The geologic history of the sphenophytes (Sphenophyta) closely parallels the pattern of evolution and diversification exhibited by the lycophytes. Sphenophytes were first encountered in the Devonian, but attained their maximum diversity during the Carboniferous. From the Carboniferous to the recent, the group has experienced a gradual decline, until today they are represented by only ~15 species in a single genus, Equisetum (horsetails) (FIG. 10.1). In some treatments, the genus Equisetum is subdivided into two subgenera, subg. Equisetum and subg. Hippochaete, based on stomatal position and stem branching (Hauke, 1963; Des Marais et al., 2005). Although sphenophytes, along with ferns, are traditionally regarded as paraphyletic successive grades of increasing complexity between bryophytes and seed plants (Rothwell, 1999), some phylogenetic analyses (Pryer et al., 2001; Wikström and Pryer, 2005) suggest that sphenophytes and ferns are a monophyletic group (see also Nishiyama, 2007).
Members of the Sphenophyta, sometimes termed the Arthrophyta or Equisetophyta, and in older treatments -Sphenopsida (Neuberg, 1964) are typically characterized by clonal growth in the form of extensive, below ground rhizome systems from which arise adventitious roots and upright (also called aerial or orthotropous) axes or shoots with monopodial branching (FIG. 10.2). There are several reports of endomycorrhizae (AM) in the roots of modern Equisetum (Koske et al., 1985; Dhillion, 1993), but these are at best facultative associations (Read et al., 2000; Brundrett, 2002). Both rhizomatous and upright axes in the sphenophytes have distinct nodes and internodes, and the upright axes produce whorls of leaves or branches at the nodes. Regularly spaced, longitudinal ribs and furrows ornament the internodal regions. Leaves of modern Equisetum are typically small and thought to represent modifications of branching systems (Rutishauser, 1999), a hypothesis which is supported by the fossil record of the group. The vascular cylinder ranges from protostelic to siphonostelic, with primary xylem maturation either exarch or endarch. Secondary tissues are present in a few taxa, most notably some Carboniferous members of the Equisetales and the Devonian genus Pseudobornia. The reproductive organs are usually loosely arranged strobili or cones that consist of
a central axis bearing whorls of modified branches, termed sporangiophores, that bear recurved, thick-walled sporangia. Some of the spores in this group are characterized by an extra-exinous layer that, in some genera, is elaborated into elongate structures termed elaters. All extant species are homosporous (Duckett, 1970), with the spores developing into unisexual gametophytes that produce either antheridia or archegonia. Sexual differentiation of the gametophytes is not genetically determined but is influenced by environmental conditions (Guillon and Raquin, 2002; Guillon and Fievet, 2003). Most fossil sphenophytes were homosporous, although a few heterosporous members are known; heter-ospory was not as extensively developed in the Sphenophyta as in the Lycophyta. In this book, the Sphenophyta includes three orders:
HIGHER TAXA IN THIS CHAPTER; Pseudoborniales (Devonian) Sphenophyllales (Devonian-Triassic) Equisetales (Devonian-recent) Calamitaceae Tchernoviaceae Gondwanostachyaceae Equisetaceae
One group that traditionally has been included in the Sphenophyta, or in their direct ancestry, is the Hyeniales (Krausel and Weyland, 1926). Several recent studies suggest, however, that the Hyeniales are more similar to ferns and fernlike plants; consequently, a discussion of several of these (e.g., Calamophyton, Hyenia) can be found in Chapter 11.
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