Selaginellales

Another herbaceous group that coexisted with the Carboniferous arborescent lycopsids is the Selaginellales (Carboniferous evidence reviewed in Thomas, 1997) (FIG. 9.99), today represented by a single genus (Selaginella) that includes around 500 species. Plants assigned to this order are herbaceous, ligulate, and heterosporous (FIGS. 9.100, 9.101), and are characterized by a creeping to erect habit. Modern members generally show leaves in four ranks, with two ranks of smaller leaves (anisophylly). Megagametophytes exhibit endosporic development (FIG. 9.102). Some extant species are capable of surviving extended periods of drought, for example S. lepidophylla ,

Selaginellales
figure 9.100 Selaginella selaginoides (Extant). (Courtesy H. Bültmann.)
Selaginella Male Female Cone
figure 9.101 Partial longitudinal section of Selaginella cone showing microsporangium (left) and megasporangium (right) (Extant). Bar = 1mm.

the so-called resurrection plant. Recent studies based on gene sequence data suggest that the xeric and woodland species evolved from ancestors in the humid tropics, but today there is less resolution in classification schemes than there was earlier understood (Korall et al., 1999; Korall and Kenrick, 2004).

The best-known fossil member, Selaginella fraipontii, represents an excellent example of the reconstruction of a complete plant based on isolated organs. For many years the generic name Paurodendron has been used for small (4 mm in diameter), anatomically preserved stems that are relatively common in coal balls from certain localities (Fry, 1954). The axes bear helically arranged, ligulate microphylls and are characterized by exarch protosteles that are stellate in cross section. The underground portion was subsequently discovered attached to a Paurodendron axis (Phillips and Leisman, 1966). It consists of an unbranched and unlobed, clavate rhizophore (root-bearing organ) from which helically arranged, monarch roots arose (FIG. 9.103). Despite the small size of the rhizophore stele, some secondary xylem is noted (FIG. 9.104). Reproductive parts of the plant are bis-porangiate cones that were initially described under the binomial Selaginellites crassicinctus (FIG. 9.105). Cones of this type are 1.2 cm long and ~5mm in diameter. Sporophylls are ligulate and attached to the axis in alternating verticils (whorls), each with four sporophylls. Megasporangia are restricted to the basal region of the cone, with each sporangium containing four or occasionally up to seven megaspores of the Triletes type. Microspores are assignable to the sporae dispersae genus Cirratriradites . Ultrastructural features of the megaspores are shared by modern members of both the Selaginellales and Isoetales (Taylor and Taylor, 1988, 1990). There have been a number of studies that have focused on determining the developmental processes involved in the formation of the complex spore wall (Hemsley et al., 1994).

Demonstration of organic attachment of Selaginellites cones to Paurodendron axes has made it possible to reconstruct the entire plant, which is now referred to as S. fraipontii (Schlanker and Leisman, 1969). The plant is reconstructed as herbaceous, sprawling, and sparsely branched; it produced cones terminally. There is some suggestion that the S. fraipontii was determinate in growth, much like arborescent lycopsids. The species is known throughout the Carboniferous, and is almost identical morphologically with many of the Selaginella species that inhabit relatively moist environments today. Extant Selaginella and the fossil differ, however, in the organization of their underground parts. In living Selaginella, roots are primarily adventitious, whereas in S. fraipontii, they are formed between adjacent older roots, resulting in a specific pattern of root formation. The

Young sporophyte

Selaginellales

Megaspore mother cells undergo meiosis, producing

Microspore mother cells undergo meiosis, producing

Fertilizatkn

Young sporophyte

Fertilizatkn

Zygote

Sperm

Zygote

Megaspore mother cells undergo meiosis, producing

Megaspore

Sperm

Megaspore

Female gametophyte

Megasporophyll „ Megaspores Archegonium containing egg

Female gametophyte

Microspore mother cells undergo meiosis, producing

Megasporophyll „ Megaspores Archegonium containing egg n

Microsporophyll And Microspores
Microsporophyll Microspores

Microspore Male gametophyte figure 9.102 Life history of Selaginella (Extant). (From Taylor and Taylor, 1993.)

n organized production of laterals is a feature commonly associated with stems, not roots. The occurrence of this feature in S. fraipontii suggests homologies with the lobed rooting systems of a number of Paleozoic and Mesozoic lycopsids and may perhaps indicate a closer affinity with the Isoetales (Rothwell and Erwin, 1985; Bateman et al., 1992), or an intermediate status between the Selaginellales and Isoetales (Bateman, 1990).

Modern-appearing Selaginella-like axes from the Rhaetian (Late Triassic) of Scania have been described as Selaginellites (Lundblad, 1950a). There are also numerous impression-compression specimens of anisophyllous lycopsids from Carboniferous rocks that have been described as either Selaginellites or placed in the extant genus Selaginella (Thomas 1992, 1997). Some of these, such as Selaginellites gutbieri, are represented as exquisite compressions that show the attachment of bisporangiate cones and details of the microphylls (Rossler and Buschmann, 1994). In this species, there appear to be at least six to seven megaspores of the Triangulatisporites type in each megasporangium; microspores can be assigned to Cirratriradites. Isolated S. gut-bieri bisporangiate cones have also been reported from Late Pennsylvanian deposits in the Czech Republic containing these same dispersed spore taxa (Bek et al., 2001). In Selaginellites primaevus (=Selaginella primaeva of Thomas, 1997), from the roof shales in the Saar Coalfield, each megasporangium contains four megaspores of the Triangulatisporites type.

Several small lycopsid cones from the Famennian (Upper Devonian) of Belgium exhibit an interesting collection of features unlike those of other known Devonian lycopsid cones (Fairon-Demaret, 1977). Barsostrobus cones are up to 14 cm long and bear helically arranged sporophylls and stalked sporangia. The sporophyll margins are evenly toothed, with the margins slightly enveloping the sporangium. The vascular

Selaginellales
figure 9.103 Suggested reconstruction of Selaginellafraipon-tii (Pennsylvanian). (Based on Phillips and Leisman, 1966; reproduced from Taylor and Taylor, 1993.)

system is that typical of lycopsid cones, and the traces to the sporophylls have centrifugal and centripetal metaxylem. Spores are 240-320 pm in diameter, trilete, and evenly ornamented. The cones are thought to have been heterosporous, although no microspores have been discovered. Features of this cone suggest affinities with members of the Lycopodiales or Selaginellales; preservation prevents the recognition of ligules. The presence of Williamson striations between the scalariform bars of the metaxylem tracheids resembles some species of Drepanophycus; the Drepanophycales, however, lack heterospory and a strobilar organization of sporangia.

Minostrobus chaohuensis is a Late Devonian cone from China that is believed to have been borne by a herbaceous lycopsid. There are four megaspores per sporangium with spores of the Lagenicula type (Y. Wang, 2001). Yuguangia ordinata is a ligulate lycopsid from the Givetian of China with bisporangiate terminal cones (Hao et al., 2007). Leaves of this permineralized form are spiny and arranged in pseu-dowhorls; megasporophylls contain four megaspores of the Triletes type, whereas the microspores are similar to Acinosporites. The occurrence of Y. ordinata suggests that small heterosporous, ligulate lycopsids had diverged from

Rhizomorph
figure 9.104 Longitudinal section of Selaginella fraipontii (Paurodendron) showing rhizomorph (R), secondary xylem (X) and numerous roots (Pennsylvanian). Bar = 3 mm.

the homosporous ligulate grade by the Middle Devonian (Hao et al., 2007).

Miadesmia membranacea (FIG. 9.106) is used for isolated cones known only from the Carboniferous of Europe. In some classifications of lycopsids, Miadesmia is included in its own order, Miadesmiales (Thomas and Brack-Hanes, 1984); but as nothing is known about the plant that bore this cone type, we will continue to include it within the Selaginellales until more information is available. The cones contain only megasporophylls which are attached to the axis at right angles (Benson, 1908). Each megasporophyll is ~3 mm long and bears a megasporangium that is attached near the proximal end of the sporophyll. Lateral laminae completely envelop the sporangium, except in the distal region. The enveloping sporophyll is divided into elongate,

figure 9.105 Longitudinal section of Selaginellites crassicinc-tus showing both microsporangia (MI) and megasporangia (ME) (Pennsylvanian). Bar = 2 cm.
figure 9.106 Longitudinal section of Miadesmia membranacea (Pennsylvanian). Bar = 1 cm.

tentacle-like extensions that project beyond the distal opening. The sporangium is somewhat flattened on the sporophyll so that the opening is directed away from the cone axis. A large ligule is present just distal to each megasporangium. Miadesmia is interesting in that the sporangium contains one large, functional megaspore and some are known with a cellular megagametophyte. In the original description, it was noted that M. membranacea occurs in the same coal balls as specimens of lepidodendrids. The small size of the sporophyll units has suggested to some that Miadesmia may represent a ligulate, heterosporous cone type within the Selaginellales that parallels the highly developed heter-ospory in the Lepidodendrales. This theory is based on the presence of one functional megaspore per megasporangium in each group and the integument-like morphology of the lateral laminae.

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  • Conrad
    Are Selaginellales are heterosporous?
    2 months ago

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