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Bryophyta |
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Bryophyta - 1 |
Bryophyta - 1
The sporophyte generation of mosses is completely dependent on the gametophyte and never makes independent contact with the substrate. Accordingly, we treat it as an anatomical extension of the gametophyte. A generalized bryophyte sporophyte can be considered as having three parts: a "foot" which connects the embryo to the gametophyte to extract nutrients, a seta which lifts the mature sporophyte clear of the gametophyte body, and a terminal sporangium capsule. Shaw & Renzaglia (2004). In addition, we might add the protective structures formed by the gametophyte.
The figure shows roughly how the structure works in development. (1)
Initially, the sporophyte is completely surrounded by the epigonium, a protective coat supplied by the gametophyte. As the sporophyte grows, it elongates and grows wider, eventually fracturing the calyptra. The last remnants form a loose, pointed apical dunce cap, the calyptra, and sometimes a ring around the base, the vaginula. At the same time (approximately -- the timing is quite variable) three other things are going on. (2) the inner sporophyte capsule (a/k/a urn or sporangium) is developing and hardening. (3) the sporophyte is levitating itself clear of the mother-ship on a long extension tube, the seta. (4) Inside the sporophyte, cells on the surface of a central columella (think of it as a little internal shoot) are undergoing meiotic divisions to create haploid spores. The small spaces where this is going on are called loculi. Loculus may be translated to mean, oddly enough, "small space." Perhaps fortunately, people tend to avoid Latin these days and refer to it as a locule.
Later, the calyptra dries up completely and falls off, and the capsule is exposed, with the end sealed by a small operculum. Eventually, this, too, is lost. Many derived mosses have yet another control on spore dispersal: a peristome with peristome teeth arranged around an annulus. The teeth are quite diagnostic of the species and are much sought after by species-level taxonomists for this reason. The teeth are so constructed that they bend outward as they dry out, causing the opening to widen and release the spores. Note that that this entire system is designed to promote spore dispersal in relatively sustained dry weather and moderate winds. That seems an odd sort of requirement for a plant that requires flowing water for sexual reproduction.
Bear in mind that that this describes sporophyte development in an imaginary "typical" moss. As we keep saying, there is a great deal of variation. Sphagnum, for example, has no seta. In Sphagnum and other basal mosses, the sporophyte is raised by the pseudopodium, a structure produced by the parent gametophyte.
These last three paragraphs will get you by the usual, garden-variety botany mid-term. Because we are kindly and avuncular, but mostly because you must be quite desperate if you are trying to prepare for an examination using this site, we will also add that most teachers of botany have their own, favorite moss -- the one they learned bryology on. It might be wise to bone up on the idiosyncrasies of that plant as well. And don't forget the mandatory life cycle diagram. Best of luck ...
Here we have deliberately picked a mixed bag of Latin endings: Sphagnum, "Andreaeids," Polytrichales, and Bryopsida. In a cladistic scheme, it makes no sense to agonize over the rank of the groups. Instead we've simply used a variety of rank suffixes to convey other information -- or possibly just to be obnoxious. In any case, Sphagnum is a single genus, so there's no sense in dressing it up as anything else. The Andreaeids are a paraphyletic collection of (probably) three small genera, two of which may have a good deal in common. We've added Takakia because it has no obvious place anywhere else. Polytrichales is a moderately diverse bunch of "typical" mosses -- probably monophyletic. Hyvönen et al. (2003). Bryopsida is an extremely speciose, but fairly uniform, set of plants -- a large and distinct monophyletic assemblage. Consequently, we've given it the highest "rank" consistent with common usage.
Sphagnum is different from all other mosses in many ways. There has never been much doubt that it is the sister group of all other mosses. Sphagnum is the moss which forms peat. There are 250-400 living species (Shaw et al., 2003) which collectively have the distinction of covering
2-3% of the entire land surface of the Earth (an oft-repeated statistic -- but how was this computed?). The plants create an environment which is antiseptic and acidic, thus the plants (and anything in them) are extremely slow to decompose.
Many of Sphagnum's unique features may be plesiomorphic "primitive") traits. The fossil record of mosses is so poor that we can't tell if these are unique specializations or traits left over from the distant common ancestor of all mosses. For example, the phyllids contain a very high proportion of hydroids mixed in with the photosynthetic cells -- to the point that the plants often look grey-green rather than the bright green of other common mosses. The spore expulsion (dehiscence) is explosive. These characters, and Sphagnum's acid, antiseptic secretions, appear highly specialized. However, unlike other mosses, Sphagnum does not retain a calyptrum "cap." You can see the naked operculum in the image. Sphagnum sporophytes also lack a peristome. On germination, the spores may (not always) form a structure which looks suspiciously like the flat thallus of a liverwort. As mentioned above, Sphagnum forms a pseudopod from the gametophyte, rather than a seta from the sporophyte.
Other peculiar features of Sphagnum have been recently summarized by Shaw et al. (2003). One deserves special mention because of its possible implications. In all mosses except Sphagnum, the spores themselves are formed from the central, invaginating endothecium tissues -- very much in the manner of mesodermal structures developing from the developing coelom in the gastrula embryo of animals. In Sphagnum, the spores develop from the outer amphithecium -- just as, in some basal, "acoelomate" animals normally "mesodermal" tissues are derived from ectoderm. ... Compare the Sphagnum sporophyte (the leftmost image in the collection above) with a typical microlecithal animal embryo at gastrula, and ponder that one a while. This is undoubtedly a case of convergence, but moderately mind-blowing all the same.
Image: Sphagnum from the website of Dr. George P. Chamuris, Bloomsburg University.
The Andreaeidae, as generally described, include Andreaea and Andreaeobryum. In most respects they created a smooth and continuous path of character acquisition from Sphagnum up through the Bryopsida. The genus Takakia was originally considered to be a very peculiar liverwort. However, when the sporophyte of Takakia was ultimately described, it became obvious that Takakia was a moss. See discussion in Renzaglia et al. (1997). This dug a raw and unseemly pot-hole in the phylogenetic pathway, since Takakia has an odd mix of characters. The current best guess is that Takakia is closest to the Andreaeidae -- but it is perhaps more bryopsid-like in some ways.
As mentioned, the Andreaeidae themselves are more cleanly intermediate forms. The gametophyte is almost indistinguishable from the gametophyte of the Polytrichales. However, the sporophyte has a number of primitive features. As in Sphagnum, the gametophytic coat over the sporophyte, the epigonium, is retained until just before dehiscence. Similarly, they the sporophytes are elevated on a pseudopodium, rather than a seta. Unlike Takakia or bryopsids, Andreaea has a relatively short sporophyte foot. However, the foot in Andreaeobryum is elongate. The Andreaeidae are also primitive in (probably) producing spores from endothecial tissue, although this tissue grows over the outer wall from the base of the columella, rather than covering the columella as in Sphagnum. The capsule wall itself is even less specialized than in Sphagnum. It rests directly on the spore mass, is single-layered, and lacks a distinct operculum. Dehiscence in all Andreaeids is accomplished by slitting the lateral walls of the capsule. Renzaglia et al. (1997).
Images: Andreaea from Images of California Bryophytes. Takakia from the Interactive Malesian Moss Database (Nat. Univ. of Singapore).
Our previous tiresome remarks notwithstanding, botanists have drawn a real distinction between Polytrichopsida and Polytrichales in that the former includes the Tetraphidales, encompassing such genera as Buxbaumia, Oedipodium, Tetraphis, and sometimes even Andreaeobryum and Takakia. Hyvönen et al. (2003). The possible inclusion of the last two genera strongly signals the likelihood that this is a garbage taxon. So, for the moment, we will ignore it and collapse Polytrichopsida into Polytrichales.
Polytrichales includes about 19 genera and perhaps 200 species. The following general description is derived from Hyvönen et al. (2003) with only minor changes. Polytrichales are typically pioneer plants of open, sometimes even dry, habitats. The group exhibits great diversity: from miniature plants with reduced leaves such as Pogonatum pensilvanicum of eastern North America, to giants of Australasia and New Zealand like Dawsonia superba (height up to 50 cm), with the best-developed gametophyte of all land plants. The most typical features of the gametophyte are closely spaced adaxial lamellae on the leaves, forming a pseudoparenchyma, and differentiation of leaves into a distinct blade and sheathing base. The calyptra is typically hairy in many common species of the Northern Hemisphere, enveloping the developing capsules of the sporophyte generation. This has given the whole group its name, although most genera have a practically naked calyptra. Capsules of the Polytrichales normally have a well-developed peristome with at least 16 teeth formed of whole cells. The epiphragm covering the mouth of the capsule is a unique character that distinguishes Polytrichales from all other groups of mosses. Size and shape of the capsule vary greatly among genera.
Image: Dawsonia superba from The Hidden Forest.
The Bryopsida include 90% of all moss species. Given this fact, and given the long history of the group, it ought to be a simple matter to list the characteristics which identify these as the definitive moss lineage. In fact, it is not. Bryopsids typically have a number of characteristics, many of which are mentioned by Renzaglia et al. 1997). Thus, bryopsid sporophytes generally have vertically aligned plates embedded between multiple layers of cells in the amphithecium. They possess stomata which may or may not be homologous with those in higher plants but Sphagnum also has stomata of a sort). The sporophytes also develop opercula, peristomes, continuous columella, and a spongy layer between the amphithecium and the spore mass. The setae frequently twist at some stage of sporophyte development.
However, only one characteristic seems to be both unique and universal among bryopsids. All bryopsid gametophytes are arthrodontous or descend from ancestors which were arthrodontous. That is, these mosses have peristome teeth which are formed by walls growing between the rows of cells making up the mouth of the spore capsule. This and other critical concepts in moss dentistry are explained and illustrated at the Tree of Life page on Bryopsida, thereby saving us the bother.
According to Newton et al. (2000) this clade is strongly supported using both morphology and molecules. Further, the basal three genera are all, like Funaria in the image, diplolepidous-opposite. That is to say, Funaria has two rows of plates (exostome and endostome) which line up to form radial teeth between them.
Image: Funaria peristome from the Bot 125 Plant Morphology site.
Most of the general features of moss evolution are implicit in the descriptions above, but we include two odd speculations that may be worth the effort of explaining.
First, we have, somewhat speciously, compared the sporophyte to the gastrula of an animal embryo. Those similarities probably reflect the inherent advantages, and limited number of ways to create, an additional population of specialized developmental cells which can do their job in a tightly controlled internal environment. The best geometrical solution to that problem is probably the one represented by the gastrula or columellar sporophyte. This creates a controlled volume, closely bordered by two layers of different types of cells. One might evolve such a system simply by having a flat embryo -- or sporophyte, as the case may be. However, the advantage of creating the space by localized invagination is that it automatically creates proximo-distal polarity, and thus a fundamental tool for differentiation between separate parts of the organism. Although plants never went this direction, the mechanism also creates the further possibility of dorsoventral polarity, if the invagination can be consistently flattened in a particular plane. In mosses the "mesodermal" compartment was co-opted almost completely for the production and storage of gametes. The same is true of many basal invertebrates. However, the elaborate peristomal structures of mosses show that mosses developed the potential of this system to some extent. Evolution is driven by low-probability random events. There does not need to be a "reason" for matters to arrange themselves in any particular way.Still, we wonder why the potential of the system was not expressed in further specializations.
Second, it is a little bit unusual to see this kind of evolutionary tree -- with a few, sometimes strongly divergent basal forms and an enormous radiation of a single derived lineage. This signals an unusually poor fossil record, which is indeed the case with mosses, and/or a very old taxon. Either can result in a "patchy" record of basal forms, with most of the early diversity completely unknown. One of the oddest things about the whole embryophyte story is, in fact, its antiquity. The first known embryophyte remains are, if correctly identified, from the Middle Ordovician, or about 470 Ma. It is usually thought that complex plant communities date from the Earliest Devonian (415 Ma) and forests from perhaps 390 Ma. This is not unreasonably slow. After all, the vertebrates probably subsisted as an only moderately successful type of oddly specialized worm for longer than 60 Ma. However, unlike vertebrates, plants were exploiting an entirely new environment. Typically, this sort of evolutionary invasion leads to explosive radiation. If mosses, liverworts, and perhaps a slew of similar forms, developed early, why didn't things move faster? We, certainly, have no answer. However, we are struck with the fact that, in the Late Ordovician, an ice age was triggered by (it is thought) a sudden draw-down in atmospheric carbon dioxide. This rapid reduction in carbon dioxide has, thus far, eluded explanation. Could these matters be related? A similar event in the Mississippian was almost certainly due to the growth of inland forests [1]. Even the "Snowball" episodes of the Cryogenian may have begun with the spread of eukaryotic algae into fresh water (small volume, but very rich in sunlight, nitrates and iron). After all, no law states that every invasion, military or evolutionary, must necessarily result in a successful or permanent conquest. History, and life, are recorded by the winners. Thus, we don't know much about the failures.
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[1] We may, one day, complete our discussion of that event. The series begins at A Lot of Rot.
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