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.[citation needed] Australia has 26 species, while the Indo-Chinese area has about 30.[citation needed] India has 9 species.[citation needed] The northernmost species (C. revoluta) is found at 31°N in southern Japan.[citation needed] The southernmost (C. megacarpa) is found at 26°S in southeast Queensland.[citation needed] Due to the occurrence of large number of Cycas species in China, Australia and India, those countries are considered as centres of Cycas diversity.[4][failed verification]
Evolution
Cycas sp.
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[5] to the Carboniferous.[6] 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.[7] 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.[8] 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.[6] 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.[9]
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.[clarification needed] 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.
Cycas 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.[citation needed]
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.[10]Cycas rumphii and Cycas pectinata have the most widespread distribution.
^ abcHill, Ken; Leonie Stanberg; Dennis Stevenson. "The Cycad Pages". Genus Cycas. Royal Botanic Gardens Sydney. Archived from the original on 2021-03-01. Retrieved 6 September 2013.
^Kramer, K.U.; Green, P.S., eds. (1990). Pteridophytes and Gymnosperms(PDF). The families and genera of vascular plants. Vol. 1. Assisted by E. Götz (illustrations). Berlin: Springer-Verlag. p. 370. ISBN978-3-540-51794-8.
Singh, R.; Radha, P.; Khuraijam, J.S. (2015). "A new species, a new combination and a new subsection of Cycas from Odisha, northern Eastern Ghats of India". Asian Journal of Conservation Biology. 4 (1): 3–14.
