Botany For Dummies

Part I

Plant Basics

In this part . . .

This part introduces you to some of your neighbors on planet earth that you might not have noticed much before. Plants seem pretty different from people, and although many people appreciate the beauty of plants, not everyone takes the time to really get to know them.

In this part, you take a closer look and discover that plants have many similarities to you. For example, like all living things, plants are made of cells, tissues, and organs. Plants even have sex! In fact, many of the foods you eat are the result of plant sexual reproduction. You can find out all about these similarities as you read this part, in which I present the fundamentals of plant structure that are essential to understanding plant function and evolution.

Chapter 1

Exploring Botany

In This Chapter

arrow Building plants one cell at a time

arrow Finding out about how plants work

arrow Connecting plants and people

Botany is the study of plants, including plant structure, function, reproduction, diversity, inheritance, and more. Plants may seem like they’re part of the background of your life, when really they’re at the center. The food you eat, the clothes you wear, the materials that make up your home — all these things depend upon plants. Plants remove carbon dioxide from the atmosphere, helping to keep your planet from getting too warm for life as you know it. They provide homes for insects and other animals, filter impurities out of ground water, and help protect shorelines from erosion.

And beyond all these useful things plants do, they’re just cool! Plants have many unique strategies that help them survive in all different kinds of environments. They trap and trick insects, grow in the ground or up in the rainforest canopy, and manage to survive everywhere from the glacial arctic to the hot, dry deserts. They seem so different from people, and yet when you really look at how plants grow and function, you’ll be surprised at how similar they are to you. This chapter offers an overview of the science of botany, giving you a peek into the mysteries of plants.

Taking a Close Look at Plant Structure

You might not think so, but plants are a lot like you. Their bodies are made of cells (see Chapter 2) that are organized into tissues (see Chapter 3) that form the familiar plant organs of roots, stems, and leaves (see Chapter 4). Plant cells use the same basic chemistry as your cells, storing information in DNA, using carbohydrates for energy, and putting proteins to work. And your cells and plant cells are both eukaryotic cells, meaning they have a similar structure that includes a nucleus and cellular organelles.

Organizing plants into roots, stems, and leaves

remember.eps Plants reach out to the sun with their leaves, absorbing light energy so that they can make sugar through the process of photosynthesis. Most leaves are flat because that’s the best shape for spreading out and catching lots of sun. But plants can also make leaves in different shapes for different purposes, such as tendrils for hanging on and climbing, spines for protecting the plant against grazing animals, or thick, fleshy leaves for storing water.

remember.eps Plant stems support the leaves, holding them in different arrangements so that they don’t shade each other and can absorb the most light. New plant growth occurs at stem tips as cells divide to make stems grow longer and to build new leaves, branches, and sometimes flowers. Some plant stems, such as those in cacti, are green so that they can do photosynthesis. Other types of stems, such as the runners of a strawberry plant, grow along the ground, sending up new plants at intervals along the horizontal stem.

remember.eps Plant roots are in charge of getting water for the leaves and stems by absorbing water from the soil. Along with the water, plant roots absorb minerals that provide them with the nitrogen, phosphorous, and other elements they need to function. Some plants, such as dandelion, have long tap roots that reach down deep into the soil, while others, such as grass, have many small fibrous roots. Plants like corn may make extra roots, called prop roots, that start from the stem above ground and then grow down to sink into the earth.

Finding ways to procreate

Plants have many ways of reproducing themselves. When plants reproduce sexually, they make special reproductive cells called spores (see Chapter 5). Many familiar plants make a structure that’s even better at starting the next generation — the seed. Seeds protect the plant embryos they carry and nourish them with stored food.

Some plants that do sexual reproduction get fancy and produce showy flowers (see Chapter 5) designed to attract animals to help spread their pollen around. Other flowering plants just dangle their flowers in the wind and let the wind do the work.

remember.eps Flowers contain the male and female parts of the plant that will participate in sexual reproduction.

Pollen comes from the male part of flowers, carrying and protecting the plant sperm. The female parts of flowers house the ovules that contain the eggs. Pollination occurs when pollen arrives at the female part of the flower. The pollen releases the sperm so that they can fuse with the egg, causing fertilization, and starting the next plant generation. After fertilization in flowering plants, the ovaries within the flowers develop into fruits (see Chapter 5). Some fruits are sweet and fleshy, inviting animals to come eat the fruit and then disperse the seeds. Other fruits are dry and designed to either float on the breeze, hitch a ride on some animal fur, or even explode to release their seeds. Whatever the method, the goal is the same — to find a nice, new home for the embryos inside the seed to grow.

Figuring Out Plant Functions

In addition to being made of cells and having similar chemistry, plants use many of the same strategies that you do to solve life’s challenges. Both you and plants need a source of building material, called matter, to build the cells of your body, and you both need a source of energy so that you can build things and move around (see Chapter 6). And just like you, plants need to transport food and fluids around their bodies. Finally, you and plants both grow and develop, responding to changes in your environment.

Making and using food

The go-to source of matter and energy for all living things is food. Of course, one big difference between you and a plant is that you have to get your food by eating another organism, while plants can make their own.

remember.eps Plants make their own food through the process of photosynthesis (see Chapter 7). Although the process of photosynthesis is pretty complex, you can get the main idea if you think of it like a recipe. The ingredients are carbon dioxide from the atmosphere and water taken up from the soil. You then follow these directions:

1. Use light energy from the sun to combine carbon dioxide and water, rearranging the atoms to form sugar and oxygen.

2. Serve sugar to all parts of the plant that need matter and energy and throw the oxygen gas away.

3. If you have leftovers, you can combine the sugars into starch to store some for later.

When plants want to use some of the sugar they’ve made to provide themselves with matter and energy, their cells do the same thing that your cells do with food, they break it down in a process called cellular respiration (see Chapter 7). Cellular respiration is a series of chemical reactions that basically unpack food molecules, making the matter and energy available to cells. When cells use cellular respiration to extract all the energy they can from food molecules, they release the waste matter as carbon dioxide and water.

Transporting materials

All the cells of a plant need food to provide them with matter and energy. Plants usually make sugars in their leaves, so they have to ship those sugars from the leaves to the rest of the plant. Likewise, plants take in water through their roots, but they need to get water to the entire plant, especially to the leaves where it’s needed for photosynthesis. So, just like you have veins and arteries to transport blood around your body, plants have vascular tissue that specializes in the transport of sugar and water (see Chapter 9).

remember.eps Plants transport dissolved sugars using a special type of tissue called phloem, and they transport water and dissolved minerals using a tissue called xylem.

Phloem transports sugar from the leaves where it’s made through photosynthesis, to all parts of the plant that need it for growth or that will store it as starch for later. Xylem transports water from the roots up through the plant to supply all the cells with the water they need.

Responding to hormones

remember.eps Yet another similarity between you and plants is that they use hormones to direct their growth and development (see Chapter 10).

Although plants never go through puberty (lucky plants!), they do undergo major developmental changes such as when a seed switches from being dormant to beginning to grow, or when a flowering plant decides it’s the right time of year to start putting on a floral display. Plant hormones also direct responses like helping plants shoots grow toward the light and causing plant roots to grow downward toward the pull of gravity.

Considering Plant Reproduction and Genetics

Plants grow like, well, like weeds. That’s because weeds are plants. (Okay, now I’m just being silly.) But seriously, plants grow when groups of cells at their tips, called apical meristems, divide in two to produce new cells. The process of cell division that adds new growth is called mitosis (see Chapter 11). Plants do mitosis pretty much the same way your cells do. Woody plants also do mitosis to grow wider, adding girth to tree trunks.

Plants also reproduce sexually, combining sperm and egg to make the next generation of plants. The sperm and egg, which are produced by a type of cell division called meiosis (see Chapter 11), carry copies of the DNA from the parent plant, passing their traits onto their offspring.

remember.eps By following the inheritance of traits from one generation to the next through the science of genetics (see Chapter 12), scientists can figure out how plant genes interact with each other to determine the traits of a plant.

Exploring the Wide World of Plants

Planet earth is filled with a glorious diversity of plants. Plants can be as tall as the mighty redwood tree, or as small as the tip of a pin. They can grow so rapidly that they go from seed to seed in a month, or they can live for over a thousand years. Since plants moved onto the land over 400 million years ago, they’ve evolved to live in every type of environment (see Chapter 13): Today, plants grow in the deserts, in the rainforests, in the ocean, and up on the mountain. Some plants make flowers, while others make cones. Plants may trap insects as a source of minerals or lure them in to help with pollination. With all these different strategies and environments, you can probably imagine that some pretty amazing plants are out there, from delicate mosses (see Chapter 15), to sturdy pine trees (see Chapter 16) and colorful flowers (see Chapter 17).

Botanists study all the different kinds of plants to understand how each one gets what it needs to survive and reproduce. They also compare the structures and DNA code of plants to figure out the relationships between plant groups and reconstruct how plants evolved (see Chapter 14).

Making Connections Between Plants and People

The lives of people are completely interwoven with the lives of plants:

check.png Plants support the ecosystems of which people are a part. Without plants to supply food to the web of life, what would you eat? (For more on this topic, see Chapter 18.)

check.png People can modify plants to make them more nutritious or so they produce medicines. Genetically modified foods are very controversial, but they have benefits as well as risks (see Chapter 19).

check.png People grow plants for food. The origins of human agriculture stretch back 10,000 years. And the switch from hunting and gathering to farming changed the entire structure of human societies (see Chapter 20).

check.png People use plants to make clothing. Cotton and flax plants are used to make cotton and linen clothing. Some of the dyes people use to give their clothing color also come from plants. (see Chapter 20).

check.png People get medicines from plants. Digitalin for heart disease, aspirin to reduce fever, and artemisinin for malaria are just a few examples of the powerful drugs people have extracted from plants(see Chapter 20).

check.png People use plants for building materials. People use wood for houses, furniture, and tools, or straw as material for roofs or bricks.

check.png People reduce their stress and improve their fitness by taking a walk and admiring the plants. Seriously, reducing stress is important. Stress has major impacts on people’s health. And for many people, nature has a soothing effect.

check.png Plants help keep water clean. You probably hear people talking about wetlands, how they’re important, and how they’re disappearing at a rapid rate, thanks to development. Wetlands are communities with certain types of plants and soils. As the rain falls across areas where humans live, it picks up lawn fertilizers, motor oil from cars, poop from pets, and more. If this runoff flows through a wetland before it enters our streams and lakes, the plants and bacteria in the wetlands will remove lots of the dangerous substances on the way. Having wetlands to slow the flow of water also helps prevent flooding.

However you look at it, from the similarities of plants to other organisms, their beauty, or their usefulness to humans, you can certainly find lots of good reasons to know a little bit more about plants.

Chapter 2

Peering at Plant Cells

In This Chapter

arrow Building molecules from atoms

arrow Discovering the molecules that make up the cell

arrow Exploring plant cells

All living things, including plants, are made of cells. Plant structure and function depend on these cells, so it’s a good idea to make sure that you have a firm grasp of cellular basics before you dive into the details of botany presented in the rest of this book. In this chapter, I give you an overview of the fundamentals of cells and the molecules they’re made of.

Making Molecules from Matter

Matter is anything that takes up space and can be weighed — in other words, it’s the stuff that makes up the living and nonliving things on planet earth.

Just like you, the air you’re breathing, and the book you’re holding, plants are made of matter. You’re probably familiar with some of the molecules that make up matter on earth — the proteins, fats, and carbohydrates in your body, the cellulose in the paper of the book, the carbon dioxide and oxygen in the air you’re breathing. Plants are made of the same kinds of molecules as you are, and they exchange carbon dioxide and oxygen with the atmosphere, too. The following sections describe these molecules, and the atoms that make them up.

Examining elements, atoms, and isotopes

Different kinds of materials, called elements, make up matter. You’re probably familiar with many common elements like copper, iron, chlorine, and calcium. Elements are pure substances because they’re made up of all the same kind of small particles, called atoms. Atoms are the smallest pieces of an element that have the characteristics of the element. For example, the element copper is a shiny metal that conducts electricity and interacts with other elements in a certain way. Iron is also a metal element, but it has different properties than copper. If you could isolate a single atom from either copper or iron metal, the individual atoms, which are so tiny you can’t even see them with a microscope, would still have the same properties as the larger metal they came from.

All the elements that have been identified so far are organized into a table called The Periodic Table of Elements (see Figure 2-1):

check.png Each row of the table is called a period. The elements in a period get heavier as you move across the table horizontally.

check.png Each column is called a family or group. Elements within the same family/group have similar properties. The size of the atoms gets larger as you move from top to bottom within each column.

Figure 2-1: The Periodic Table of the Elements.

Elements and atoms

Each element is different from the other elements because of the structure of its atoms. Atoms are made of smaller components (see Figure 2-2A):

Figure 2-2: Atoms and chemical bonds.

check.png Protons and neutrons make up the core of an atom, called the atomic nucleus. Both of these particles have mass and contribute to the weight and size of the atom. Protons have a positive electrical charge, while neutrons have no charge.

check.png Electrons are negatively charged particles that form a cloud around the atomic nucleus. Electrons have no mass.

Ions and isotopes

remember.eps The number of protons in an atom, called the atomic number, determines the type of element. All iron atoms (atomic symbol: Fe), for example, have 26 protons, while all copper atoms (atomic symbol: Cu) have 29 protons.

While the number of protons in an element are always the same, the numbers of neutrons or electrons may change:

check.png When an atom gains or loses electrons, it becomes an ion. In an atom of an element, the number of positively charged protons is equal to the number of negatively charged electrons. The opposite electrical charges cancel out each other, and the atom has no net charge. If an atom gives up electrons to another atom, then the atom will have a net positive charge, becoming a positively charged ion. Likewise, if an atom gains electrons from another atom, then the atom will have a net negative charge, becoming a negatively charged ion. For example, when sodium and chlorine atoms interact, sodium gives an electron to chlorine (see Figure 2-2B), forming a positively charged sodium ion and a negatively charged chloride ion.

check.png Atoms with the same number of protons, but different numbers of neutrons, are isotopes of each other. Neutrons have mass, so isotopes of an element are heavier or lighter than each other. Atoms of the element carbon (atomic symbol: C), for example, always have 6 protons — that’s what makes them carbon atoms. Most carbon atoms also have 6 neutrons, giving them a total mass, called their mass number, of 12. Some carbon atoms, however, have 6 protons and 8 neutrons, giving them a mass number of 14. Thus, carbon-12 and carbon-14 are isotopes of each other.

technicalstuff.eps Scientists have estimated the ratio of different isotopes of each element found on Earth and used that ratio to calculate the average atomic mass of each element. Most carbon atoms on Earth have a mass number of 12, but a small fraction have other mass numbers, like 13 and 14. If you average the mass numbers, taking into account the proportion of the different isotopes, you get an atomic mass for carbon of 12.01. So, in any sample containing carbon atoms, you’d expect the mass of the carbon atoms to average out at 12.01.

Getting atoms together to form molecules

Atoms join together to form molecules, chemical structures made of two or more atoms. The way an atom interacts with other atoms depends upon the number of electrons it has. Atoms will give, take, or share electrons with other atoms in order to achieve a stable electron arrangement. The attractions that join atoms together to form molecules are called chemical bonds.

Four types of chemical bonds are particularly important in plant cells:

check.png Ionic bonds are the electrical attractions between oppositely charged ions (see Figure 2-2B). Ionic bonds can be very strong in dry environments like salt crystals, but they’re weak in the watery world of plant cells.

check.png Covalent bonds form when atoms share electrons (see Figure 2-2C). Covalent bonds are the strongest bonds in plant cells, creating the backbones of the molecules that form the cell.

check.png Polar covalent bonds are a special category of covalent bonds, forming when atoms share electrons unequally. In water molecules, for example, the oxygen nucleus has a stronger attraction for the shared electrons than does the hydrogen nucleus. The electrons spend more time hanging around the oxygen side of the molecule, giving that side a slight negative charge. Due to the absence of the electrons, the hydrogen side of the molecule has a slight positive charge (check out Figure 2-3).

check.png Hydrogen bonds are weak electrical attractions between polar groups. Polar covalent bonds create polar molecules that have slight electrical charges. You can think of these molecules like they’re electrically sticky — positive sides of one molecule are attracted to the negative sides of other molecules (like the water molecules in Figure 2-3, where δ+ marks the weak positive charges near the hydrogen atoms, and δ– marks the weak negative charge near the oxygen atom.). The hydrogen bonds between water molecules give water many of its unique properties, like the fact that ice floats or that liquid water has surface tension. Hydrogen bonds between polar groups within a molecule can also help give individual molecules useful shapes.

Figure 2-3: Polar covalent bonds within water molecules and hydrogen bonds between water molecules.

Making Acids and Bases

Molecules can change when they’re mixed with water to make a solution, and can even change the properties of the solution itself.

Molecules called acids and bases release ions into a solution. When solutions become acidic or basic, they can damage cells:

check.png Acids are molecules that release hydrogen ions (H+) into solutions. When hydrochloric acid (HCl) is added to a solution, it splits apart into hydrogen ions (H+) and chloride ions (Cl-), increasing the number of hydrogen ions in the solution.

check.png Bases are molecules that release hydroxide ions (OH-) into solutions or that remove hydrogen ions (H+) from the solution. When the base sodium hydroxide is added to a solution, it splits apart into sodium ions (Na+) and hydroxide ions (OH-). The hydroxide ions can combine with hydrogen ions in the solution, forming water.

The ions that acids and bases release into solution can interfere with the chemical bonds that hold molecules together, leading to cell damage. You can measure the potential for cellular damage by looking at the pH of a solution, which is a measure of the concentration of hydrogen ions (H+) in the solution. Increasing hydrogen ions makes a solution more acidic, while decreasing hydrogen ions makes a solution more basic.

The pH scale, shown in Table 2-1, shows the pH of some common substances. Most cells, including yours and the cells of plants, have a neutral pH right in the middle of the scale at pH7. Acidic substances like lemon juice have a lower pH, while basic substances like oven cleaner have a higher pH. The farther the pH of a solution gets from pH 7, the greater the potential that it can disrupt the molecules that make up cells and do cellular damage.

Building Cells from Four Types of Molecules

Molecules are the building blocks that make up cells. You can think of molecules like little chemical Legos that are arranged and rearranged to build the structure of each living and growing cell. In multicellular organisms like plants, cells join together to form the tissues that make up the structure of the organism.

remember.eps The cells of all living things, including plant cells, are primarily made of four types of big molecules, called macromolecules:

check.png Carbohydrates

check.png Proteins

check.png Nucleic acids

check.png Lipids

Carbohydrates

Carbohydrates are commonly referred to as sugars, and the foods you can think of that are naturally sweet — like fruit, for example — are probably high in carbohydrates. Cells use carbohydrates for storing energy and building materials and also to provide structure to the cell.

Several types of carbohydrates are important to cells:

check.png Many sweet tasting carbohydrates are smaller, simple sugars, called monosaccharides by scientists. Glucose is an example of a monosaccharide. Glucose is extremely valuable to cells because it can be used as a fast source of energy. Glucose can exist in the linear form shown in Figure 2-4a, but in the water-filled environment of a cell, the molecule loops around and binds to itself, forming a ring-shaped structure (shown in Figure 2-4b).

check.png Monosaccharides may form bonds with each other to form larger structures.

• When glucose bonds with fructose, the sugar found in fruit, they form the disaccharide sucrose, otherwise known as common table sugar (see Figure 2-4b). Plants make sucrose in their green structures and then ship it all around the plant body to provide matter and energy to all their cells.

• Short chains of monosaccharides are called oligosaccharides (see Figure 2-4c). Oligosaccharides send signals to plant cells, triggering growth responses and defense mechanisms.

• Long chains of monosaccharides form polysaccharides (see Figure 2-4d. You may have heard polysaccharides referred to as complex carbohydrates. Like monosaccharides, polysaccharides are important molecules for storing energy and building materials and then making them available to cells. For example, the starch found in rice, pasta, breads, and potatoes is a polysaccharide that’s an important source of energy for both plants and people. Plants reinforce the structure of their cells with the polysaccharide cellulose, which is one of the major components of the cell wall that surrounds and supports plant cells.

Figure 2-4: Carbo-hydrates.

Get your fiber here!

Fruits and vegetables, as well as other plant foods like nuts and whole grains, are an excellent source of fiber. But what is fiber? And why do plants have it? Fiber is the common name for a plant polysaccharide called cellulose that surrounds and supports plant cells. Just like the starch you eat as a source of energy and building materials for our cells, cellulose is a long chain of glucose molecules strung together. However, there’s an important difference between the way the glucose molecules are attached to each other in starch and the way they’re attached together in cellulose. Well, this difference is important to people anyway, because humans can break the links in starch molecules, but not in cellulose molecules. So, when you digest starch, your body separates it into individual glucose molecules that you can easily break down for energy or rearrange to build your cells. But when cellulose, or fiber, hits your digestive system, it just passes on through as long chains. Your body can’t access the glucose molecules at all. So, all that undigested fiber passes into your large intestine and helps give mass to your, er, waste, which helps keep your large intestine healthy and functioning normally. Fiber can help lower blood cholesterol, control blood sugar, and help people lose weight. So, be sure to include plenty of plants in your daily diet!

Proteins

Plant cells couldn’t function without proteins. That’s because proteins perform essential jobs in cells, moving materials around, supporting the cell, helping chemical reactions, controlling information flow, and sending signals.

Each protein has a unique shape that helps it do its job. To make a protein, cells link amino acids with covalent bonds called peptide bonds (shown in Figure 2-5), forming long chains of amino acids called polypeptide chains. The polypeptide chains fold up, either singly or in groups, to form the final shape of the functional protein.

Figure 2-5: Amino acids link together to form proteins.

Proteins have so many functions in plant cells that a list could go on for two pages. Rather than overwhelm you with all those functions at once, I hit a few of the most important functions here and then introduce specific proteins as they’re needed throughout the book:

check.png Enzymes are proteins that speed up chemical reactions. As they live and grow, plants are constantly building new molecules and breaking other molecules down. The speed of these chemical reactions by themselves wouldn’t be fast enough to keep up with the pace of life. So plant cells, just like all cells, use enzymes to make those reactions happen exactly when plants need them.

check.png Structural proteins support the cell. Protein cables inside plant cells, called cytoskeletal proteins, provide supportive scaffolding from the inside. (For more details on cytoskeletal proteins, see the upcoming section Scaffolding and railroad tracks: The cytoskeleton.) Outside the cell, proteins are woven into the plant cell wall, a protective layer that encases plant cells. (You can find out more about plant cell walls in the section Rebar and concrete: Cell walls and extracellular matrices.)

check.png Transport proteins move materials into and within plant cells. Plants need to move molecules in and out of their cells. Transport proteins located at the boundary of the cell help create passageways for these materials. Inside the cell, molecules and structures may use cytoskeletal proteins as tracks that allow them to move around the cell.

check.png Receptor proteins help plant cells communicate. In order to receive signals, such as hormones, plant cells need receptors that specifically recognize each signal. Receptors, which are usually proteins, can be located on the surfaces or insides of cells.

Nucleic acids

Even if you haven’t heard the term nucleic acids before, I’m sure you’ve heard of DNA, which is short for deoxyribonucleic acid. Nucleic acids like DNA are molecular specialists in information: The molecules are a chemical code that store information and can transfer it from one generation to the next.

Two types of nucleic acids are found in cells:

check.png DNA