Botany: An Introduction to Plant Biology - Mauseth, James D. 2017

Plant Structure

Part Opener Image: The structure of this poppy fruit is adapted such that it aids the survival of the plant. While immature, the fruit protected and nourished the seeds as they developed. Now that the seeds are mature and must be released, pores (holes) have formed near the top of the fruit. During the formation of the pores, some cells broke down, creating flaps, then cells at the base of each flap contracted, pulling the flaps outward and downward, opening the pores. As the fruit sways in the wind, seeds will be thrown out through the pores in various directions.

Plant structure is the physical, material body of a plant, composed of carbohydrates, proteins, lipids, minerals, water, and other components. In terms of both material and energy, it is expensive for an organism to build a body—all of the energy and resources used by a tree to construct its body could have been used instead for reproduction, increasing its number of offspring. The theory of evolution by natural selection predicts that organisms must receive enough benefit from their structure to offset the cost of building it: It must be more advantageous selectively to have a body than not to have one. The benefit is that the structure is the framework in which metabolism occurs and the means by which metabolism interacts with the environment. This can be more easily understood by considering the alternative—organisms with very little structure.

Several organisms have almost no structure whatsoever: viroids, viruses, and mycoplasmas. All are parasites that live, grow, develop, and reproduce only if immersed within the body of a host organism. The host provides a benign environment with enough nutrients for the parasite while also being free of harmful chemicals. The temperature of the host’s body is ideal for the parasite, and adequate water is available. Parasites cannot live outside the host-supplied environment. With minimal structure, they have minimal capacity to interact well with the heat, cold, drought, and scarcity of nutrients in a natural environment.

Organisms whose bodies are more structured and complex are able to resist temporary adverse environmental conditions as well as exploit optimal ones. A simple bacterium can absorb mineral nutrients from certain environments; but, by constructing a root system, a plant can spread its mineral-absorbing metabolism throughout an extensive, deep volume, growing past dry, rocky, mineral-poor soil and tapping moist, rich, fertile soil. The investment for the plant is great, but sufficient reward is achieved in having an adequate, secure source of water and minerals.

The structures discussed in this part of the text are important to the plants themselves, but they are also important to us because we rely on them for food and many products, and as we cultivate them we affect the lives of many other organisms. Most of the food we eat is some part of a plant body. For example, we eat the leaves of lettuce, spinach, and cabbage; roots of carrots, beets, and radishes; stems of asparagus and potatoes; and fruits and seeds such as apples, oranges, peas, beans, and almonds. Some of our foods are so highly processed we may not immediately realize they were once part of a plant body: Bread, doughnuts, cakes, and pasta are made with flour (the ground seeds of wheat); beer is brewed from barley (also seeds) and hops (flowers); and wine is fermented grape juice. Our very lives depend on these plant-based foods because they supply us with energy, vitamins, and minerals. Look at your hand: Every atom and molecule, every bit of your body was at one time part of some plant. Vanilla, chocolate, cinnamon, sage, black pepper, and chili peppers are spices that do not really provide us much in the way of nutrition, but they make eating fun and few people would want to give them up. The cotton in our clothes is a plant structure, as are the fibers in paper and burlap bags and the lumber used for construction or for making guitars and pianos.

Our reliance on plant bodies goes beyond these fundamental needs. The grass lawns on which we play or relax on are the leaves and stems of plants, and trees provide us with shade and protection from weather. We love the beauty of flowers, and almost certainly there will be some point in your life when something wonderful or tragic will have happened and you just won’t know what to say. At that point you will probably give someone flowers to express emotions you cannot put into words. We celebrate weddings and birthdays with flowers, we give flowers to friends when they are ill or just for friendship, and we rely on flowers to express sympathy at funerals.

Because we rely on plants for so many things, we need to cultivate them on farms, orchards, and tree plantations, and this raises important issues with the rest of the world. Agriculture began as early as 9,000 years ago and has been affecting the world ever since. The first step of agriculture then as now is to clear a piece of land of its natural vegetation and animals. If we apply fertilizers or pesticides to crops, rain may wash these chemicals from the fields into nearby streams, polluting them and harming fish, frogs, and other aquatic life. Crops must be harvested, processed, and transported to markets, all of which require roads, fuel, and machinery and result in pollution. Our use of plant structures for so many things in our lives results in negative impacts on the ecology and well-being of many organisms.

Plant structures, however, may also provide solutions to these problems. Scientists are conducting research to find plants that provide more abundant or more nutritious food, better fibers, or natural resistance to insect pests. Many of our crop plants now produce much greater harvests than did the crops available just a few decades ago. For example, a fast-growing plant called kenaf can produce many more tons of fibers per acre than do pine trees, and thus less land is needed to make the same amount of paper: It may even be possible to convert some farm land back to a more natural condition suitable for wildlife.

Most plants around us are angiosperms, commonly called the “flowering plants.” Most plants discussed in this text are angiosperms, and to a lesser extent we discuss ferns (which reproduce with spores) and conifers (which have cones, such as pines and spruces). One of the most noteworthy features of angiosperms is the diversity of bodies that occur in this group. Some angiosperms are ordinary herbs or shrubs or trees, but some are desert succulents (cacti), vines (grapes), parasites (mistletoe), bulbs (onions), and tubers (potatoes). Ferns and conifers are not so diverse, and this text would have been much shorter if it had been written 135 million years ago before angiosperms had come into existence.

Angiosperms are one of the newest groups of plants. The first true plants originated about 420 million years ago, but the oldest fossils of angiosperms are only about 135 million years old. At the time angiosperms originated the world was a very different place: The air and land were much warmer, as were the oceans. Consequently, much more water evaporated from the oceans into the air and then fell as rain on land; warm, moist conditions allowed the growth of lush vegetation that consisted mostly of ferns and conifers. The dominant land animals were dinosaurs. During the last 100 million years Earth warmed even more as volcanoes and lava released large amounts of the greenhouse gas carbon dioxide into the air. However, gradually plants took carbon dioxide out of the air as they used it in photosynthesis and built their bodies with it. As plants removed carbon dioxide from the air, less heat was trapped and Earth began to cool, starting about 35 million years ago; it entered the Pleistocene ice age 2.5 million years ago. It was during this time of changing temperatures and climates that the angiosperms originated and evolved to have such diverse bodies.

Continental drift is another factor that affected the evolution and diversification of the flowering plants and the structures that make up their bodies. Continents are constantly changing their position on Earth’s surface as hot, fluid rock flows in Earth’s mantle below its crust (we live on the top of the crust). As angiosperms were diversifying, North America broke away from Europe and drifted westward, whereas South America separated from Africa and Antarctica and migrated west and northward. Today, North and South America are colliding, pushing up a range of mountains we call Central America. As the continents change their position, they move from warm areas near the equator to cold areas near the poles or vice versa, and they block circulation of ocean currents. These currents contain millions of tons of water that is either warm, if it is flowing from the equator, or cold, if coming from the poles; as continents shift they redirect the flow of that water and that heat. For example, Alaska is not as cold as it would be if warm water did not flow from the equator, past Japan and up to Alaska’s southern coast. Thus, the changing climate of Earth and the changing position of the continents created an extremely varied and variable environments to which plants had to become adapted; those that did not adapt became extinct.

As you study the material in this part, think about how the structures discussed facilitate the metabolism that occurs inside them: How do the structures make the metabolism more efficient? What are the selective advantages of the structures? Are alternative structures possible for a particular metabolism? If so, what are the consequences of each? The structures that make up the body of a plant must work together to make a functional plant and they must evolve; they evolved over long periods of times and in conditions that were very different from those we live in now. If the past climate of Earth had been different, the structures covered in this text would also have been different.