Isoetes
From Palaeos
Isoetes (quillworts) is a cosmopolitan genus of small aquatic to semiaquatic plants, the only living members of the order Isoetales. While superficially rush-like in appearance, Isoetes are actually members of the Lycopodiopsida and so related to the clubmosses. Of the three living genera of Lycopodiopsida (Lycopodium, Selaginella and Isoetes), Isoetes is the most closely related to the extinct Lepidodendrales, the scale trees of the Carboniferous (Doyle, 1998), and shares features with them such as secondary tissue development (wood and bark) in the modified shoot system that acts as the rooting system. The earliest known fossils of Isoetes from the Triassic are little different from living exemplars (Retallack, 1997).
Description
Parent taxa: Lycophytina, Lycopodiopsida, Isoetales.
Age: earliest Triassic to Recent (Retallack, 1997).
Description: (from Floras of North America and Retallack, 1997) Rootstock 2(-3)-lobed, nearly globose to horizontally spindle-shaped and proliferous, corky. Leaves several to many, erect to spreading, straight to recurved, 1-100 cm; ligule with broadly inset base (glossopodium), deltate to cordiform, 1-6 mm, membranous. All leaves fertile sporophylls with embeddedsporangia and undifferentiated leaf blades extending beyond sporangia, megasporophylls near base and microsporophylls near apex of plant. Sporangia ovoid to ellipsoid or oblong, 3-15 mm, walls unpigmented or brown-streaked to completely brown, traversed internally by trabeculae (internal partitions). Obscured sporangial apex (velum). Megaspores white, gray or black, globose, mostly 300-700 µm diameter, trilete, each with equatorial ridge and 3 converging proximal ridges, smooth or with spines, tubercles or ridges. Microspores grayish or brownish in mass, reniform, mostly 20-50 µm, monolete, smooth or textured with spines, tubercles or ridges.
Comments: From a phylogenetic perspective, perhaps the most interesting feature of Isoetes is their development of woody tissue in the base of the plant which demonstrates their relationship with extinct tree-like lycopods, and their relictual status as part of a once much more diverse radiation. On this basis, it has been suggested that Isoetes might represent a greatly reduced descendant of arborescent ancestors such as the palm-like Pleuromeia. However, phylogenetic analysis suggests that Isoetes is rather the sister-group to a clade containing both arborescent forms such as Pleuromeia as well as herbaceous forms such as Tomiostrobus, suggesting that Isoetes rather than Pleuromeia more closely resembles the ancestral form (Retallack, 1997).
References
Doyle, J. A. 1998. Phylogeny of vascular plants. Annual Review of Ecology and Systematics 29: 567-599.
Retallack, G. J. 1997. Earliest Triassic origin of Isoetes and quillwort evolutionary radiation. Journal of Paleontology 71 (3): 500-521.
Credits
Christopher 22:28, 7 July 2008 (PDT)
