What is a plant?
Everybody knows what plants are. They’re green organisms that set down roots, spread out leaves and turn sunlight into sugar. So a simple statement like “plants are the living organisms of the Earth that photosynthesize” ought to do for a definition. That would probably suit ecologists, for it simply and clearly defines a distinct class of organisms according to their role in the ecosystem. But if you have already glanced uneasily at the length of this post, you know that it’s not going to be that simple.
Everyone will agree that the trees, shrubs, and herbs that populate our gardens and forests are plants, but the inclusion of other plant-like organisms is debatable. What about algae, for example, or fungi and other plant-like organisms that don’t photosynthesize? All have been called plants at one time or another. The word “plant” is a common term, and there are no rules for what it may include, and so it has been used in a variety of ways. Nevertheless, many of my colleagues argue that the term should be used in a fairly restrictive sense, almost as if it were a formal taxonomic category, e.g. as in the “Plant Kingdom.” Ecologists with some savvy of the complexities of biological classification might simply describe the first link of the global food chain as consisting of “photosynthetic organisms” and thereby avoid taxonomic confusion.
Modern biological classifications are based on relationship rather than ecological profession. A Smith is always a Smith, even if a member of that family decides to grow cabbages rather than shoe horses. Likewise, some plant families contain rogue members who have abandoned solar sugar-making for other life styles. My goal here is to clarify the modern definition of “plant,” but along the way have some fun looking at various plant pretenders and why they have been excluded from the Plant Kingdom.
Indian pipes and snow plants, for example, live like fungi, extracting energy from organic matter, yet are closely related to blueberries and Rhododendrons (family Ericaceae). Fungi have been excluded from the Plant Kingdom because genetically they are more closely related to animals. Snow plants and Indian pipes, however, have flowers similar to those of the blueberry and DNA sequencing confirms that they belong in the same family. They are still plants. Many plants in a variety of families have similarly found it more lucrative to siphon off sugar made by other organisms than to make their own. Wikipedia lists 57 genera of plants, including both eudicots and monocots, that obtain organic energy from underground fungi. They are particularly common among orchids. Others are parasitic directly on the stems or roots of other plants. Dodder is a parasitic vine in the morning glory family (Convolvulaceae), and the kiss-mandating mistletoe is a hemiparasite as it does some photosynthesis. The largest flower in the world, Rafflesia, is also a parasite. All of these are plants because they are related to “normal” plants.
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| The snow plant, Sarcodes sanguinea, growing in the Sierra Nevada, California. This is a non-photosynthetic member of the Blueberry Family, Ericaceae. |
Is a sea slug a plant?
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| A photosynthetic sea slug, Elysia chlorotica. Photo by Skip Pierce, University of South Florida. |
OK, so parasitic plants are a minor deviation that we can deal with, but there are more serious photosynthetic criminals afoot. There are organisms that have no business being photosynthetic at all: sea slugs for example. Sea slugs are mollusks - relatives of snails, clams, and squids. They are clearly animals. They move around and eat stuff. They have nerves, muscles, eyes, etc. Nothing could be more unrelated to plants than a sea slug. Yet there are certain species of sea slugs that are “photosynthetic.” The most extensively studied are in the genus Elysia. Their tissues are filled with chloroplasts that make sugar and release oxygen. The chloroplasts can sustain the animals for prolonged periods of time when there is nothing to eat. Doesn’t that make sea slugs plants?
If you’ve read about such creatures before, you’re thinking to yourself “but they cheat! They don’t have chloroplasts of their own, they steal them from the algae they eat.” And you’d be correct. Sea slugs are among a surprisingly long list of “kleptoplastic” organisms, a term coined by biologist Kerry Clark1. These animals eat algae but retain their functioning chloroplasts (kleptoplasts) within certain of their cells, and even have evolved a flat, leaf-like shape.2 The chloroplasts eventually wear out and are replaced through ingestion.
Other animals, such as nudibranchs, giant clams (Tridacna), corals, a sea anemone (Aiptasia) and even a spotted salamander, Ambystoma maculatum - the only known photosynthetic vertebrate3, maintain whole algal cells (called zooxanthellae), within their tissues, and are able to share their photosynthetic product. That is a slightly different sort of symbiosis, but the net result is similar. In all these animals, the relationship is temporary. The young animals are not born photosynthetic, and must they ingest algae in their food to obtain their first chloroplasts. Zooxanthellae may persist indefinitely as self-reproducing populations within their hosts, but can also be expelled. Photosynthetic animals are photosynthetic in the same sense that a greenhouse is photosynthetic. Photosynthesis is being done by what’s inside, not the structure that houses them.
In “true plants,” chloroplasts reproduce under the control of the host cells and are passed on from generation to generation as the cells divide. Chloroplasts have been integral parts of plant cells for hundreds of millions of years, and could not live on their own. Non-photosynthetic plants still have ‘plasts’ (technically plastids), but they have been modified for other purposes, such as storing pigment (chromoplasts) or storing starch (amyloplasts).
Plants therefore cannot be defined simply by the presence of photosynthesis. Animals are not plants, even if they are photosynthetic, and non-photosynthetic organisms are plants if they are related to plants. What then are the true plants? Certainly the mosses, ferns, gymnosperms and flowering plants are, but who else?
What about algae? They have permanent, reproducing chloroplasts that are passed on from generation to generation. Are they plants? They certainly have been considered part of the Plant Kingdom in the past, but modern studies of evolutionary relationship, involving structure, chemistry, and DNA sequencing, have revealed a very complicated picture, with some groups of algae closely related to land plants and others not related at all. Keep in mind that by current taxonomic standards, a recognized group of organisms must be related by common ancestry.
The term “algae” is generally applied to photosynthetic members of the broad grab-bag of organisms referred to as protists. Protists are generally simple and generally aquatic, and do not protect or nourish their embryos. Some organisms that fit that description have been revealed to be kleptoplastic. There are a number of predatory, animal-like protists (“protozoans”), including some foraminifera, dinoflagellates, radiolarians, and ciliates, who incorporate small algae or chloroplasts into their cells, just like sea slugs. They generally have non-photosynthetic relatives, and so can be excluded on the same grounds as sea slugs.
Now for a more troublesome case: certain species of the unicellular protist genus Euglena have permanent, reproducing chloroplasts within them, and so might be considered plants. Some related euglenoids, however, are non-photosynthetic foragers, consuming food like miniature animals. Whether these organisms are plants or animals has puzzled biologists for centuries. Both zoologists and botanists have classified them within their respective kingdoms. Are the photosynthetic euglenoids kleptoplastic predators or plants? It turns out that their ancestors acquired chloroplasts the same way that sea slugs did, but the fact that their chloroplasts are permanent and passed on from one generation to the next makes these organisms decidedly more plant-like. Their kleptoplastic origin from unrelated predatory cells, however, banishes them from the Kingdom.
What about other algae? Brown algae, like the giant kelps that form “forests” off the coast of California are surely plants, right? Sorry, no. They are the greatest plant pretenders of all. Brown algae, and their ubiquitous cousins the diatoms, have fully integrated, reproducing, heritable chloroplasts, like the photosynthetic euglenoids, but these too were stolen, probably from red algae, by their early ancestors. Dinoflagellates, the group that includes the organisms responsible for “red tide,” are even more complex, having multiple kleptoplastic events in their history. The ancestors of all these kinds of algae were predators that obtained kleptoplasts, and again not related to the trees and grasses of the terrestrial environment. Modern classifications are based on the relationships of the host cells, not on the chloroplasts they contain, so all algae that originated through kleptoplasty are excluded from the plant kingdom.
So what’s left? We must see where chloroplasts came from in the first place to finalize the roster of true plants.
The first chloroplasts
Endosymbiosis is the creation of a close, mutually beneficial relationship between two previously independent organisms, with one residing inside the other. Delving deep into evolutionary history, we find that the first chloroplasts originated from whole photosynthetic bacterial cells related to modern Cyanobacteria. This is referred to as primary endosymbiosis, to distinguish it from kleptoplastic secondary endosymbiosis. Mitochondria, incidentally, also resulted from a primary endosymbiosis between aerobic bacteria and ancient eukaryotic cells.
The ancient photosynthetic cyanobacteria that became the first chloroplasts left many free-living relatives that are still abundant today. Their common ancestor invented oxygen-releasing photosynthesis over 3 billion years ago, and the chloroplasts in all photosynthetic organisms today are descended from them. Cyanobacteria (previously known as “blue-green algae”) were once classified in the plant kingdom, along with all groups of algae, fungi, and well just about anything that clearly wasn’t an animal. But since they are bacteria with simple prokaryotic cell structure, they are even further from being related to plants than sea slugs, euglenoids or brown algae. (Prokaryotes have simple cell structure without the nuclei or other complex internal organelles found in the eukaryotic cells of plants, animals, fungi and protists) Bacteria and eukaryotes are classified in different domains (bigger than kingdoms). Once again, organisms are classified according to their main cell structure, not what might be in them or how they make a living.
So, who is left (we are almost there!)? Evidence suggests that red algae, green algae, and terrestrial plants form a natural group descended from the first plant-like cells with captured cyanobacteria. Is this our Plant Kingdom? Together, they are officially named Archaeplastida ("ancient plastids"), without committing to a particular rank. Logically and according to the rules, this could be considered the Plant Kingdom. Or we could restrict membership to just part of that large clade: the green algae and land plants, together known as the Viridiplantae ("green plants"). Most practicing biologists today, however, prefer to include only the complex, primarily terrestrial plants, the Embryophyta ("embryo plants"), in the formal Plant Kingdom, and that’s where it sits at the present. The alternate suggestions are quite valid, however. Even with classifications based on true relationship, names and ranks are arbitrarily assigned and there are different ways that can be done. I will most likely devote a future post two to the ins and outs of formal classification.
We could turn this completely around, however. The chloroplasts of all modern plants, algae, and photosynthetic animals are direct descendants of cyanobacteria. No eukaryotic organism evolved photosynthesis on its own, not even those officially designated as plants. An oak tree is therefore no more legitimately photosynthetic than a sea slug! One could argue then that cyanobacteria and their descendant chloroplasts are the only true plants, the cells that contain them are just packaging.
Well, one could argue it, but in modern classification, the packaging takes precedence. Eukaryotic organisms are classified according to the structure and nuclear DNA of their cells, not according to the chloroplasts they contain. Though their descendants are found in every photosynthetic eukaryotic organism, the cyanobacteria cannot be classified with any of them.
1. Clark, K. B., K. R. Jensen, and H. M. Strits (1990). "Survey of functional kleptoplasty among West Atlantic Ascoglossa (=Sacoglossa) (Mollusca: Opistobranchia).". The Veliger 33: 339–345.
2. http://www.seaslugforum.net/showall/elyschlo (see this site for more pictures and discussion of photosynthetic sea slugs)
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