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 khoïx, 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 (text with tooltip) Ducts or channels in the axes of cycads and ginkgophytes that contain mucilage, a water-soluble substance which solidifies upon exposure to the air and likely serves to protect against invasion of the tissue by fungi and bacteria. through the large cortex (text with tooltip) Cortex is a tissue of parenchymal cells that surrounds vascular tissue. . Cycads are dioecious (text with tooltip) Dioecious organisms have separate male and female individuals. and produce seeds (text with tooltip) Unit of sexual reproduction in some plants. Formed when an ovule is fertilized and comprised of outer coat that encloses stored food and an embryo. 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 (text with tooltip) A strobilus is an axis of fertile appendages. A simple strobilus is an axis of sporophylls. A compound strobilus is an axis of simple fertile axes. Sometimes the compound cones have simple fertile axes that are reduced to a single sporophyll and appear to be 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 (text with tooltip) The collective mass of grains produced within the anthers of flowering plants or the male cones of a gymnosperm. In all seed plants, pollen is generated by the development of a microspore into a microgametophyte. The germination of the pollen grain leads to the development of a pollen tube, which delivers two sperm or sperm nuclei to the egg in the ovule. In flowering plants, mature microgametophyte has only two cells, a tube cell and a generative cell. . The pollen finds its way into the micropyle (text with tooltip) An opening in the integuments of an ovule that exposes part of the megasporangial wall (a chamber called a pollen chamber in gymnosperms). Thus, in gymnosperms, pollen enters the micropyle and germinates in the pollen chamber. However, because the micropyle is not exposed in flowering plants, their pollen germinates on the stigma. The pollen tube grows through the style, and enters the ovule through 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).

FIGURE 1. A habit photograph of Cycas. Image from http://horticulture.missouri.edu/trinklein/cycas-r.htm	FIGURE 2. Cycas producing ovulate (text with tooltip) An ovule is a structure that contains the megagametophyte in seed plants. The megagametophyte remains within the megasporangium (the nucellus), which is surrounded by layers of integuments. After fertilization, the ovule develops into a seed. leaves (megasporophylls). Image from http://biotech.tipo.gov.tw/plantjpg/1/Cycas%20revoluta.jpg	FIGURE 3. The ovulate cone of Zamia. Image from http://botit.botany.wisc.edu/courses/img/bot/401/Cycadophyta/Zamiaceae/Zamia/	FIGURE 4. The staminate cone of Zamia. Image from http://www.csupomona.edu/~jcclark/classes/bot125/resource/graphics/cyc_zam_pollcone.html

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)

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.

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