Cycadopsida

The cycads (taxa Cycadidae, Cycadopsida, also sometimes as division Cycadophyta) with their single order Cycadales, are an ancient group of plants that have retained clearly primitive characters, such as sperm motile. Its origin probably goes back to the Carboniferous, although the unquestionably oldest remains date from the Permian, about 280 million years ago, and reached a peak of abundance and diversity in the Mesozoic era. They served as food for herbivorous non-avian dinosaurs. Today cycads are found mainly in relicts in the Southern Hemisphere, and many of their species are endangered or endangered, consisting of about 185 species and 9 genera. According to the 2021 Red List, 64% of cycads are threatened with extinction. Cycads today are found in the Southeastern United States, Mexico, Central America, some Caribbean islands, South America, East and Southeast Asia, Australia, and in parts of Africa. They are one of the four divisions of bare-seeded plants that fit into the traditional concept of gymnosperms.

Cycas circinalis

The group is monophyletic, as judged by its synapomorphies in structural characters, such as its girdle-like foliar traces, its characteristic pattern of vascular bundles on the petiole (like an omega), the presence of mucilage channels, meristems distinctive, as well as specific poisonous compounds: cycasins. These and other poisonous compounds have been important in the evolution of cycads as a defense against bacteria and fungi. Cycad toxins have occasionally caused partial or complete paralysis of the hind legs of cattle.

Another synapomorphy of cycads is coraloid roots, named for their similarity to a sea coral. These roots harbor nitrogen-fixing cyanobacteria, such as legume bacteria. Cyanobacteria convert gaseous nitrogen (which cycads cannot use) into a form of nitrogen that cycads can use, allowing them to grow in nutrient-poor soils that cycads can thus occupy.

Cycads often resemble palm trees, with their erect unbranched stem up to 18-20 m tall (as in the misnamed genus Microcycas), and large, pinnately compound leaves. bunched in a spiral at stem apex, or fern-like, with subterranean stem and pinnately compound leaves (only the genus Bowenia from Australia has pinnate leaves). Many cycads bear cataphylls, which are scale-like leaves that occur between normal leaves and often provide a protective function. The trunk of cycads usually does not exhibit lateral (axillary) branches, so the loss of axillary branching on the aerial trunk may be a cycad apomorphy. Interestingly, cycad leaves have circinate vernation, as in ferns, perhaps the remnant of a primitive character that was lost in other spermatophytes. Cycads are slow growing, some as little as 1 m in 500 years.

Cycad reproductive structures occur in strobili consisting of a spindle and megasporophylls (ovule-bearing leaves) or microsporophylls (pollen-bearing leaves). These simple structures contrast with the complex seed cones of conifers. All cycads have pollen strobili, and all except Cycas have ovule strobili. Although cycads produce abundant, dusty pollen suggesting wind pollination, insects are the main vectors of pollen. Beetles (and to a lesser degree bees) are either vectors of pollen or move it to the ovule once it has been carried by the wind to the ovule strobila from another plant. Pollination and fertilization can however be separated by up to 7 months.

Cycad seeds often have a brightly colored outer covering (pink, orange, or red) and are commonly dispersed by birds, bats, weasels, turtles, and many other animals. The brightly colored megasporophylls of some cycads also attract dispersal agents. Ocean currents carry floating seed cycads due to a spongy outer covering, while gravity disperses the seeds of the others.

All cycads are dioecious (with either male or female feet) and can be governed by sex chromosomes. Chromosome number varies considerably between genera of cycads (but not within them) and is taxonomically useful.

The cycad clade consists of 2 families, 10 or 11 genera, and about 300 species. Cycadaceae only comprises Cycas, while the rest of the genera are found in Zamiaceae, including Stangeria, which was sometimes found in its own family Stangeriaceae. Cycads are distinguished by not forming a female cone. In Cycas species, the seeds are produced by the lower margin of numerous female sporophylls (also called megasporophylls) that congregate at the apex of the trunk in dense masses. Cycas species do have male cones, which are found in all cycads. The Zamiaceae family differs from Cycadaceae in having both male and female cones, also called strobili.

Nilssoniocladus, dicada extinta del Cretaceico.

Taxonomy

Cycads sometimes form a division (Cycadophyta), with a single class (Cycadopsida), today it is preferred to place them in the division Spermatophyta in the class Gymnospermae, in the subclass Cycadidae with a single order (Cycadales).

Classification according to Christenhusz et al. (2011)

The classification, according to Christenhusz et al. 2011, which also provides a linear sequence of gymnosperms through genus:

Subclass Cycadidae Pax in K.A.E. Prantl, Lehrb. Bot. ed. 9:203 (1894). Type: Cycadaceae. Synonym: Zamiidae Doweld, Tent. syst. Pl. Vasc.: xv (2001). Type: Zamiaceae.

• Order Cycadales Pers. ex Bercht. " J. Presl, Přir. Rostlin: 262 (1820). Type: Cycadaceae. Synonyms: Zamiales Burnett, Outl. Bot.: 490 (1835). Type: Zamiaceae. Stangerials Doweld, Tent. Syst. Pl. Vasc.: xv (2001). Type: Stangeriaceae.

• Family Cycadaceae Pers., Syn. Pl. 2: 630 (1807), Nom. cons. Type: Cycas L.

1 genus, about 107 species, East Africa to Japan and Australia.

• Cycas L., Sp. Pl. 2: 1188 (1753). Type: C. circinalis L. Synonyms: Todda-Pana Adans, Fam. 2: 25. (1763), nom. illeg. by typification. Type: Cycas circinalis L. Dyerocycas Nakai, Chosakuronbun Mokuroku [chuckles]Ord. Fam. Trib. Nov.] 208 (1943). Type: D. micholitzii (Dyer) Nakai (Cycas micholitzii Dyer). Epicycas de Laub. en D.J. de Laubenfels & F.A.C.B. Adema, Blumea 43: 388 (1998), nom. illeg. Type: E. micholitzii (Dyer) from Laub. (≡ Cycas micholitzii Dyer).

• Family Zamiaceae Horan. Prim. Lin. Syst. Nat.: 45 (1834). Type: Zamia L.

9 genera, about 206 species, tropical and subtropical Africa, Australia and America. The phylogenetic tree followed here is that of Zgurski et al. (2008). Synonyms: Encephalartaceae Schimp. & Schenk in K.A. Zittel, Handb. Palaeontol, Palaeophyt. 2: 215 (1880). Type: Encephalartos Lehm. Stangeriaceae Schimp. & Schenk in K.A. Zittel, Handb. Palaeontol, Palaeophyt.: 216 (1880). Type: Stangeria T. Moore. Boweniaceae D.W. Stev., Amer. J. Bot. 68: 1114 (1981). Type: Bowenia Hook.f. Dioaceae Doweld, Tent. Syst. Pl. Vasc.: xv. (2001). Type: Dioon Lindl. Microcycadaceae Tarbaeva, Anat.-Morf. Str. Sem. Cycad.: 19 (1991). Type: Microcycas (Miq.) A.DC.

• Dioon Lindl., Edwards's Bot. Reg. 29 (Misc): 59 (1843), as ' Dion ' Nom. et orth. cons. Type: D. edule Lindl. Synonyms: Platyzamia Zucc., Abh. Math. -Phys. Cl. Königl. Bayer. Akad. Wiss. 4(2): 23 (1845). Type: P. stiffness Zucc.
• Bowenia Hook. F., Bot. Mag. 89: adt. 5398 (1863). Type: B. spectabilis Hook.f.
• Macrozamia Miq., Monogr. Cycad. 35 (1842). Type: M. spiralis (Salisb.) Miq. Zamia spiralis Salisb.)
• Lepidozamia Regel, Bull. Soc. Imp. Naturalistes Moscou 30: 182 (1857). Type: L. peroffskyana Regel. Synonym: Catakidozamia W. Hill, Gard. Chron. 1865: 1107 (1865). Type: C. hopei W. Hill
• Encephalartos Lehm., Nov. Stirp Pug. 6: 3 (1834). Type: E. caffer (Thunb.) Lehm. (≡ Cycas caffra Thunb.)
• Stangeria T. Moore, Hooker's J. Bot. Kew Gard. Misc. 5: 228 (1853). Type: S. paradox T.Moore
• Ceratozamia Brongn., Ann. Sci. Nat. Bot, be. 35: 7 (1846). Type: C. Mexican Brongn.
• Microcycas (Miq.) A.DC., Prodr. 16: 538 (1868). Type: M. calocoma (Miq.) A.DC. (≡ Calocoma Miq.)
• Zamia L., Sp. Pl.ed. 2, 2: 1659 (1763), Nom. cons. Type: Z. pumila L. Synonyms: Palma-Filix Adans, Fam. 2: 21, 587 (1763), No. Aulacophyllum Regel, Gartenflora 25: 140 (1876). Type: A. skinneri (Warsz.) Regel (≡ Zamia skinneri Warsz.) Palmifolium Kuntze, Rev. Gen. 2: 803 (1891), nom. illeg. (≡ Palma-Filix Adans, No. ≡ Zamia L., Nom. cons.) Chigua D.W. Stev., Mem. New York Bot. Gard. 57: 170 (1990). Type: C. restrepoi D.W. Stev. (≡ Zamia restrepoi (D.W. Stev.) A.J. Lindstr., see Lindstrom (2009).

Economic importance

Invernadero de cycadales en el Real Jardín Botánico Juan Carlos I.

Some Cycadophyta are poisonous, Dioon edule can occasionally kill a herbivore as large as a cow.^{[citation needed]}

Some species are cultivated as edibles, the stem and seeds of Cycas produce starch (sago or sago). However, the toxic substances (glycosides) must be removed from the seeds. The pith is exploited from the stem, from which the sago is obtained, typically collected from the apex of the trunk just before a crown of leaves or from the reproductive structures.

Old and year male cones on a foot Cycas circinalis.

Some species, especially Cycas revoluta, are cultivated in horticulture.