Equisetopsida Wednesday, July 1, 2009

Equisetopsida Fossil range: Late Devonian[1] to Recent
Equisetum telmateia
Scientific classification
Kingdom: Plantae Division: Pteridophyta * Class: Equisetopsida C. Agardh
Orders
Equisetales Archaeocalamitaceae † Calamitaceae † Equisetaceae Pseudoborniales † Sphenophyllales †
Synonyms
Sphenopsida

Equisetopsida, or Sphenopsida, is a class of plants with a fossil record going back to the Devonian. Living species are commonly known as horsetails^[2] and typically grow in wet areas, with needle-like leaves radiating at regular intervals from a single vertical stem. Equisetopsida is placed in the botanical division of ferns (Pteridophyta),^[3] though sometimes regarded as a separate division Equisetophyta (also as Sphenophyta or Arthrophyta).

Morphology

The Sphenophytes comprise photosynthesising, "segmented", hollow stems, sometimes filled with pith. At the junction ("node", see diagram) between each segment is a whorl of leaves. In the only extant genus Equisetum, these are small leaves (microphylls) with a singular vascular trace. However, sphenophyte leaves probably arose by the reduction of a megaphyll, as evidenced by early fossil forms such as Sphenophyllum, in which the leaves are broad with branching veins.^[4] The plumbing of these leaves is interesting: the vascular traces trifurcate at the junctions, with one thread going to the microphyll, and the other two moving left and right to merge with the new branches of their neighbours. The vascular system itself curiously resembles that of the vascular plants' eustele, which evolved convergently. A primary xylem contains carinal canals; in the Calamitales, secondary xylem (but not secondary phloem) can be secreted as the cambium grows outwards, producing a woody stem, and allowing the plants to grow as high as 10m. The cortex itself contains valecular canals; due to the softer nature of the phloem, these are very rarely seen in fossil instances.

The plant does not bear a coherent root system but underground rhizomes, from which roots and aerial axes emerge.

The plant contains an intercalary meristem: that is to say, each segment of the stem grows as the plant gets taller. This contrasts with the seed plants, which contain an apical meristem - i.e. new growth comes only from growing tips (and widening of stems). Growth was determinate - i.e. the plants' phenotype dictated a maximum height, which the plant would grow to then get no higher.

Sphenophytes bear cones (technically strobili, sing. strobilus) at the tips of some stems. These cones comprise spirally arranged sporophores, which bear spores in four clusters, and in extant sphenophytes cover the spores externally - like four sacs hanging from an umbrella, with its handle embedded in the central cone body. In extinct groups, further protection was afforded to the spores by the presence of whorls of bracts - big pointy microphylls protruding from the cone.

The spores themselves bear characteristic elaters, distinctive spring-like attachments which are hygroscopic: i.e. they change their configuration in the presence of water, helping the spores move and aiding their dispersal. Dispersal is aided in the first instance by laterally dehiscing sporangia, which pop open and scatter spores.

Vegetative stem: N = node, I = internode, B = branch in whorl, L = fused microphylls

Cross-section through a strobilus; sporophores, with attached congregations of spores, can be discerned.

Strobilus of E. telmateia, terminal on an unbranched stem

The extant horsetails are mostly homosporous, but this is conspicuously not the case in the past.

Fossil record

The extant horsetails represent a tiny fraction of Sphenophyte diversity in the past. There were three orders of Equisetopsid; the Pseudoborniales, which first appeared in the late Devonian.^[1] Second, the Sphenophyllales which were a dominant member of the Carboniferous understory, and prospered until the mid and early Permian respectively. The Equisetales existed alongside the Sphenophyllales, but diversified as that group disappeared into extinction, gradually dwindling in diversity to today's single genus Equisetum.

The organisms first appear in the fossil record during the late Devonian,^[1] a time when land plants were undergoing a rapid diversification, with roots, seeds and leaves having only just evolved. (See Evolutionary history of plants) However, plants had already been on the land for almost a hundred million years, with the first evidence of land plants dating to 475 million years ago.^[5]

Systematics

The horsetails and their fossil relatives have long been recognized as quite distinct from other seedless vascular plants.^[6] In fact, the group is so unlike other living and fossil plants that its relationship to other plants has long been considered problematic.^[7]

Because of the unclear relationships of the group, the rank botanists assign to it varies from order to division. When recognized as a separate division, the literature uses many possible names, including Arthrophyta^[7], Sphenophyta^[1][8], or Equisetophyta. Other authors have regarded the same group as a class, either within a division consisting of the vascular plants or, more recently, within an expanded fern group. When ranked as a class, the group has been termed the Equisetopsida^[9] or Sphenopsida.^[10]

Recent genetic testing has produced evidence that this group of plants accurately belong within the fern clade of vascular plants. There is some minor morphological evidence to support this. However, this assignment is not universally agreed upon, because of this group's highly distinct and peculiar morphology in many respects, and its ancient and distinct lineage. The genetic evidence could possibly be explained by one of three mechanisms other than this relatedness: 1) that it represents convergent evolution; 2) that it represents an ancient, conserved condition from the root clade, Euphyllophyta, that branched into both seed plants and ferns, or 3) that this result represents ancient genetic crossover, unusual but not impossible.^{[citation needed]}

References

1. ^ ^{a b c d} Taylor, Thomas N.; Edith L. Taylor. (1993). The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. pp. 303–305. ISBN 0-13-651589-4.
2. ^ "horsetail". Merriam-Webster Online Dictionary. http://www.merriam-webster.com/cgi-bin/dictionary?book=Dictionary&va=horsetail.
3. ^ Smith, Alan R.; Kathleen M. Pryer, Eric Schuettpelz, Petra Korall, Harald Schneider, & Paul G. Wolf (2006). "A classification for extant ferns". Taxon 55 (3): 705–731. http://www.pryerlab.net/publication/fichier749.pdf.
4. ^ Rutishauser, R. (1999). "Polymerous Leaf Whorls in Vascular Plants: Developmental Morphology and Fuzziness of Organ Identities". International Journal of Plant Sciences 160 (6): 81–103. doi:10.1086/314221. http://links.jstor.org/sici?sici=1058-5893(199911)160%3A6%3CS81%3APLWIVP%3E2.0.CO%3B2-T. Retrieved on 2008-01-31.
5. ^ CH Wellman, PL Osterloff, U Mohiuddin (2003). "Fragments of the earliest land plants". Nature 425 (6955): 282–285. doi:10.1038/nature01884. PMID 13679913. http://sherpa.leeds.ac.uk/archive/00000106/01/wellmanch1.pdf.
6. ^ Eames, Arthur J. (1936). Morphology of Vascular Plants (Lower Groups). New York and London: McGraw-Hill Book Company. pp. 110–115.
7. ^ ^{a b} Bold, Harold C.; C. J. Alexopoulos, & T. Delevoryas (1987). Morphology of Plants and Fungi (5th ed.). New York: Harper-Collins. pp. 371–387, 478, 506–514. ISBN 0-06-040838-1.
8. ^ Gifford, Ernest M.; Adriance S. Foster (1988). Morphology and Evolution of Vascular Plants (3rd ed.). New York: W. H. Freeman and Company. pp. 175–207. ISBN 0-7167-1946-0.
9. ^ Kenrick, Paul; Peter R. Crane (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. pp. 241–242. ISBN 1-56098-730-8.
10. ^ Stewart, Wilson N.; Gar W. Rothwell (1993). Paleobotany and the Evolution of Plants (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-38294-7.