DIVERSITY OF LIFE

DESCRIPTION OF THE PHYLUM CYCADOPHYTA (NATHORST 1902)

EUKARYA>ARCHAEPLASTIDA>VIRIDIPLANTAE>STREPTOBIONTA>EMBRYOPHYTA>TRACHEOPHYTA>SPERMOPHYTA>CYCADOPHYTA

Cycadophyta (si-ka-DA-fa-ta) is formed from two Greek roots that mean palm (khoix -χοιζ); and plant (phyto -φυτο).  The reference is to a palm-like plant.  However, the common transliteration (kykas) is a corruption of kho´x, a name that Theophrastos gave to Hyphaene thebaica, the Ethiopian Palm.  Linnaeus thought that Theophrastos was describing the plant that he (Linnaeus) called Cycas.

 

INTRODUCTION TO THE CYCADOPHYTA

The cycads resemble ferns with their frond-like leaves (Figure 1), barrel-shaped trunks covered with persistent leaf bases.  The leaves are very tough with a thick hypodermis layer and sunken stomata.  The roots tend to be coraline and some taxa have endosymbiotic cyanobacteria in the superficial roots.  The stem axes grow slowly, are eustelic, and have mucilage canals through the large cortex.

Cycads are dioecious and produce seeds on specialized leaves.  Some taxa like Cycas produce megasporophylls that look like slightly modified vegetative leaves (Figure 2).  Others like Zamia and Dioon produce highly modified megasporophylls in simple strobili (Figure 3).   All cycads bear their microsporophylls in simple staminate strobili (Figure 4).  The mature seed begins with the formation of the ovule with its megaspore mother cell and the microsporangia with microspore mother cells.  The microspore mother cells undergo meiosis forming microspores, with are released as three-celled pollen.  The pollen finds its way into the micropyle and germinates in the pollen chamber of the ovule.  The microgametophyte matures and forms a haustorium, called a pollen tube, that slowly feeds on the megasporangial wall.  In the mean time, the megaspore mother cell undergoes meiosis and aborts three of the daughter megaspores.  The resulting megaspore begins to grow and develop into a megagametophyte.  In the case of Zamia, the megagametophyte develops from December to June at which point multiple archegonia reduced to an egg with two neck canal cells.  The pollen tube enters the archegonial chamber and multiflagellated sperm, two per pollen grain, swim to the archegonia.  Fertilization occurs and in June, and the embryonic dicotyledonous plant develops within the maturing seed until November.  The seeds are released and germination of the seed occurs several months after that (Figure 5).

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FIGURE 1

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FIGURE 2

 

FIGURE 5. Zamia Life History.  a. Staminate plant, b. Microsporophyll, c-d. Microsporangium.  A. Ovulate strobilis, B. Megasporophyll, C. Ovule with a megaspore mother cell, D. Linear tetrad of megaspores following meiosis, E. Pollination, F. Pollen tube from microgametophyte, G. Megagametophyte with several archegonia, H. Mature microgametophyte, I. Fertilization, J. Seed with embryo, K. Germinating seed.

From Norstog (1976)

 

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FIGURE 3

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FIGURE 4

 

SYSTEMATICS OF THE CYCADOPHYTA

The descriptions of Bold et al. (1987) and Bierhorst (1971) follow the classical view that the cycads are the most primitive of the living seed plants.  Current analyses suggest the same thing.  For example, the Crane (1996, Tree of Life Project) and Doyle (2006) illustrate the cycads as surviving seed ferns.  Tudge (2000) has a similar (but more truncate) scheme.  The fossil history of the group suggests that this could be true.  Cycads appear in the Carboniferous and became very abundant during the Mesozoic (Jurassic to Cretaceous) followed by a decline through the Tertiary.  Curiously, Nagalingum et al. (2011) found through fossil-calibrated molecular phylogenies that the cycads experienced a brief renaissance during the Miocene followed by a decline to the present level of diversity (~300 species) such that the divergence of all living species is no older than 12 million years old.   The few survivors today are dioecious and rarely dominant in those environments where they occur.  The molecular  evidence of Chaw et al. (2000) also is consistent with the seed fern connection in that that the cycads are basal in their gymnosperm clade (Figure 6).  Pearson (1995)  departs from the classical view in that he shows the cycads as a major group in the line that leads to the flowering plants and not basal in the gymnosperms.

The plastid based phylogenetic analysis of Zgurski et al. (2008) suggests that there are two major groups of cycads (Zamia +Microcycas+Ceratozamia+Stangeria and Encephalantos+Lepidozamia+Macrozamia) with Cycas and Dioon basal to the living cycads.  Nagalingum et al. (2011) used fossil-calibrated molecular phylogenies to demonstrate a cycad diversificationshow that Clearly, the cycads, remnants of a once diverse group of seed plants, require much more work to produce a phylogenetic taxonomy.  Until the systematics of the cycads stabilizes, we will use the classical system of Bold et al. (1987) and Bierhorst (1971).

 

 

FIGURE 6.  The relationships between spermophytes (seed plants) is an integration of molecular studies (Chaw et al. 2000, Soltis et al. 2002, Matthews 2009, Zhong et al. 2010, Zhong et al. 2011,  Ran et al. 2010, Rai et al. 2008), anatomy and fossil evidence (Doyle 2006, Hilton and Bateman 2006, and Tomescu 2008).  In this cladogram, the cycads (taxa in shaded box) are embedded in seed fern taxa and are the most basal of all living spermophytes.

 

 
FURTHER READING:

INTRODUCTION TO THE DOMAIN EUKARYA

 

LITERATURE CITED

Banks, H. P. 1975. Reclassification of Psilophyta. Taxon. 24: 401-413.

Bierhorst, D. W. 1971. Morphology of Vascular Plants. In: N. H. Giles and J. G. Torrey. The MacMillan Biology Series. The MacMillan Co. New York.

Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York. 

Cantino, P., J. A. Doyle, S. W. Graham, W. S. Judd, R. G. Olmstead, D. E. Soltis, P. S. Soltis, and M. J. Donoghue. 2007. Towards a phylogenetic nomenclature of Tracheophyta. Taxon 56(3): E1-E44.  

Chaw S.-M., C. L. Parkinson, Y. Cheng, T. M. Vincent, and J. D. Palmer. 2000. Seed plant phylogeny inferred from all three plant genomes: Monophyly of extant gymnosperms and origin of Gnetales from Conifers. Proceedings of the National Academy of Sciences (USA) 97:4086-4086. 

Crane, P.. 1996. Spermatopsida. Seed Plants. Version 01 January 1996 (temporary). http://tolweb.org/Spermatopsida/20622/1996.01.01 in The Tree of Life Web Project, http://tolweb.org/

Dittmer, H. J. 1964. Phylogeny and Form in the Plant Kingdom.  Van Norstrand Company, Inc. New York.

Doyle, J. A. 1998b. Phylogeny of vascular plants. Annual Review of Ecology and Systematics. 29:567-599.

Doyle, J. A. 2006. Seed ferns and the origin of angiosperms. Journal of the Torrey Botanical Society. 133(1):  169-209. [C]

Kenrick, P. and P. R. Crane. 1997b. The Origin and Early Diversification of Land Plants: A Cladistic Study. Smithsonian Institute Press. Washington, DC.

Nagalingum, N. S., C. R. Marshall, T. B. Quental, H. S. Rai, D. P. Little, and S. Matthews. 2011. Recent synchronous radiation of a living fossil.  Science. 334(6057): 795-799.

Northington, D. K. and J. R. Goodin. 1984. The Botanical World. Times Mirror/Mosby College Publishing, St. Louis.

Pearson, L. C. 1995. The Diversity and Evolution of Plants. CRC Press. New York. 

Rai, H. S., P. A. Reeves, R. Peakall, R. G. Olmstead, and S. W. Graham. 2008. Inference of higher-order conifer relationships from multi-locus plastid data set. Botany. 86:658-669.

Ran, J-H., H. Gao, X-Q. Wang. 2010. Fast evolution of the retroprocessed mitochondrial rps3 gene in Conifer II and further evidence for the phylogeny of gymnosperms. Molecular Phylogenetics and Evolution. 54: 136-149.

Soltis, D. E., P. S. Soltis, and M. J. Zanis. 2002. Phylogeny of seed plants based on evidence from eight genes. American Journal of Botany. 89:1670-1681.

Soltis, D. E., P. S. Soltis, and M. J. Zanis. 2002. Phylogeny of seed plants based on evidence from eight genes. American Journal of Botany. 89:1670-1681.

Tomescu, A. M. F. 2008. Megaphylls, microphylls and the evolution of leaf development. Trends in Plant Science. 14(1): 5-12

Zgurski, J. M., H. S. Rai, Q. M. Fai, D. J. Bogler, and J. Francisco-Ortega. 2008. How well do we understand the overall backbone of cycad phylogeny? New insights from a large, multigene plastid data set. Molecular Phylogenetics and Evolution. 47: 1232-1237.

Zhong, B., T. Yonezawa, Y. Zhong, and M. Hasegawa. 2010. The position of Gnetales among seed plants: overcoming pitfalls of chloroplast phylogenomics. Molecular Biology and Evolution. 27(12): 2855-2863.

Zhong, B., O. Deusch, V. V. Goremykin, D. Penny, P. J. Biggs, R. A. Atherton, S. V. Nikiforova, and P. J. Lockhart. 2011. Systematic error in seed plant phylogenomics. Genome Biology and Evolution. 3: 1340-1348.

 

By Jack R. Holt.  Last revised: 04/08/2013