Embryophyta
From Paleos
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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.
