Conclusion Of Lycophyta

The lycopsids have an extensive geologic history, extending back into the Late Silurian (Kotyk et al., 2002). They were widespread during the Late Mississippian and most of the Pennsylvanian, representing the dominant group of plants in most of the vast Euramerican paleoequatorial coal-swamp ecosystems. The widespread coal-mining operations that have uncovered Carboniferous rocks have been responsible, in large part, for the abundant, well-preserved fossil specimens of this group. The paleoecology of these plants has been extensively studied, and many constitute the focal point of ancient landscape reconstructions in museums around the world. The comprehensive geologic history of the lycopsids and numerous, exquisitely preserved specimens have provided paleobotanists with the opportunity not only to trace the evolution of the group but also to investigate some basic facets of their biology.

The Lycophyta are monophyletic and basal within the vascular plants (Bateman, 1996b); together with the Zosterophyllophyta (Chapter 8), they comprise a clade (Lycophytina of Kenrick and Crane, 1997a) ; Both lycopsids and zosterophylls occupied the same habitats (floodplain and channel margins) during the Devonian (Gensel, 1992). The common names of this group have become somewhat confusing in recent years. Among paleobotanists, lycopods has been the traditional name for all of the Lycophyta (without the zosterophylls), and you will find this name in the older literature; some neontologists, however, use this name to refer only to members of the Lycopodiales. More recently, lycopsids has been widely used. The term lycophytes has been used to refer not only to the Lycophyta but also to the clade composed of the lycopsids and the zosterophylls together. Kenrick and Crane (1997a) classified the lycophytes as the subphylum (subdivision) Lycophytina, containing two classes, Lycopsida and Zosterophyllopsida. We will use the name lyc-opsids to refer to the plants discussed in this chapter. According to Gensel and Berry (2001), the lycophytes represent a broader concept, which includes traditional lycopsids as well as the zosterophylls and transitional taxa such as Asteroxylon and Drepanophycus. Synapomorphies of the lycopsids include helically arranged microphylls and sporangia borne in the axil or on the adaxial (upper) surface of sporophylls. Most have exarch primary xylem maturation and, like the zosterophylls, lycopsid sporangia dehisce into two valves. In addition, metaxylem trac-heids contain Williamson striations (discussed below).

Currently available evidence suggests that the lycopsids originated from the zosterophylls, although these groups were coeval in the Siluro-Devonian (Gensel and Berry, 2001;

Kotyk et al., 2002). From these beginnings, the lycopsids radiated extensively during the Carboniferous, in terms of both diversity and distribution, and then began to diminish in numbers of taxa and abundance toward the end of the Carboniferous, as the climate began to change and the extensive, equatorial peat swamps diminished (DiMichele et al., 2001a) . Today the clade is represented by only seven genera in most treatments, of which three genera, Huperzia, Lycopodium (club mosses), and Selaginella (spike mosses), contain most of the —1600 species (gbif.org). The group is characterized by either dichotomous branching or a combination of dichotomous and monopodial branching. Stems are densely covered with true leaves termed microphylls. Microphylls are generally small (except in some of the extinct arborescent lycopsids), helically arranged, and vascu-larized by a single bundle that does not produce an interruption (leaf gap) in the stele of the stem when it separates. The roots in the lycopsids, as in other vascular cryptogams, are primarily adventitious.

One of the most diagnostic features of the group is the position of the sporangium, which is borne in the axil or on the upper surface of a modified leaf or sporophyll. In some members sporangia are borne on short stalks. Sporophylls may be interspersed among photosynthetic microphylls, or they may be non-photosynthetic and aggregated into loosely constructed strobili or more consolidated cones. In most species the sporophyte produces only one type of spore and these plants are therefore regarded as homosporous. In some lycopsids, however, two types of spores are produced, and they not only look different but also function differently in the life history of the plant. Small spores (microspores, developed in microsporangia on microsporophylls) produce the male gametophyte, whereas the mega- or macrospores (developed in megasporangia on megasporophylls) germinate to produce the female gametophyte. Living heter-osporous members of the Lycophyta produce both types of spores in the same strobilus (bisporangiate); in the heter-osporous fossil representatives, microspores and megaspores were produced either in the same or in different cones (monosporangiate).

All extant lycopsids are herbaceous and do not produce secondary vascular tissue, although many fossil forms are known to have been arborescent. Despite the large size of some of the arborescent members (>40 m tall), the amount of secondary vascular tissue is small compared with the total stem diameter. Maturation of the primary xylem is exarch in most lycopsids with scalariform wall thickenings, the most common type of secondary wall pattern on the tracheids. Vascular organization ranges from protostelic to siphonostelic.

Although leaf gaps are not produced, interruption in the vascular cylinder occurs when branch traces are produced.

The classification of lycopsids, similar to many groups of fossil plants, is a difficult task, in part because of the large number of fossil taxa for which there is a limited amount of information, especially reproductive characters. Thomas and Brack-Hanes (1984) have suggested the formation of what they term satellite taxa to accommodate various plant organs that cannot be accurately placed in well-defined families, after the initial concept of satellite genera was proposed by Meyen (1978). Their classification is similar to the one used in this volume, with the exception that we prefer the order Lepidodendrales rather than the Lepidocarpales, and we continue to include Miadesmia within the Selaginellales. In some phylogenies, heterospory and the presence of a ligule are used to group certain taxa; in others herbaceous versus arborescent habit has been used to define hierarchy. DiMichele and Bateman (1996) included all the rhizomor-phic lycopsids (Lepidodendrales, Isoetales) in a single order, the Isoetales. The rhizomorphic lycopsids are those with a stigmarian-type rooting system, which can be either laterally extensive or small and lobed (discussed below). As research continues with the lycopsids, it is increasingly clear that delineation into major clades is not easily resolved, and that transformation series leading to the origin of some modern forms that at one time seemed well defined, are today more difficult to resolve (Gensel and Berry, 2001) . The following traditional classification of the lycopsids into seven orders is intended to provide a framework for discussion of this group of plants. As is true of most groups with fossil members, there are several enigmatic forms that do not fit precisely into this classification.

HIGHER TAXA IN THIS CHAPTER:

Lycophyta Lycopsida Drepanophycales (Devonian) Protolepidodendrales (Devonian-Mississippian) Lepidodendrales (Devonian-Permian) Lepidodendraceae, Diaphorodendraceae, Sigillariaceae Lycopodiales (Pennsylvanian-recent)

Lycopodiaceae Selaginellales (Pennsylvanian-recent)

Selaginellaceae Pleuromeiales (Triassic-Cretaceous) Isoetales (Upper Devonian-recent) Isoetaceae, Chaloneriaceae

+1 0

Post a comment