Cycas

Cycas is a genus of cycad, and the only genus in the family Cycadaceae. About 113 species are accepted, which are native to the Indo-Pacific, East Africa and Madagascar. Cycas circinalis, a species endemic to India, was the first cycad species to be described in western literature, and is the type species of the genus. The best-known Cycas species is Cycas revoluta.
Range
The genus is native to the Old World, with the species concentrated around the equatorial regions - eastern and southeastern Asia including the Philippines with 10 species (9 of which are endemic), eastern Africa (including Madagascar), northern Australia, Polynesia, and Micronesia. Australia has 26 species, while the Indo-Chinese area has about 30. India has 9 species. The northernmost species (C. revoluta) is found at 31°N in southern Japan. The southernmost (C. megacarpa) is found at 26°S in southeast Queensland. Due to the occurrence of large number of Cycas species in China, Australia and India, those countries are considered as centres of Cycas diversity.
Evolution
Cycas is thought to have a deep split from all other living cycads, with estimates of the timing of the split ranging from the Jurassic to the Carboniferous. Fossil seeds from the Middle Jurassic of England and British Columbia were suggested in a 2017 study to be more closely related to Cycas than other cycads, and were assigned to the same family, Cycadaceae. However, a later study suggested that these seeds could not be assigned to the stem-group of Cycas with confidence due to lacking the double vascular system that characterises the seeds of all living cycads. The leaf fossil genus Paracycas known from the Jurassic and Cretaceous of Europe has been suggested to be early representatives of the Cycas lineage by cladistic analysis. The earliest fossils assignable to Cycas are known from the Paleogene of East Asia, such as Cycas fushunensis from the Eocene of Northeast China, with East Asia likely representing the ancestral homeland of the genus.
Morphology
The plants are dioecious, and the family Cycadaceae is unique among the cycads in not forming seed cones on female plants, but rather a group of leaf-like structures called megasporophylls each with seeds on the lower margins, and pollen cones or strobilus on male individuals.
The caudex is cylindrical, surrounded by the persistent petiole bases. Most species form distinct branched or unbranched trunks but in some species the main trunk can be subterranean with the leaf crown appearing to arise directly from the ground. There are two types of leaves - foliage leaves and scaly leaves. The foliage leaves are pinnate (or more rarely bipinnate) and arranged spirally, with thick and hard keratinose. They are not permanent and fall off leaving back leaf-bases. The leaflets are articulated, have midrib but lack secondary veins. The scaly leaves are persistent, brown in colour and protective in function. Megasporophylls are not gathered in cones. Pollination takes place by air.
Reproduction
Cykas take about 10 years to reach sexual maturity, after years of exclusive vegetative growth, which occurs by bulbils arising at the base of the trunk.
Conservation status
Cycas species are threatened worldwide and almost all the species are listed in the IUCN Red List. Cycas beddomei is the only species of the genus Cycas listed in Appendix I of CITES. All other members of Cycadaceae are listed under Appendix II. Cycas rumphii and Cycas pectinata have the most widespread distribution.
Phylogeny
Other species:
Cycas aenigma Hill & Lindström
Cycas apoa Hill
Cycas arnhemica Hill
Cycas badensis Hill
Cycas bglswamyi Pant ex Agrawal, Akhtar & R.C.Srivast.
Cycas brunnea Hill
Cycas campestris Hill
Cycas candida Hill
Cycas cantafolia Jutta, Chew & Saw
Cycas changjiangensis Liu
Cycas chenii Gong & Zhou
Cycas collina Hill, Nguyên & Lôc
Cycas cupida Forster
Cycas darshii Srivastava & Jana
Cycas desolata Forster
Cycas dharmrajii Singh
Cycas distans P.I.Forst. & B.Gray
Cycas divyadarshanii Khuraijam & Rita Singh
Cycas elephantipes Lindström & Hill
Cycas falcata Hill
Cycas flabellata Agoo, Madulid & Callado
Cycas fugax Hill, Nguyên & Lôc
Cycas furfuracea W.Fitzg.
Cycas glauca Miquel
Cycas hoabinhensis Lôc & Nguyên
Cycas inermis Oudem.
Cycas javana (Miquel) de Laubenfels
Cycas lacrimans Lindström & Hill
Cycas lane-poolei
Cycas laotica Nguyên & Nguyên
Cycas litoralis Hill
Cycas mindanaensis Agoo, Madulid & Callado
Cycas miquelii Warburg
Cycas montana Lindström & Hill
Cycas nayagarhensis Rita Singh, Radha & Khuraijam
Cycas nathorstii Schuster
Cycas orixensis (Haines) Rita Singh & Khuraijam
Cycas pachypoda Hill
Cycas panzhihuaensis Zhou & Yang
Cycas papuana von Mueller
Cycas pectinata
Cycas pranburiensis Yang et al.
Cycas pruinosa Maconochie
Cycas sainathii Srivastava
Cycas sancti-lasallei Agoo & Madulid
Cycas saxatilis Hill & Lindström
Cycas scratchleyana von Mueller
Cycas seemannii A.Braun
Cycas seshachalamensis Rao et al.
Cycas sexseminifera Wei
Cycas shanyaensis Fu
Cycas sundaica Miquel ex Lindström & Hill
Cycas terryana Forster
Cycas tuckeri Hill
Cycas xipholepis Hill
Cycas yorkiana Hill
Cycas yunnanensis Cheng, Fu & Cheng
Cycas zambalensis Madulid & Agoo
Cycas zeylanica (Schuster) Lindström & Hill
References
External links
Hill, K.D.(1998–2004) The Cycad Pages, Royal Botanic Gardens Sydney. http://plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/index.html
Virtual Cycad Encyclopedia edited by the Palm & Cycad Societies of Florida
David, J. de Laubenfels, Cycas Taxonomy
Singh, R & JS Khuraijam (2013-) Cycads of India. http://www.cycadsofindia.in/
Terrence Walters & Roy Osborne (eds.) (2004), Cycad Classification: Concepts and Recommendations, CABI publishing,