The Diversity of Plants
From Paleos
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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
