology, and technology to research important questions about plant life, seed science, agriculture, classification, and conservation.
The biggest seed in the world, from the double coconut palm, weighs 45 pounds - that is as much as a five-year-old child!
The smallest seed in the world belongs to certain orchid flowers. One million of these seeds weighs as much as one grape!
The strawberry is the only fruit that bears its seeds on the outside. The average strawberry has 200 seeds!
Most plants grow flowers each year, but some take much longer. The century plant grows only one flower after many years and then it dies. Most bamboos flower once every 60 to 100 years, and one plant in South America grows a flower when it is 150 years old!
Bamboo, a fast growing woody plant, can grow 35 inches in one day! On the other hand, the saguaro cactus grows only one inch in a year.
In southern Africa there is a plant called the tree tumbo, which only has two leaves in its whole life. Each leaf can be over 27 ft (8 m) long - longer than four bathtubs!
In one year, the Amazon water lily produces leaves that are 8 ft in diameter - a child can stand on one leaf! On the other hand, the leaves of the watermeal plant are only 1 millimeter across.
Not all flowers smell good. The corpse flower smells like a rotting dead body! The bloom is over 8 feet tall and 12 feet around. Their rotten-flesh smell attracts flies as pollinators. People have been known to pass out from the smell!
Hummingbirds hover in front of flowers while they collect nectar. They use so much energy to do this that it would be like you needing to eat 330 pounds of hamburgers every day!
Do plants like music? There have been a few experiments suggesting that plants grow better and are healthier if they are exposed to classical or soothing music.
Plants are the only living organisms that can 'eat' sunlight to make food. The average global solar energy capture by photosynthesis is about six times larger than the current power consumption of human civilization!