What is a plant?
Is this a plant? This moss looks very strange. That’s not what I think plants look like.
What is a plant? We all have an intuitive sense of what plants are, but there are all kinds of strange things living on this earth that are hard to classify. A tree is a plant. A carrot is a plant. A flower is a plant. But what about algae? What even is algae? Is seaweed a plant? Is moss a plant? Mushrooms? Lichen? Is a sea sponge a plant? Living things take many forms, and it can be difficult to tell what else they are related to. I wanted to explore the form, function, and evolution of plants—even though I study plants, there is a lot of diversity out there that I do not understand.
The hallmarks of plants are that they are green and photosynthesize. They have chlorophyll pigments that allow them to use energy from the sun to transform CO2 into sugars. Lesser known features that all green plants share are cell walls made of cellulose and double membranes around the chloroplast (site of photosynthesis). There are many traits that they don’t all share: for example, not all plants have flowers, and not all plants even have leaves or roots. We can take a quick trip backwards through the tree of life to see where things fall. Anything that flowers is a definitely a plant (lilies, asters, grass, deciduous trees, fruits, vegetables; Angiosperms). Anything green with cones is a plant (conifer trees, cycads; Gymnosperms). Ferns are plants (Polypodiaphyta, seedless vascular plants) and some other small, unique-looking species also fall into this category, such as clubmoss and horsetails. Moss is a plant (along with hornworts and liverworts; Bryophyta, seedless vascular plants).
From the Kew Royal Botanic report on the State of the World’s Plants. There is a nice interactive version here: https://stateoftheworldsplants.org/2017/describing-the-worlds-plants.html Willis, K.J. (ed.) 2017. State of the World’s Plants 2017. Report. Royal Botanic Gardens, Kew.
We know most of these groups from everyday life, and these are the major categories known as “land plants” on the tree of life. Once we leave land, we get to algae, and things get more complicated. Green algae has some pretty big differences from the land plants we know and love, but it’s the ancestor of land plants, and considered a “green plant.” Algae is generally aquatic. Green algae often live in shallow water in both marine and fresh water, and can even live on land. Algae has chlorophyll but not vascular support tissues, which is why they flop around and grow close to the surface on land. Moss doesn’t have vascular tissue either, which is also why moss never gets very tall. Hornworts, cousins of moss, basically look like land algae. These are all of the categories that compose “green plants,” the group that most closely maps onto our everyday idea of what plants are.
This is a hornwort (Megaceros flagellaris). You can see how much it looks like some kinds of seaweed. Photo by Scott Zona, CC license
With Red alage, we are still in algae world, but out of the traditional realm of plants. Red algae evolved before green algae and uses different photosynthetic pigments (red phycobilins), and it is generally found in marine environments. You might know red algae from foods such as nori, laver, or agar. Differences between red and green algae include different pigments, different starch storage compounds and cellular structures. There are significant differences between red and green algae, but they are in the same evolutionary lineage. You could consider red algae a “plant” in a broad sense. It’s on the cusp.
We are not done with algae. Before we go any further, let’s go back to the story of how plants got the amazing power to be self-sustaining via photosynthesis. One-celled bacteria, called cyanobacteria, aka blue-green algae, are the original photosynthesizers. Other one-celled organisms with nuclei (eukaryotes) were swimming around and could not make their own food but instead had to eat other organisms for energy. The theory is that one of these larger, hungry cells engulfed a photosynthetic cyanobacterium, but the bacterium wasn’t digested. The larger cell harnessed the powers of photosynthesis from the cyanobacterium inside it. Eventually the cyanobacterium and the host cell and became inseparable, and the first algae was the result. This process is known as “endosymbiosis.” Organisms that developed photosynthesis in this way are “primary” endosymbionts. This group includes red and green algae.
The story doesn’t end there. Just as red and green algae developed the ability to photosynthesize by incorporating a photosynthetic bacterium, other lineages developed the ability to photosynthesize by incorporating a photosynthetic red or green algae into their cells. That gave rise to many unrelated organisms that all have the ability to photosynthesize or have some of the photosynthetic machinery, but are significantly different from plants. These species are known as “secondary endosymbionts.” Examples are kelp, a kind of brown algae; most phytoplankton, composed of diatoms and dinoflagellates; and the green pond organism euglena. These are all known generically as algae. You can see that the term “algae” is not very helpful in understanding the relationships between organisms. Instead it is a catch-all term for organisms that photosynthesize (or used to) and live in water, and it includes a lot of things that are not on the plant branch of the tree of life.
This is the Eukaryote tree of life. It shows all life that we know of excluding bacteria and archaea. Land plants are in the Archaeplastida branch of this tree (in green here), along with red and green algae. Other kinds of algae are found throughout the other branches of life; within Cryptista, Haptista, Discoba, and all three of the 'Sar' groups. Image courtesy of Alastair Simpson, 2019.
I meant to send you off today with answers, but I’m afraid I may have sent you away with more questions. Algae are deeply strange, and there is no simple way to explain or identify their place on the tree of life. Our intuition about the natural world is shaped by the three kingdoms from the game “20 questions:” animal, vegetable, and mineral. This was the original framework for the taxonomy of living things proposed by Carolus Linnaeus, and it does not include tiny half-plant, half-swimming cell organisms. The three kingdoms way of thinking is ingrained in our everyday understanding of the categories of life. Our knowledge of evolutionary relationships has exploded in the last 50 years, but it will take a lot longer for that knowledge to penetrate our intuitive categories.
Let’s see about some of these mystery categories:
Algae: Some algae are plants. I’m not totally sure about this but my sense is that most algae that are not plants are microscopic. Most macroscopic algae is a plant, but there are many exceptions.
Seaweed: Most seaweed is a plant. Seaweed is macroscopic algae, which is mostly plants. There are also exceptions.
Mushrooms: Not a plant. It’s fungus. Fungus is more closely related to humans than plants and needs to “eat,” get nutrition from external sources.
Sea sponge: Not a plant. In fact it’s animal. Sea sponges are the original animals. They don’t do much so they seem more like plants than animals in their life styles. However, they are filter feeders, and depend eating other organisms for nutrition.
Lichen: Not a plant. It’s a composite organism composed of a fungus and green algae or cyanbacteria living together. The parts you see is mostly fungus, but the greenish cast comes from a photosynthesizer. They are part plant.
Harmful algal blooms: Not plants. Caused by cyanobacteria (blue-green algae) or dinoflagellates (non-plant algae).
What else are you curious about? Send me your “is this a plant?” questions!
