CYCADS: CHARACTERISTICS, TAXONOMY, HABITAT, REPRODUCTION - BIOLOGY - 2025

The cycads (Cycadophyta) are a group of gymnosperms survivors of the first primitive group seminiferous the late Paleozoic species. In fact, they are considered living fossils due to the fact that they currently retain the same characteristics as prehistoric plants.

Indeed, the Triassic and Jurassic periods are called the "Cycad Era" because they dominated the planet's vegetation. Their distribution was so wide that today they are located in places as distant as Polynesia, Madagascar, Mexico, Cuba and Australia.

Cycads (Cyca revoluta). Source: flickr.com

They are primitive gymnosperms plants, which reproduce by seeds developed in typical conical structures located at the end of the stem. They inhabit tropical and subtropical areas, from warm regions to very cold territories.

These species show arboreal growth, with a robust unbranched stem crowned by a cluster of compound leaves. Currently, 170 species comprising 11 genera have been described, however, in the Jurassic, they encompassed most plant species.

Cycads have managed to survive various geological eras, currently man is the cause of their possible extinction. The increase in deforestation of natural habitats, the expansion of agricultural activities and furtive trade are reasons to classify them as threatened species.

Indeed, the few living species are considered the trunk of the evolutionary tree of today's seed plants. The anatomy of these structures is comparable to the fossil record of the seeds of the early late Paleozoic ferns.

They are plants prized for their high ornamental and commercial value, and many of the species grown in parks and gardens come from their natural habitat. Because they are endangered species, in many countries they have legal protection to prevent their extraction and commercialization.

characteristics

Cycads are a group of dioecious gymnosperms of tropical and subtropical origin. They are characterized by having a thick, unbranched stem, with a soft and spongy texture, with poorly developed wood of the mannoxylematic type.

They are arborescent plants, with tall stems with more than 10 m, or short and underground where only the leaves are observed. Slow growing, they are of extreme longevity, living more than 1,000 years.

Cyca rumphii Source: flickr.com

It has pinnate-compound leaves forming a crown of fronds at the top. They are commonly classified as palms, however they do not have any botanical relationship to these species.

The roots are of the coraloid type, they grow close to the surface of the ground, they are wide and they arise from the same place. They are well-developed roots, which have the ability to develop nitrifying nodules by forming symbiotic relationships with some soil bacteria.

These species are dioecious, that is, they have separate male and female sexes. The reproductive parts form a type of cone called a strobilus, in a terminal or subminal position, generally brightly colored.

The flowers are simple, it has pollen sacs and seminal primordia on its corresponding microsporophylls and megasporophylls. The seeds are large, with a fleshy cover and bright yellow tones, adapted to being dispersed by external agents.

Its habitat is located in tropical and subtropical areas, in humid and dry forests, in cloudy and temperate forests, in savannas and scrublands. They are currently distributed throughout South America, Central America, Mexico and Southeastern North America, West Africa, Southeast Asia, and Australia.

In some regions its smooth stem and strobilus are eaten fresh or used to produce flour with high nutritional value. However, it contains toxic substances with neurological effects, in fact, there are few fauna species that prey on them.

Taxonomy

• Kingdom: Plantae
• Subkingdom: Viridiplantae
• Underkingdom: Streptophyta
• Superdivision: Embryophyta
• Division: Tracheophyta
• Subdivision: Spermatophytina
• Class: Cycadopsida Brongn. 1843 orth. emend.
• Subclass: Cycadidae Pax in Prantl 1894
• Order: Cycadales Dumortier 1829

The clade or branch of the cycads phylogenetic tree is made up of two families, eleven genera and approximately 300 species. The Cycadaceae family includes only the Cycas genus, while the Zamiaceae family encompasses the remaining genera, including the Stangeria genus.

Classification according to Christenhusz

The classification of Christenhusz et al. (2011) establish a linear order from gymnosperms to genus. This classification includes cycads in the subclass Cycadidae.

Subclass Cycadidae Pax in KAE Prantl, Lehrb. Bot. ed. 9: 203 (1894).

• Order Cycadales ex Bercht. & J. Presl, Přir. Rostlin: 262 (1820).
  • Cycadaceae family, Syn. Pl. 2: 630 (1807). It includes a genus and approximately 107 species. Located from Africa to Japan and Australia.
  • Family Zamiaceae, Prim. Lin. Syst. Nat.: 45 (1834). It comprises nine genera and 206 species. Located in America, Australia and tropical and subtropical Africa.

Subtypes

At present the classification of cycads is carried out based on the veining of the leaves. The recognized cycads are grouped into three families: Cycadaceae, Stangeriaceae, and Zamiaceae.

Cycadaceae

Species with perceptible midrib on leaves and lacking lateral veins. It is made up only of the genus Cycas, which includes some 20 species located in Asia and East Africa.

The species C. revoluta is the most representative of this family, highlighting also C. circinalis, C. media, C. riuminiana, and C. rumphii.

Stangeriaceae

Cycad family with evident presence of the central and lateral nerves. It is grouped only by the genus Stangeria, represented in the natural South African species S. eriopus, with fern-like leaves.

Stangeria eriopus. Source: wikimedia.org

Zamiaceae

Plants with a large number of parallel, longitudinal, simple or forked nerves in their leaves. This family of cycads is the most numerous, comprising nine genera and more than 80 species native to America, Africa and Australia.

In the tropical and subtropical zone of America the genus Zamia is located, characterized by its almost imperceptible short stem. Representative species are Z. pumila, Z. furfuracea, Z. floridana, Z. latifolia, and Z. angustifolia.

In Cuba the species Microcycas calocoma is located, which grows more than 6 meters in height, it is currently in danger of extinction. The short-stemmed genus Macrozamia is native to Australia, comprising the species M. communis, M. diplomera, M. heteromera, M. lucida, M. moorei, and M. stenomera.

Distribution and habitat

At present, approximately 1700 species survive, distributed in 11 genera, which inhabit exclusively tropical and subtropical areas. The genera Encephalartos and Stangeria are located in central and southern Africa.

Zamia pumila. Source: flickr.com

Australia is the region where the greatest variety of species is found, including the Bowenia, Cycas, Lepidozamia and Macrozamia genera. In Southeast Asia there is a wide distribution of the genus Cycas.

For its part, in America, in the area from the Bolivian Amazon to southern Florida in North America, the genera Ceratozamia, Dioon and Zamia are located. Of these regions, Mexico is characterized by the presence of various endemic species.

Reproduction

Cycads are dioecious, that is, they are species that present individuals with separate male and female reproductive organs. In this context, they are plants that reproduce in a sexual and asexual way.

Strobilus of Chicada. Source: flickr.com

The reproductive structures are formed in the upper part or apex of the stem, where the characteristic cones of each sex develop. In fact, the female or male cones are practically the only structures that allow differentiating the sex of each plant.

These species develop a conical fruit of bright yellowish colors called strobilus. In female plants it is known as ovuliferous strobilus, and in male plants polliniferous strobilus.

Sexual reproduction takes place when pollen from the male plant fertilizes the female's egg through pollination. This process occurs thanks to the intervention of the wind, or of insects, mainly coleopterans.

Pollen penetrates the pollen chamber through a pollinating drop or nectar, the formation of which is a cyclical phenomenon that culminates when pollination occurs. The time that pollination and subsequent fertilization takes is five months.

Asexual reproduction occurs when the plant develops lateral shoots at the base of the stem, called “children”. These structures are genetically identical to the mother plant.

Current situation

Today cycads are classified as threatened species, various studies based on the analysis of geographic patterns and the evolutionary history of the species confirm this.

At the international level, they have protection through the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). For decades the various species that comprise cycads have been looted from their natural habitat.

Mexico was one of the main sources of illegally extracted material, damaging the native flora and fauna with this type of exploitation. Fortunately, the application of legal instruments and awareness raising has drastically reduced these practices.

One of the projects that has allowed its conservation is sustainable management through wildlife management units. The idea is to maintain individual nurseries of each species in its natural habitat, producing new individuals in a controlled manner.

Through this technique, the loss of illegally stolen plants has been corrected. In effect, establishing and interpreting the guarantor factors of each species will increase the chances of success of conservation strategies.

References

1. Cycadopsida (2018) Wikiwand Wikipedia Modernized. Recovered at: wikiwand.com
2. Domínguez, LM, Morejón, FN, Silva, FV, & Stevenson, DW (2018). Cycads and Genetic Bar Codes. Science and Development, 64.
3. Iglesias Andreu, LG, Octavio Aguilar, P., Sánchez Coello, N., Baldo Romero, Á., & Casas Martínez, JL (2012). The determination of sex in Cycads (Cycadales).
4. Rivadeneyra-Domínguez, E., & Rodríguez-Landa, JF (2014). Cycads and its relationship with some neurodegenerative diseases. Neurology, 29 (9), 517-522.
5. Sánchez de Lorenzo-Cáceres, Jose M. (2003) Las Cícadas. Fossils of the past. Recovered in: arbolesornamentales.es
6. Vovides Andrew P. (2000) Mexico: second in the world in diversity of Cycads. CONABIO. Biodiversitas 31: 6-10.