Chlorobionta
From Paleos
Contents |
Chlorobionta (Green Plants)
|
LIFE (=Archaea?) |--Eubacteria `--Eukarya |--+--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
Green Algae
The Green Algae - the Chlorophyta and Charophyta - include a number of mostly aquatic forms, including some unicellauar 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 higher plants in the kingdom Plantae. Molecular and cellular similarities between green algae, particularly the charophytes, and land plants include the following:
(1) Both the green algae and plants have chlorophyll b and beta-carotene
(2) Green algae and plants both have special intracellular membranes (the thylakoid membranes) which contain the chlorophyll stacked into grana.
(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.
Plants Conquer the Land
The Early Devonian Rhynie Chert Flora - from Life Before Man by Zdenek V. Spinar, illustrated by Zdenek BurianIf the great evolutionary radiation of metazoa (multicellular animals) in the earliest Cambrian oceans was the first great dramatic even of the Phanerozoic era (indeed ushering in the Phanerozoic), the conquest of land by multicellular plants was the next, and of equal importance. Indeed, without the plants no animals would ever have been able to survive on land.
But whereas the Cambrian explosion was very rapid, in the order of perhaps 3 to 5 million years for the origin of all major phyla (and many others now extinct), the colonization of the land by vegetation was a much slower and more protracted. The reason for this is not hard to understand. Cambrian animals were moving into a favorable new environment with no competitors. Plants had to brave desiccation, extremes of temperature, and harsh ultra-violet radiation.
Enigmatic traces are known from the early and middle Ordovician, These are fossils of spores, cuticles, and tubes and don't reveal much about the structures or nature of these plants. All we can say is that these plants were probably of a bryophyte grade of evolution - small, non-vascular, and lacking morphological differentiation into roots, stems, and leaves, like modern mosses and liverworts.
The first unambiguous record of land plants is from the Silurian period. They were mostly small, primitive forms, dependent on the proximity of water, and with the most rudimentary stem and leaf structure.
A common middle Silurian to early Devonian plant is Cooksonia, which had dichotomous branching and terminal sporangia (spore cases) at the tips of its green leafless stems. It is not known whether Cooksonia was a proper vascular (tracheid-bearing) plant. True vascular plants evolved and began to diversify during the Latest Silurian and Early Devonian.
The Devonian Period
Devonian plant root depthThe Devonian period. marked a major shift in plant evolution and terrestrial ecosystems. Early Devonian plants such as the rhyniophytes, zosterophyllophytes and lycophytes have features such as vascular tissue, stomata, a cuticle to protect against drying, rhizoids, and sporangia at the tips of short lateral branches instead of terminal as in Cooksonia. These forms were small, non-rooted or shallowly rooted, lacked woody tissue and hence were unable to grow beyond the height of small bushes. These plants reproduced by means of spores, which requires a moist habitat. They were therefore confined to moist, lowland habitats, thus having little effect on their physical environment
The first shrub and tree-like plants, such as Progymnosperms and lycopsids, had evolved by the middle Devonian. By the late Devonian the first real trees, such as Archaeopteris ("ancient fern" - not to be confused with Archaeopteryx, "ancient wing", the first bird!), had appeared. Trees have special vascular systems to allow for water circulation and nutrient flow against the pull of gravity. At the very end of the Devonian seed-bearing (gymnosperm) plants appeared for the first time, breaking free of the dependence on moisture that limits spore-bearing (pteridophyte) plants. Along with these developments came the development of advanced root systems and the production of soils, increased weathering, and huge ecological feedback.
The black & white figure shows the increasing terrestrial plant root depth penetration with time during the Devonian, leading to increasing soil depth and weathering. "Rhyniophytes" are a basal radiation of land plants such as Aglaophyton or Horneophyton. Trimerophytes include such plants as Psilophyton. Lycophytes arrived in the Middle Devoinian. They originally appeared as low-lying herbaceous forms, such as Asteroxylon or Drepanophycus. Tree-sized lycopods (e.g., Lepidosigillaria and Cyclostigma) appeared by the end of the Middle Devonian. Progymnosperms, such as Tetraxylopteris, arose in the Frasnian. By the Famennian, Archaeopteris forests are common. At the very end of the Devonian, Archaeopteris is found together with early gymnosperms, such as Elkinsia and Moresnetia, and zygopierid ferns such as Rhacophyton.
The Carboniferous Period
Despite the origin of the seed habit, the majority of Carboniferous plants reproduced by spores. The moist swampy environments of the time provided a nurturing environment. Lycophytes (scale trees and club mosses), which had evolved as small plants during the late Silurian? or early Devonian, and diversified greatly during the succeeding Devonian period, continued and thrived throughout the Carboniferous, but being dependent on water and moist conditions, most died out with the increasing aridity at the end of the Paleozoic, only a few small ones making it through. Calamites and ferns were other spore-bearing plants that appeared during the Devonian and flourished during the following Carboniferous period.
The Diversity of Plants
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 differnce 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.
Chlorobionta
Halosphaera viridisThe 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 mixotropic 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 Fm. 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.
Chlorophyta
UlvaWithin the Chlorobionta are two large clades making up the "green algae." The green algae, as currently conceived, have no formal taxonomic name. We will define the group as Quercus + Chlamydomonas. The corresponding stem clades are Chlorophyta (Chlamydomonas > Quercus) and Charophyta (Quercus > Chlamydomonas). "Chlorophyta" is also the old name for all green algae, so this is perhaps unnecessarily confusing. Tough luck. The ambiguity is now so embedded in the literature that there's nothing anyone can do about it.
The Chlorophyta have largely been delineated by molecular techniques, so it is a bit difficult to describe their characters. We know of two possible synapomorphies of the Chlorophyta. First, chlorophyte sexual forms bear paired apical flagellae usually separated by 180�, but sometimes at the same end. Second, they retain the nuclear envelope during mitosis. Indeed, chlorophytes seem to be distinguished by a variety of bizarre variations on the usually pedestrian theme of mitosis; however those variations are not entirely consistent within the group.
Like the land plant lineage, they tend to form large aggregates, with some tissue differentiation (primarily holdfasts and reproductive structures). They are very often found in terrestrial and fresh water environments, with a distinct preference for very cold environments, such as under snow cover, or even within Antarctic ice. Various species are important in forming symbiotic relationships with fungi, i.e., lichens. As with all green algae, chlorophytes tend to have a double cell wall -- an inner wall of cellulose and an outer gelatinous wall of protein, particularly pectin, known in higher plants as a marker for parenchyma. Starch stored in pyrenoids, located inside the chloroplasts.
Charophyta (= Streptophyta)
KlebsormidiumThe Charophyta are the other lineage of green algae, the group which includes the land plants. Karol et al. (2001). As mentioned above, our working definition is Quercus (oak) > Chlamydomonas. The Charophyta have recently been referred to as the Streptophyta, but the reasons given for this change in nomenclature are probably insufficient. Unfortunately, the name is also frequently, and wrongly, used in place of Charophycea or Charales to describe the stoneworts -- one of several distinct groups of charophytes.
The synapomorphies of the group are said to include the the dissolution of the nuclear membrane during mitosis and the presence of paired flagella (when flagella are present at all) directed perpendicularly to each other. In addition, the charophytes are strongly inclined toward growth as long filaments.
Embryophyta
LiverwortThe Embryophyta constitute the terrestrial or land plants, the first representatives of which appeared during the Silurian or possibly even the Middle or Late Ordovician period. The most primitive of these are nonvascular land plants, a group that classically includes liverworts (Hepatophyta / Hepaticopsida), hornworts (Anthocerotophyta / Antheroceratopsida) and mosses (Bryophyta). The majority of land plants however are included within the huge and diverse clade traditionally called Tracheophyta, or Vascular Pants, and which we will refer to as the Rhyniophyta.
We treat Embryophyta in a specialized sense, as Quercus + moss. This may be a mistake, as this definition probably excludes the liverworts (see image) and perhaps even the hornworts. Both of these groups have traditionally been thought of as embryophytes.
Embryophytes (including liverworts) have the following synapomorphies: 1) a life cycle with alternation of generations 2) apical cell growth (some kind of meristem-like growth organization), 3) cuticle (needed to control water loss on land), 4) antheridia (male gametophyte organs), and 5) archegonia (female gametophyte organs). The more derived embryophytes are vascular plants. Vascular plants have an elaborate system of conducting cells, consisting of xylem - in which water and minerals are transported) and phloem (in which carbohydrates are transported). This method of internal support enables them to stand and grow upright and pull up nutrients against the force of gravity. There are two developmental grades - those that reproduce by means of spores, and hence are dependent on water or extensive moisture (e.g. ferns), and those that reproduce by means of seeds (e.g. conifers and flowering plants). The most primitive forms reproduce by means of spores (haploid (1N) spores). They generally require a moist environment, because the flagellated sperm require water for fertilization.
The Embryophytes, then, are plants with an alternation of generations and some ability to live on land. The basal embryophytes were still not land plants, since they required, and still require, open water to propogate. As we define the Embryophyta, they split basally into mosses (Bryophyta) and land plants (Rhyniophyta). The Rhyniophytes two important groups: the Lycophytina (lycopods and the extinct zosterophylls) and the Euphyllophytina (ferns and seed plants). Embryophyta
Embryophyta
Bryophyta
Bryophyte Life Cycle If the mosses had not survived into the present, we would be forced to invent them as just the sort of intermediate we might expect between essentially aquatic algae and fully terrestrial plants. Mosses do have differentiated stems. Although these are generally only a few millimeters tall, they are still designed to provide mechanical support against gravity without help from water -- the first such structure in any kingdom. Bryophytes also have leaves. These are typically one cell thick and lack veins, although they may have a central thickening for support. Mosses also have rhizomes. These may have some function in extracting soil nutrients, although their primary function seems to be mechanical attachment to the substrate. Thus they are not true roots, but do approach that condition.
The bottom line is that, structurally, mosses really differ from rhyniophytes in only one aspect: mosses lack specialized vascular tissues. That alone is sufficient to explain the lack of big leaves, long stems, and true roots. This whole complex of characters is thus probably primitive. The other distinctive character of mosses is that the plant we normally observe is the haploid, gametophyte stage. But this character is shared with liverworts (basal embryophytes) and so is also probably plesiomorphic.
Curiously, in hornworts (also basal embryophytes) the sporophyte generation is dominant. In addition, it turns out that the leaves of moss probably evolved independently from the leaves of higher plants. So the relationships of the mosses and basal embryophytes are still uncertain. What really does seem to set mosses apart is their unique form of leaf. What really seems to unite mosses with higher plants is (a) the presence of stomata to control water loss and (b) meristem (apical growth) in the sporophyte generation. See, Friedman et al. (2004). Phylogenetically, we treat Bryophyta as Moss > Quercus.
Rhyniophtya
HorneophytonSee Rhyniophyta. That section covers the basal rhyniophytes, such as Horneophyton, which were the first real land plants. These probably evolved in the Ludlow and formed the stem group for all other land plants. Consequently, they are paraphyletic. Rather than abandoning this name and its rich history, we use it to mean all land plants. Our working phylogenetic definition is definition is Quercus > moss.
This group is characterized by the ability to reproduce without open water. Anatomically, in all rhyniophytes, the (diploid) sporophyte generation is dominant, and the sporophyte is branched. For this reason, the taxon is often referred to as the Polysporangiophytes. In addition, the archegonium develops inside the body of the plant, rather than being superficial as in mosses and most basal embryophytes. Kenrick & Crane (1997).
Horneophyton and a few other basal forms lack tracheids. That is, they are avascular plants. However, almost all other rhyniophytes have some development of specialized vascular tissues. The most basal tracheid type, present in most stem rhyniophytes, appears to be the S-type tracheid.
Lycophytina
The Lycophytina includes the lycopods, zosterophylls, and related forms, including (probably) a number of plants often treated as basal rhyniophytes, such as Baragwanathia. Kenrick & Crane (1997). Since they are a complex group and are treated extensively elsewhere, we will defer discussion to a revision of the existing materials. Euphyllophytina
The clade that unites oak trees and ferns is Euphyllophytina = Quercus + Equisetum. The two complementary stem clades are Moniliformopsida and Spermatophytata. Euphyllophytines are characterized (Kenrick & Crane, 1997) by monopodial or pseudomonopodial branching, helical arrangement of branches, small, pinnule-like vegetative branches, the branch apex is recurved or coiled, paired sporangia which split open along one side through a single slit, and radially-alligned xylem in the larger axes. Only early euphyllophytines have P-type tracheids. Kenrick & Crane identified this clade based entirely on morphological characters. However, Euphytophytina has also been recovered, with essentially the same structure, using ssu rDNA. Duff & Nickrent (1999).
Moniliformopses
Psilotum nudumThe Moniliformopses are the horsetails and ferns, including the Psilotidae (whisk ferns). They are closely related to the seed plants. Pryer et al. (2001). So, for example, they exhibit apical growth (meristem) in both sporophyte and gametophyte generations. They have well-developed roots megaphyllous leaves and the vascular system needed to make use of both. However, both may have been evolved independently of higher plants. Friedman et al. (2004). In addition, Moniliformopses lack a complete vascular cambium, and growth of xylem is restricted to lobes of the primary xylem strand.
Since this is a new clade -- discovered, for all practical purposes, by Preyer's group, we have little to say about Moniliformopses as a taxon, and defer discussion to a fuller consideration of its three component parts. The Psilotidae are the most basal, followed by the horsetails, then the remainder of the ferns.
We apply a crown group defiition to Moniliformopses: Equisetum + ferns.
Spermatophytata
PsilophytonThe clade that unites oaks and lycopsids is Euphyllophytina. The two complementary stem clades are Lycopsida and Spermatophytata = Quercus > Lepidodendron. A second way to look at Spermatophytata is as the stem group leading to angiosperms. It includes Trimerophyta and the progymnosperms, in fact everything up to and including the seed plants (Spermatopsida). However, we will only be concerned with the more basal forms for now. A third way of considering Spermatophytata is as the seed plants. However, this applies only to living forms. The basal Trimerophyta and their immediate descendants (assuming Trimerophyta is paraphyletic) lacked seeds, true leaves, or even, perhaps, roots. It is quite likely that virtually all the important land plant adaptations were independently developed in the moniliformopsid and spermatophytate lineages.
What seems to have set Spermatophytata apart quite early is not, in fact, the development of seeds, but the evolution of a full vascular cambium which permitted secondary growth. Early plants with apical growth were able to use that trait to grow taller and (a) get more sunlight (b) shade their competition and (c) have a better shot at spore dispersal. However, supporting a long stalk is much easier with a wider central column. Less derived groups either had no way to do this, or developed lateral lobes of the apical meristem. The latter worked, but required the tree to grow wide before it grew tall. The evolution of a complete vascular cambium permitted the tree to grow just wide enough to suit its height -- growing continuously wider as it grew tall.
The evolution of seeds follwed this innovation. Seeds are embryonic sporophytes, held in a sort of metabolic stasis and provided with enough food to get started once their growth has been re-stared by exposure to suitable growing conditions. Well adapted seeds combined sexual reproduction with spore-like wide dispersal and so made the alternation of generations obsolete. However, early seeds, which might lack these refinements, probably evolved on tall trees which gave any sort of propagule a head start in dispersal.
The Spermatophytata are the stem group for our next major division, the Spermatopsida.
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.
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.
