Chlorobionta

From Paleos

Chlorobionta (Green Plants)

  |--+--Rhodophyta
  |  `--CHLOROBIONTA
  |     |--Chlorophyta
  |     `--Charophyta 
  |        |--(various green algae)
  |        `--Embryophyta
  |           |--Bryophyta 
  |           `--Rhyniophyta
  |              |--Lycophytina
  |              `--Euphyllophytina
  |                 |--Moniliformopses 
  |                 `--Spermatophytata 
  |                    |--trimerophytes
  |                    `--Spermatopsida
  `--+--Fungi
     `--Metazoa
        |--Deuterostomata
        `--Protostomata

  Lists
  Glossary
  Taxa
  References
  "The Wearing of the Green"
  Evolution of Land Plants
  Green Algae
  Plants Conquer the Land
  The Devonian Period
  The Carboniferous Period
  The Diversity of Plants
  Chlorobionta (Prasinophyta)
  Chlorophyta
  Charophyta
  Embryophyta
  Bryophyta
  Rhyniophyta 
  Lycophytina  
  Euphyllophytina 
  Moniliformopses  
  Spermatophytata  
  Links

Lists

A. Glossary of terms and abbreviations. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

B. Taxon Index: alphabetical list of taxa. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

C. References: literature citations by author. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

"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 Gelidium coulteri 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 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 Quercus alba 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.

For that reason, we should be careful, as well as explicit, in framing the definition and articulating the underlying hypothesis. Here, the hypothesis is that red algae, in a colloquial sense, are closely related to green plants, in an equally colloquial sense. This then allows us to define both rigorously in terms of that relationship. Strictly speaking, we should do so in terms of particular anchor taxa, just in case either group turns out to be polyphyletic (which is possible). By all means, then, let's do so. On the red algae side, we'll pick 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.

Evolution of Paleozoic Land Plants

(See article for details)

The Devonian Period

(See article for details)

The Diversity of Plants

(See article for details)

Embryophyta

(See article for details)

Links

Introduction to the Plantae - The green kingdom

Integrative Biology 181/181L - Paleobotany

Land Plants On-line - covers recent plants only, links to images etc

International Plant Taphonomy Meeting - The purpose of the International Plant Taphonomy Meetings is to stimulate scientific research and to promote contacts among scientists engaged in the study of plant taphonomy including living and fossil plants of all geological periods.

Botany

Web Sites by Subject

Excellent annotated list of links to Botany and related subjects - note, some of these links are no longer current.

A BASIC BIOLOGICAL CLASSIFICATION OF PLANT-LIKE ORGANISMS

A History of Palaeozoic Forests - Hans Kerp - very informative - originally published in German.  Deals with forests of the Devonian, Carboniferous, and Permian periods.

Hans' Paleobotany Pages - info on the earliest land plants and on the lycopod Lepidodendron

Carboniferous Forests Ralph E. Taggart - good non-technical intro, covers main groups of Carboniferous plants, also brief mention of insects, amphibians, and reptiles'

The Biota of Early Terrestrial Ecosystems: The Rhynie Chert - includes useful information on Early Devonian plants from this location

The First Land Plants - The Conquest of the Land - gives a good introduction to basic concepts regarding the transition of plants from water to land

Orto Botanico - somewhat technical but not too difficult coverage of plants and paleobotany. Includes glossary.

Integrative Biology 181/181L - Paleobotany - at UC Berkeley - includes material on Paleozoic plants.  A bit technical but if you stick at it you will learn a lot.