What Is a Plant, and Why Should I Care? part two

“Organisms green with chlorophyll appeared pretty early in Earth history, diversified, and adapted to oceanic, coastal, and finally terrestrial environments. As this took place, the Earth turned green.” – Joseph E. Armstrong, How the Earth Turned Green

world turned green

The Earth not only turned green, but the composition of its atmosphere dramatically shifted. Thanks in part to photosynthesis, Earth’s atmosphere went from having virtually no free oxygen to being composed of about 21% oxygen. The increasing availability of oxygen helped facilitate the evolution of more and more diverse forms of life. Had photosynthesis (specifically oxygen-producing photosynthesis) never come about, the Earth would not be anything like it is today.

There are organisms in at least three taxonomic kingdoms that have the ability to photosynthesize: Bacteria, Protista, and Plantae. A book itself could be written about how photosynthesis developed and how it differs among organisms. The important thing to note in a discussion about plants is that the type of photosynthesis that occurs in cyanobacteria is the same type that occurs in the chloroplasts of plants and green algae. Additionally, pigments called chlorophyll are only found in cyanobacteria and the chloroplasts of plants and green algae. As Joseph Armstrong puts it in How the Earth Turned Green, “evidence strongly supports the hypothesis that chloroplasts were free-living photosynthetic bacteria that became cellular slaves within a host cell.”

In Part One, we established that green algae are closely related to plants, and that a subset of green algae colonized the land and evolved into modern day plants. Plants are green because of cyanobacteria via green algae; however, cyanobacteria are not plants, and green algae may or may not be plants depending on your preference. Classification is not nearly as important as determining evolutionary relationships.

So, again, what is a plant? K. J. Willis and J. C. McElwain offer this summary in their book, The Evolution of Plants: “Plants are relatively simple organisms with a common list of basic needs (water, carbon dioxide, nitrogen, magnesium, phosphorous, potassium, some trace elements, plus various biochemical pathways necessary for photosynthesis). This list has remained almost unchanged from the first land plants to the present.” In Part One, we also listed three major features that all plants have in common: multicellularity, cell walls that contain cellulose, and the ability to photosynthesize.

Photosynthesis is a big one, because it means that plants make their own food. They are autotrophs/self-feeders/ producers. This sets them apart from heterotrophs, organisms that consume other organisms in order to obtain energy and other essential nutrients. Plants are at the bottom of the food chain, providing energy and nutrients to all other organisms that either directly or indirectly consume them. In Armstrong’s words:

“Eating and being eaten is a fact of life, a process by which the light energy captured by green organisms is passed through a series of consumers, a food chain, before eventually being lost as heat, which dissipates. Everything else is recycled with the able assistance of decomposers, primarily fungi and microorganisms, heterotrophs who obtain their food from dead organisms or their metabolic wastes. A large part of ecology concerns such trophic or feeding interactions, the energy transfers that result, and the cycling of biogeochemicals, the elements of life.”

Their ability to photosynthesize, among other things, gives plants a prominent role in the world’s ecosystems. Much more will be said about that as we continue, but first there are a few other things about plants worth mentioning.

Plants exhibit modular growth. While animals generally produce all of their body parts early on in life and rarely reproduce new body parts in replacement of lost ones, plants can continue to reproduce and replace body parts. Even at maturity, plants maintain embryonic tissues, which allows them to regenerate body parts as needed. This is one reason why so many plants can be propagated asexually via stem, root, and/or leaf cuttings. Roots can be encouraged to grow from unlikely places, and a whole new plant can be produced as a result.

Plants are generally stationary. Rooted in place, they must obtain everything necessary for life, growth, and reproduction by accessing whatever resources are in their immediate vicinity. Roots search the soil for water and other nutrients, and leaves harvest sunlight and carbon dioxide to make sugars. Relationships are maintained with soil fungi to aid in the search for water and nutrients, but otherwise, plants are largely on their own. Since they cannot run or hide, they must stand and fend for themselves when insects and other herbivores come to devour them. They have adapted a variety of chemical and physical defenses to address this.

Despite being largely immobile during their juvenile and adult phases, plants can actually be incredibly mobile during their embryonic stage (or in other words, as seeds/spores/progules). Employing biotic and abiotic resources, seeds and spores can potentially move miles away from their parent plants, enjoying a freedom of movement they will never know again once they put their roots down.

It is estimated that the total number of plant species on the earth today is around 400,000. (For reference, see this BGCI page and this Kew Gardens page. See The Plant List for up to date plant species names.) The first land plants evolved around 450 million years ago. It wasn’t until around 160 million years ago that the first flowering plants appeared, yet about 90% of the plants in existence today fall within this group. How many tens of thousands of species of plants have existed on Earth throughout history? I don’t think we can say. So many have come and gone, while others have radiated into new species. Exploring life that currently exists on this planet is an enormous pursuit on its own; add to that the exploration of life that once existed, and your pursuits become endless.

Sticky purple geranium (Geranium viscosissimum) one species of around species of extant flowering plants.

Sticky purple geranium (Geranium viscosissimum) is just one of more than 350,000 species of extant flowering plants.

At the close of the first chapter of his book, Armstrong highlights eight major historical events that have brought us plants as we know them today: “the origin of life itself, the development of chlorophyll and photosynthesis, the advent of the eukaryotic (nucleated) cell, the development of multicellular organisms, the invasion of land, the development of vascular tissues, the development of seeds, and the development of flowers.”  Consider that a brief synopsis of all we have to cover as we continue to tell the story of plants.

What Is a Plant, and Why Should I Care? part one

I want to tell the story of plants. In order to do that, I suppose I will need to research the 4 billion year history of life on earth. And so I am. Apart from satiating my own curiosity, studying and telling the story of plants advances me towards my goal of creating a series of botany lesson themed posts. Botany 101 and beyond, if you will. An ambitious project, perhaps, but what else am I going to do with my time?

So what is a plant anyway? We all know plants when we see them, but have you ever tried to define them? They are living beings, but they are not animals. They are stationary – rooted in the ground, usually. Most of them are green, but not all of them. They photosynthesize, which means they use water, carbon dioxide collected from the atmosphere, and energy harvested from the sun to make food for themselves. No animal can do that (okay…a few sort of can). They reproduce sexually, but many can also reproduce asexually. They are incredibly diverse. Some grow hundreds of feet into the air. Some barely reach more than a few centimeters off the ground at maturity. They have discernible parts and pieces, but they can also lose parts and pieces and then grow them back. There aren’t many animals that can do that. They have been on this planet for hundreds of millions of years, colonizing land millions of years before animals. Plants helped pave the way, and if it weren’t for plants, animals may not have stood a chance.

I don’t mean to pick on animals, it’s just that for a long time, humans grouped living things into just two kingdoms: Plantae and Animalia. Stationary things that appeared to be rooted to the ground or some other surface were classified as plants. Green things that lived in the water were also considered plants. Thus, lichens, fungi, algae, and everything we consider to be a plant today were placed in kingdom Plantae. Everything else was placed in kingdom Animalia. This, of course, was before much was known about microorganisms.

Dichotomous classification was reconsidered as we learned more about the diversity of organisms in each kingdom, particularly as the theory of evolution came into play and microscopes allowed us to observe single celled organisms and chromosomes. Eventually, fungi was awarded its own kingdom, which includes lichens – organisms composed of both fungi and photosynthetic species but classified according to their fungal components. Most of the algae was placed in a kingdom called Protista, a hodgepodge group of unicellular and unicellular-colonial organisms, some of which are animal-like and some of which are plant-like. Two kingdoms were also formed for prokaryotic organisms (organisms with cells that lack membrane bound organelles): Bacteria and Archaea.

Illustration of one current itteration of kingdom classification system (illustration credit: wikimedia commons)

Taxonomic kingdoms as we currently consider them (illustration credit: wikimedia commons)

In short, the answer to what is a plant seems to be whatever organisms humans decide to put in kingdom Plantae. One problem with this answer is that some chose to include certain species of algae and others don’t. But why is that? It has to do with how plants evolved and became photosynthetic in the first place.

Microorganisms developed the ability to photosynthesize around 3.5 billion years ago; however, the photosynthetic process that plants use today appeared much later – around 2.7 billion years ago. It evolved in an organism called cyanobacteria – a prokaryote. Eukaryotic organisms were formed when one single cell organism was taken inside another single cell organism, a process known as symbiogenesis. In this case, cyanobacteria was taken up and the eukaryotic organisms known today as algae were formed. The incorporated cyanobacteria became known as chloroplasts.

Not all algae species went on to evolve into plants. A group known as green algae appears to be the most closely related to plants, and a certain subset of green algae colonized the land and evolved into modern day plants (also known as land plants). That is why some taxonomists choose to include green algae in the plant kingdom, excluding all other types of algae.

Common stonewort (Chara vulgaris, a species of green algae (photo credit: www.eol.org)

Common stonewort, Chara vulgaris, a species of green algae (photo credit: www.eol.org)

The term land plants refers to liverworts, hornworts, mosses, ferns, fern allies, gymnosperms, and flowering plants – or in other words, all vascular and non-vascular plants. Another all encompassing term for this large group of organisms is embryophytes (embryo-producing plants).

Still confused about what a plant is? Three main features can be attributed to all plants: 1. They are multicellular organisms. 2. Their cell structure includes a cell wall composed of cellulose 3. They are capable of photosynthesis. Many species of green algae are unicellular, which is an argument for leaving them out of kingdom Plantae. Certain parasitic plants like toothwort, dodder, and beech drops have lost all or most of their chlorophyll and no longer photosynthesize, but they are still plants.

Deciding what is and isn’t a plant ultimately comes down to evolutionary history and common ancestry. As Joseph Armstrong writes in his book, How the Earth Turned Green, “Our classifications of human artifacts are totally arbitrary, but to be useful scientifically our classification of life must accurately reflect groupings that resulted from real historical events, common ancestries.”

Obviously this is going to be a multi-part series, so I will have much more to tell you about plants in part two, etc. For now, this You Tube video offers a decent summary.

President Obama’s Lichen

It is a presidential election year in the United States of America and, as per usual, it’s a circus. Prolific coverage of the surrounding events is hard to avoid. President Barack Obama is in the final year of his second term, which means that 8 years ago he was in the same position as today’s presidential hopefuls. Ultimately Obama was elected President, but during that lively process something else was afoot.

Kerry Knudsen is the lichen curator at the University of California Riverside Herbarium. In the final weeks of the 2008 campaign season, Knudsen was making collections of a species of lichen that he had discovered a year earlier. As Obama was being elected President, and (as Knudsen terms it) “the international jubilation” surrounding the event proceeded, Knudsen was drafting a paper describing and naming the newly discovered species. The final draft was completed during President Obama’s inauguration, and so it seemed fitting to Knudsen that he name the lichen after Obama. Caloplaca obamae it was – named after the 44th President of the United States, in honor of “his support of science and scientific education.”

President Obama's lichen - Caloplaca obamae - discovered and described by Kerry Knudsen (photo credit: UCR Herbarium/J.C. Lendemer

President Obama’s lichen – Caloplaca obamae – discovered and described by Kerry Knudsen (photo credit: UCR Herbarium/J.C. Lendemer)

Caloplaca obamae is a rare find. It is endemic to Santa Rosa Island, a member of the Channel Islands off the coast of Southern California near Santa Barbara. Cattle ranching and the introduction of elk and deer nearly removed it from existence. Now that cattle ranching has ceased and elk and deer are being removed, the lichen has a chance of survival.

Lichens are unique organisms. They are the result of a symbiosis between fungi and algae and/or cyanobatcteria. In this symbiosis, a mycobiont (the fungus) is essentially farming a photobiont (the algae/cyanobacteria) in order to feed off the sugars produced when the photobiont photosynthesizes. Photobionts in turn receive protection as well as water and other nutrients collected by the mycobiont.

There are at least 17,000 species of lichens known to science. They occur throughout the world in all manner of habitats from low to high elevation, and they adhere to virtually any stable surface including glass, plastic, and rubber. Lichens are ancient organisms, having existed for as long as 300 million years, with early lichens – or protolichens – dating back at least 400 million years. They are also very slow growing and can be incredibly long-lived.

Lichens are named after the fungal component, which can cause confusion since a particular species of fungus may form lichens with more than one species of algae or cyanobacteria. One way lichens are classified is according to their growth form, which is determined by their thallus – their non-reproductive, vegetative tissues. Three common thallus forms are fruticose (shrub-like), foliose (leaf-like), and crustose (crust-like).

While unassuming and benign in appearance, lichens have great ecological importance. They are involved in soil formation, the water cycle, and nitrogen fixation. They are homes to insects and microorganisms and are used as food by some animals and nesting materials by others. Some species of lichens are even consumed by humans. Lichens have also been used to develop medicines and dyes. Lichens are sensitive to air pollution, and are used to help determine the environmental health of urban areas. If your neighborhood has a healthy lichen population, chances are your air is pretty clean.

Santa Rosa Island - home to Caloplaca obamae (photo credit: wikimedia commons)

Santa Rosa Island – home to Caloplaca obamae (photo credit: wikimedia commons)

Caloplaca obamae is an orange, crustose lichen. It is terricolous, which means that it grows on soil. It is part of a community of soil dwelling lichens and bryophytes that form a biological soil crust on the Pleistocene soils of Santa Rosa Island. This sensitive community is easily disturbed by activities like grazing, which is why removing cattle, deer, and elk (all of which were introduced by humans to the island) is important for its survival.

Lichens are great, and they deserve much more attention than they get. A lichen named after President Obama is also pretty cool. However, as I researched this story the thing that impressed me the most was Kerry Knudsen himself. Knudsen is a retired construction worker with no academic degrees. He started studying lichens on his own after a medical condition forced him into early retirement. His initial interest grew into an obsession, and he is now among the few lichen experts in the world. He has added thousands of lichens to The Lichen Herbarium at UCR and has helped describe and name dozens of new species. He currently studies and collects lichens in California and the Czech Republic. You can read more about Knudsen in this 2004 article in the Los Angeles Times.

Selected Resources: