o Archaeplastida |--Rhodophyta `--o CHLOROBIONTA |==Micromonadophyceae |--Ulvophyta `--o Streptobionta |--Chlorokybales `--+--Klebsormidiales `--+--Zygnematales `--+--Charales [Charophyceae] `--+--Chaetosphaeridiaceae |--Coleochaetaceae `--Embryophyta
As defined here, the Chlorobionta are the green plants, including both green algae - a paraphyletic assemblage of unicellular and multicellular forms - and the Embryophyta or land plants, which could be considered the evolutionary culmination of the former. The Chlorobionta therefore are among the most successful and diverse of the Eukarya. Altermative names are Viridiplantae and Chloroplastida.
"The Wearing of the Green"
Beginning in the Archean era, Cyanobacteria evolved photosynthesis, which enabled them to use sunlight to draw carbon dioxide from the atmosphere and convert it to oxygen, water and glucose (a simple carbohydrate). These could be considered the first simple "plants" Plants therefore might be seen as any organism that is able to use sunlight, carbon dioxide, and water, to manufacture its own food, that is, as a special class of autotroph. However, that's far too broad. It would include all kinds of things like diatoms, chromists, and photosynthetic bacteria which have nothing to do with plants in a phylogenetic sense. They are, to be sure, all within the subject matter of a General Botany class. All of these groups share some essential biochemistry. However, what they don't share is a common ancestor to the exclusion of all other organisms. This similarity arises from (a) convergent evolution and (b) the exchange of plastids.
The description above also fails because it is only partially correct, even as a general description. Plants not only breathe out (respire) oxygen, but parts of their tissues also respire carbon dioxide, just as animals (heterotrophs) do. These processes provide the plant with energy for growing and maintaining its life support systems, and go on at all times. During the sunlit day, more carbon dioxide is consumed than is released in respiration, but at night photosynthesis ceases and the plant respires only carbon dioxide, returning a portion of its carbon to the atmosphere.
One better approach to defining "plants" is the "Chlorobionta" hypothesis, as used on the Tree of Life site:
- There are two major lineages of green plants. One consists of most of what have been classically considered "green algae" -- mostly microscopic freshwater forms and large seaweeds. The other lineage contains several groups of "green algae" that are more closely related to land plants. Because these two lineages are monophyletic, they have been placed in a single monophyletic group called green plants, or, in technical parlance, the subkingdom Chlorobionta ...
This suffers only from being vague. Is there anything else in the box besides green algae and land plants? The ToL authors don't suggest any other content. Alternatively, this might be an attempt to suggest a crown group: "the last common ancestor of Chlorophyceae and evergreens and all of its descendants," or something like that. That sounds like a workable definition, but that can't be right, since they include the prasinophytes among the Plants. Some, but not all, prasinophytes would be excluded from the plants by a crown group definition. We think what the ToL authors actually had in mind is an even better choice: the stem group "green algae > red algae". This includes all of the prasinophytes, all other green algae and all plants, as those terms are normally used, but not much of anything else.
Why do we care about definitions? The price of admission to doing good science is taking an explicit position, so that others can prove you wrong. A vague definition, such as ToL's original formulation, is not good science. Unless we know precisely what they mean by "plant" we can't really make testable statements about what are or are not plants, nor about what characteristics plants have or do not have, nor about whence they might have derived their characteristics. Without really crisp definitions, these issues quickly get bogged down in semantics and arm-waving. Arguably that is exactly what happened to the whole business of taxonomy for the better part of a century.
Of course, definitions can never be "wrong," in a logical sense. However, they can be useless, if they fail to draw lines within our area of interest. A vague definition is always useless because it draws no line at all. Phylogenetic definitions have revived the whole business of evolutionary systematics because they are quite precise and refer to historical events (e.g., the evolution of red and green algae from a common ancestor), rather than to some man-made list of (sometimes fuzzy) characteristics. However, this precision also comes at a price. A phylogenetic definition is built around a phylogenetic hypothesis. Unlike a definition, a hypothesis can be wrong. If so, any definition based on that hypothesis usually must be abandoned, and a lot of good work may go down the tubes.
Suppose for example, that we interested in the evolution of birds. Our hypothesis is that birds are the sister group of dinosaurs, and that some "dinobird" was their last common ancestor. We thus define birds as Struthio (ostrich) > Struthiomimus (a theropod dinosaur which looked like an ostrich) and dinosaurs as Struthiomimus > Struthio. Sadly, after years of frustrating labor sorting out the characteristics of the supposed dinobird ancestor, we realize that birds are dinosaurs. Oops. Our definition of "bird" turns out to include embarrassingly unbirdlike things like therizinosaurs, while our definition of "dinosaur" includes only tyrannosaurids and ornithomimosaurs. How to explain this little faux pas to those notoriously humorless folk whose grants supported our research the last three years? Again, that is simply the price of doing good science.Gelidium coulteri, a randomly chosen species of a well-known and very successful genus of red algae. On the green plant side, let's use an angiosperm, a highly derived group, and Quercus albus, because (as any citizen of the state of Connecticut will know) it symbolizes the willingness to take risks to vindicate historical truth. Based on our phylogenetic hypothesis, our working definitions are Chlorobionta (plants) = Q. alba > G. coulteri, and Rhodophyta (red algae) = G. coulteri > Q. alba.
Was that so hard? Of course not. But then, unlike ToL, we are not subject to the temptations to waffle which come with peer review and the caprice of granting agencies. Lest we be misunderstood, we support both peer review and post hoc review by grantors as excellent things for science; but they are not unmixed blessings. The inducements to please everyone may become irresistable. Now, unlike ToL, the purpose of Palaeos is only to amuse those who write it. However, if we can, occasionally, counterweight the temptation for others to hide behind intentionally vague and inconsistent pronouncements made in the service of their own comfort, perhaps it may serve another purpose as well.
ATW041201. Text public domain. No rights reserved.
The Green Algae - the Chlorophyta and Charophyta - include a number of mostly aquatic forms, including some unicellular and primitive colonial forms, and other multi-cellular types that however lack a true root system. They are very closely related to (and probably the ancestors of) the land plants in the Embryophyta. Molecular and cellular similarities between green algae, particularly the charophytes, and land plants include the following:
(3) Charophytes have a cellulose content of 20 to 25% of the cell wall, a composition similar to that of plants.
(4) Cell division in green algae is very similar to that of land plants. Both use microtubules to bring vesicles containing new material in to form the cell plate which will divide the cell into two.
(5) Nuclear genes and RNA are similar between charophytes and plants.
The Diversity of Plants (part 1)
We will cover the higher taxa of lower plants in two blocks: Chlorobionta and Embryophyta. The prasinophytes (basalmost chlorobionts), chlorophytes and charophytes are essentially algae, which normally impinge on our consciousness just long enough to apply a little wasabi and shoyu. Arigato, and next I'll have ni unagi, kudasai. Don't try that with an embryophyte. There's a difference between sushi and soba. Embryophytes are mostly land plants, and it was the ability of plants to live on land that allowed all the other branches of life to live on land as well. In fact, only the plants can really be said to have adapted to land. With few exceptions, the rest of life simply adapted to plants.
The general characteristics of the green plants are touched on above. The purpose of this section is to introduce the prasinophytes. These are a paraphyletic group of green algae which radiate from the base of the Chlorobionta. Most are photosynthetic flagellates. In addition, the prasinophytes are the only mixotrophic plants, i.e., they obtain food both by photosynthesis and phagotrophy. This is, presumably, how they obtained chloroplasts in the first place.
The phycomate prasinophytes (those with large, thick-walled floating stages, or "phycomata") have received special attention because of their extremely long fossil record. Phycomata are known as acritarchs well into Proterozoic time. One genus (Tasmanites) dates back to 600 Mya. Javaux et al. (2004) have turned up an entire menagerie of forms from the Mesoproterozoic, and even beyond (at least 1500 Mya), which are almost certainly eukaryotic and could well be prasinophytes, or somewhat stemward of the plants. They cannot be too distantly related, as the presence of thick organic walls, with extreme resistance to degradation, seems to be a trait of the plant-chromist lineage. One of these in particular, Leiosphaeridia crassa, from the c. 1460 Mya Roper Formation of northern Australia, is being investigated as a possible green alga. Interestingly, in Recent or merely Paleozoic forms, these relatively large, thick-walled morphs are associated with moderately anoxic conditions and nutrient exhaustion during algal blooms.
Opening paragraph MAK061014; introduction ATW041201 Palaeos com; diversity ATW041212.