I’ve recently been approached by a publicist representing the Plant Kingdom, and on behalf of all the plants in the world, I have been requested to assist in setting the record straight. The plants of the world feel strongly that they have not been getting enough credit. It seems that very few people have a full appreciation of the roles that plants have played — and continue to play — in making the world what it is today. The issue is not just that plants are the ultimate source of everything we eat — which means that without plants we would not be here. It’s also that plants have played an absolutely essential role in so many other aspects of our development, and we seldom talk about most of these other roles.
But first, before we get into the big stuff, I’ve been asked by the publicist to air a pet peeve of the angiosperms (also known as flowering plants). These guys are sick and tired of hearing that “bees make honey”. According to the official representative of the Flowering Plants Union, it is actually flowering plants that make honey, although they prefer to call it “nectar”. The bees simply collect the nectar, and then concentrate it (slightly) by evaporating some of the remaining water. The bees do this by fanning the collected nectar with their wings. The plants go to considerable effort to provide nectar that is already quite concentrated — otherwise it would not be energy-efficient for the bees to collect: The bees would spend more energy buzzing their wings than they would get from the honey. In contrast, butterflies and hummingbirds are perfectly happy to accept watery nectar that contains a lower concentration of sugars. This demonstrates that flowering plants go out of their way to meet the finicky demands of bees.
Imagine if a plant were to ask you what maple syrup is. I don’t think you’d say “Maple syrup is a concentrated sugar-water solution made by humans.” No, you would say that maple syrup is the concentrated sap of maple trees. During the process of converting that sap into maple syrup, humans have to evaporate a lot more water than bees do when “making honey”. (Maple sap is typically around 5% sugar and 95% water, while the nectar that flowers produce for bees is typically more than 50% sugar.) And yet we have the good sense to give credit to the effort of the maple trees. The flowers involved in the honey trade — such as clover, orange blossoms, blueberries, buckwheat, sourwood, alfalfa, tupelo, and many, many others — simply request the same due respect and credit that maple trees already receive.
(For more details on bees, see What Does “Save the Bees” Really Mean?)
You are undoubtedly aware that people — and virtually all other animals — require oxygen in the air in order to survive. If our atmosphere did not contain oxygen, then we humans would not exist. But how did the oxygen get into the air in the first place? Plants put it there! Before plants began their tremendous task of transforming the earth’s atmosphere, there was no oxygen in the air. But thanks to plants, our atmosphere is now 21% oxygen — a very comfortable level for humans and other animals.
I should mention that the Greater Brotherhood of Green Plants allows any living thing that contains chlorophyll to become a member — and therefore to be counted as a plant. This includes not only the things that we normally think of as being plants, but also microscopic creatures, even one-celled creatures such as blue-green algae — which are actually related to bacteria. What all of these organisms have in common is that they contain chlorophyll, which allows them to capture the energy of sunlight. The captured energy is stored in the form of sugar molecules — as we saw in the case of flower nectar and maple sap. What may not be obvious is that all green plants produce sugars. They do this by using the energy of sunlight to convert molecules of water and carbon dioxide into molecules of sugar — a process that requires a lot of energy. Later on, whenever a plant needs any energy, it simply converts some of that sugar back into water and carbon dioxide, releasing the energy for whatever purpose it is currently needed.
Every time a plant creates a molecule of sugar, six molecules of carbon dioxide (CO2) are removed from the air. Simultaneously, six molecules of oxygen (O2) are released into the air as a byproduct. Early in the history of life on earth — when every living creature in the world was still microscopic — there were so many busy little plants on earth that they gradually changed the atmosphere from completely anoxic (without oxygen) into the atmosphere that we have today. The process took a very long time — more than a billion years — but the little plants kept at it until the job was done. And now the Greater Brotherhood of Green Plants simply wants a little credit for what they have done — an acknowledgement that their efforts produced a world in which animals can successfully live and breathe.
(For more details on why blue-green algae — the little guys that produced so much of that oxygen — might or might not be considered a plant, see How Many Kinds of Living Things Are There?)
Imagine how many molecules of sugar were produced in order to fill the earth’s atmosphere with 21% oxygen. Keep in mind that each time a sugar molecule is turned back into carbon dioxide and water — thereby releasing the stored energy — six oxygen molecules from the atmosphere get used up. Even if the plant itself does not use the energy, because an animal ate the plant, oxygen is used up when the animal uses the energy. Even if no animal eats the plant, but the plant simply dies and decomposes, then oxygen is used up (and energy released) when the plant decays. Therefore all of those oxygen molecules in the atmosphere correspond to sugar energy that was never used. Where did all of that sugar energy go?
Sugar is an example of what is called an organic compound. A “compound” is any substance in which all of the molecules are identical — in other words, the substance consists of only one kind of molecule, instead of a mix of molecules. The word “organic” can be a bit confusing, because it can have very different meanings in different contexts. An “organic compound” is any compound in which the molecules contain atoms of carbon. In most organic compounds, the carbon atoms are linked together into a chain — which can be a straight chain, a branched chain, or even a ring. The simple sugars produced by photosynthesis are chains of six carbons each, attached to some additional atoms of oxygen and hydrogen. (Therefore sugars are carbohydrates.) Plants and animals can convert these sugars into other organic molecules without releasing the energy that is stored in the bonds. As a result, organic molecules of all kinds contain stored energy.
The bodies of most plants and animals consist primarily of organic compounds and water. All of this mass is sometimes called biomass. The term “biomass” refers not only to the mass of all the living creatures in the world, but also to the mass of anything that has died but not yet decomposed, such as the leaf litter on a forest floor, or dead tree trunks. The biomass on land surfaces is more than 99% plant material, and less than 1% animal material. (There is also biomass in the oceans.) If you consider all of the biomass currently on the surface of the earth (including the oceans), then you can account for part of the sugar energy that has been stored away by plants over the eons. All of this biomass corresponds to carbon dioxide that has been removed from the atmosphere, and to oxygen that has been released into the atmosphere. If all of this biomass were to burn up or decompose, then part of the oxygen in the atmosphere would be replaced with carbon dioxide.
And yet, even considering all of the biomass on the surface of the earth, we still have not accounted for all of the organic compounds stored away by billions of generations of plants. To account for the rest, we must consider fossil fuels — coal, petroleum, and natural gas. Fossil fuels are the remains of biomass that got buried in the earth without decomposing. Therefore these fuels consist of organic molecules that never turned back into carbon dioxide and water. These molecules never released their stored energy, and they never used up the oxygen that was released when the energy was first captured.
And now we come to another pet peeve of the Plant Kingdom. Nothing drives those guys up the wall more than hearing the often-repeated notion that petroleum comes from the remains of dinosaurs. This really infuriates many of the green plants, especially the lycopsids — club mosses and their relatives. The ancient biomass deposits accumulated in soggy areas where a lack of oxygen prevented decomposition. Huge, thick layers of biomass accumulated over time. But back then, just like today, more than 99% of the biomass was of plant origin, and less than 1% was of animal origin. As this plant biomass became buried and compressed, most of the organic molecules never decomposed, although the carbohydrates gradually changed into hydrocarbons. Thus we owe these energy-rich buried treasures to plants, not to animals.
Our modern civilization is quite dependent upon fossil fuels. Even though we are trying to reduce this dependence — primarily because of the consequences of increased carbon dioxide in the atmosphere — we still generate a majority of our electric power using fossil fuels. We still rely on fossil fuels for the vast majority of the energy used in transportation — to drive our cars, trucks, trains, ships, and airplanes. Imagine what life would be like today if plants had never created this vast store of chemical energy. Our modern civilization simply would not exist.
How did civilization first begin? What was the first step in the long transformation of humankind from just another smart animal into a species that can shape and control the entire earth? Most of us would consider that first step to be when humans learned to control fire. By mastering the use of fire, we were able to warm ourselves and to cook our food. Eventually we learned to use fire to produce metals and durable ceramics — and these skills allowed us to create the tools to advance ourselves even more. Fire has been an essential partner in every stage of the advancement of civilization.
And yet without plants, we would have nothing to burn. At first we depended entirely upon wood and other plant materials as our fuels. Eventually we came to rely more and more on fossil fuels — but these also come from plants. Without plants, humans would never have had fire, and therefore we would never have had civilization.
Of course, fire sometimes causes problems too. Wildfires are a natural and even essential part of most terrestrial ecosystems, and yet we justifiably fear such fires, because they can kill us or destroy our property. But consider this: In a wildfire, it is plant material that burns — primarily trees, shrubs, and grasses. If there were no plants, then there would be no wildfires.
If a house should catch fire, then what parts of the house are actually flammable? It is not the bricks or stone, nor the metals or concrete or mortar. In many cases, a large percentage of the flammable material is the wooden framing. Inside the house we have materials made from wood, paper, textiles, and plastics — all of which quickly burn. The wood and paper are obviously made from plants. The textiles and the flooring are most likely made from either plant materials or plastics. But the plastics are made from petroleum products, which come from fossil fuels. In short, all of the flammable materials in a house consist of organic compounds that can be traced back to plants.
In essence, all fires are the indirect result of plants having captured the energy of sunlight in the form of sugars. These sugars ultimately get converted into a wide range of organic compounds — some natural and some man-made — without losing their stored energy. But when these compounds catch fire, they are converted back into carbon dioxide and water (assuming complete combustion), releasing all the stored energy and consuming oxygen. Therefore nearly all fires — whether controlled or wild — are ultimately due to plants. If plants had never existed, then there would be no fire in this world, because there would be no fuel to burn.
Of course, if plants had never existed, then there would be no oxygen in the atmosphere, which also means that there would be no fire. In capturing the energy of sunlight, plants flooded the world with organic compounds and filled the atmosphere with free oxygen. Fire is simply a rapid reversal of the process.
(For more details on fire, see Fire Is Not What You Think It Is.)
Steel and Chemicals
Two of the hallmarks of recent civilization are the production of steel and chemicals. Of course we use a lot of energy to create steel and chemicals — and that energy usually comes from fossil fuels. But there is another dimension to how plants are involved in the process.
The production of steel not only requires a lot of heat (usually produced by coal), but it also requires carbon — because steel is an iron alloy that contains around 1% to 2% carbon. That carbon comes from metallurgical grade coal — which in turn comes from ancient plants. So without plants, we would not have steel.
Human industry produces a bewildering variety of chemicals. Some of the chemicals with the longest history — such as turpentine — are extracted directly from plants. We have also learned to synthesize chemicals — converting one set of chemicals (serving as the raw ingredients) into a different set of chemicals. Certain ingredients come from minerals, such as salt. But much of our chemical industry is built around the production of organic compounds — including all of our plastics. Most of the raw ingredients are themselves organic compounds, which are sometimes extracted from plants, but are often extracted from petroleum — which ultimately comes from plants. The upshot is that most of our chemical industry relies on raw materials that can be traced back to plants. Without plants, we would not have plastics, nor would we have most of the other man-made organic compounds that we churn out in great quantities.
And now we come to the elephant in the room — although I have been asked to refer to it as the elephant grass in the room. Many of us know that we ultimately rely on plants for all of our food — but a surprising number of people have not fully picked up on this message. Even if you eat a lot of meat, you probably realize that the animals you eat are primarily fed with plants. And even if you love to eat fish that primarily eat other fish, you can always trace the food chain back to plants.
The irony is that carnivores are even more dependent upon plants than herbivores, although indirectly. All of the food energy in an ecosystem depends upon the capturing of sunlight by plants (with a few very rare exceptions). But as you go up the food chain, from herbivores to carnivores to top carnivores, you lose at least 90% of the available energy at each level. Therefore it requires at least ten times as much plant material to sustain an herbivore-eating carnivore as it does to sustain an herbivore of the same size. And it requires at least one hundred times as much plant material to sustain a top carnivore as it does to sustain a similar sized herbivore.
All animals need food for two different reasons — for the energy stored in the food, and for an assortment of organic compounds that are needed to build bodies and control the functions of life. In particular, we need sugars from carbohydrates, fatty acids from fats, and amino acids from proteins. Starting with these basic building blocks, our bodies (and the bodies of other animals) can synthesize most of the thousands of organic compounds that our bodies need to survive. But we are unable to build any of these compounds from scratch — as plants do — starting with carbon dioxide and water as the two principal raw ingredients. We require plants to assemble the carbon into chains, producing the sugars, fatty acids, and amino acids that we can then repurpose and convert into other organic molecules. Without plants, we would have no source for the energy we need to survive, and we would have no source for the organic molecules we need to survive.
(For more details on food molecules, see Digesting Food Is Like Playing with Lego Blocks.)
Evolution of Animals
It is not just human civilization that required the presence of plants in order to evolve. The entire evolutionary history of the animal kingdom depended upon the continued presence of plants. Moreover, each new species of animal evolved in a context in which certain kinds of plants were available, presenting opportunities for food, nesting, and other uses — but also presenting challenges to overcome, such as thorns or toxic compounds. Furthermore, throughout evolutionary history, the mix of plants in any given spot kept changing — evolving to be better adapted to current local conditions.
This led to a constant evolutionary spiral, in which evolutionary changes in the plants led to evolutionary changes in the animals, and vice versa. For example, the rapid emergence and spread of angiosperms (flowering plants) in the mid-Cretaceous period provided lots of new food sources for many species of dinosaurs, helping to fuel a boom in dinosaur evolution. However, the heavy predation on the plants led many of them on an evolutionary path to improve their defenses, resulting in new species that were unpalatable or difficult to consume. As a result, new animals evolved that could overcome these defenses.
(For more details on plant and animal evolution, see Snapshots of Prehistory: A Vacation in the Geologic Past.)
Interactions between plants and animals are not limited to predator-prey relationships. Mutually beneficial relationships exist as well. The best-known examples involve flowering plants and their pollinators, especially bees. There are countless examples of co-evolution among flowers and pollinators, where the species evolve together to become ever more specialized and dependent upon one another. For example, if you should find a trumpet-shaped wildflower with a narrow 3-inch corolla tube, then you can be sure that some sort of pollinator with a 3-inch tongue has evolved to make use of that flower. Furthermore, the wildflower is almost certainly dependent upon that pollinator in order to get pollinated.
(For more details on pollination, see How Plants Solve Their Transportation Needs.)
The upshot is that the entire history of animal evolution has been heavily shaped by the details of the plants with which each species of animal interacts. Of course, not all animals eat plants, but nearly all of them must interact with plants — hiding in tall grass, climbing into trees, building nests from plant material, and so on. Therefore the animals evolve to become better at all of these tasks, based on the plants that are locally available.
I must emphasize that I am not a paid representative of the Plant Kingdom. Not a single species of plant has paid me a single cent as an incentive to write this essay. After their publicist (who admittedly receives a stipend) brought this issue to my attention — that plants fail to receive adequate credit for what they have done and continue to do — I felt that it was my duty to speak out to the world, calling attention to the injustice of this situation.
I hope that you now realize that plants have played a huge role in making the world what it is today. There are so many ways in which plants have made Earth habitable for animals — and especially for humans. We can thank plants for the food we eat, the oxygen we breathe, and the fuels that we burn. We can thank plants for the ingredients that made civilization possible — such as fire, agriculture, and steel. We can thank plants for the environments that led to the evolution of all of our favorite species of animals, including ourselves. And I haven’t even gotten to some of the topics that I could have covered. For example, the rich topsoil in which we grow our crops is due to the incorporation of organic matter from plants. Our clothes are all made from materials that can be traced back to plants. The list goes on and on. It is high time that we finally give plants the credit that they deserve.
And don’t forget that plants are the ones that actually make honey. Bees just collect it.
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