Embryophyta

From Palaeos

Chlorobionta
  |--Chlorophyta
  `--Charophyta
       `--Embryophyta
            |--Marchantiophyta
            |--Bryophyta
            |--Anthocerotophyta
            `--Tracheophyta
                 |--Lycopodiopsida
                 `--o--o--Equisetopsida
                    |  `--Pteridopsida
                    `--Spermatophytata

Plants Conquer the Land

If the great evolutionary radiation of metazoa (multicellular animals) in the earliest Cambrian oceans was the first great dramatic event 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 hepatophyte 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

The 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 zygopterid 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 (part 2)

Embryophyta

The 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 / Antheroceropsida) and mosses (Bryophyta). The majority of land plants however are included within the huge and diverse clade traditionally called Tracheophyta, or vascular plants. [NB. Be prepared for a rather confusing array of multiple names for taxa in higher plant taxonomy. Different authors have often treated taxa as being at different ranks, and the Botanical Code has standardised suffixes for different ranks. Also, many clades are referred to both by an official typified name and a traditional non-typified name. So flowering plants, depending on whom you ask, may be known as Anthophyta, Magnoliophyta, Angiospermae or Magnoliopsida - CKT061105]

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 propagate. As we define the Embryophyta, they split basally into mosses (Bryophyta) and land plants (Rhyniophyta). The rhyniophytes contain two important groups: the Lycophytina (lycopods and the extinct zosterophylls) and the Euphyllophytina (ferns and seed plants).

Bryophyta

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.

Rhyniophyta

See 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.

Spermatophytata

Spermatophytata is the total group that contains the seed plants (Quercus > Equisetum + Lycopodium). Apart from the seed plants (Spermatopsida), it includes Trimerophyta and the progymnosperms. We will only be concerned with these more basal forms for now. Another 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 followed 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.

Phylogeny

References

Friedman, W. E., R. C. Moore & M. D. Puruggnan. 2004. The evolution of plant development. American Journal of Botany 91: 1726–1741.

Kenrick, P., & P. R. Crane. 1997. The Origin and Early Diversification of Land Plants: A cladistic study. Smithsonian Inst. Press, 441 pp.

Credits

Evolutionary history MAK?; Embryophyta MAK02?, revised ATW041215; Bryophyta ATW041215; Rhyniophyta ATW041229; Spermatophytata ATW041216.