Cycads are a group of gymnosperm trees and shrubs. They are woody, seed producing plants with no flowers or fruit. Cycads often look similar to palm trees with branchless stems and a crown of leave at the top of the tree, but they are not at all closely related to palm trees.
They form the sub-class Cycadidae which belong to the class Pinopsida and includes all the gymnosperms plants. The cycads are widely distributed around the world but are mostly found in tropical areas. Many species are threatened with extinction due to habitat loss.
Cycad Growth form
Cycad plants grow as trees and shrubs. They typically have short trunks topped off with a green crown of large compound leaves. In appearance they closely resemble palm trees, however, they are not closely related.
Most species do not grow more than a few meter tall. Hope’s cycad of Australia is one of the tallest species and is known to reach 20 m (65 ft.) in height. More primitive cycads were often much taller than the majority of cycad species that currently exist.
Stems are usually unbranched and fallen leaves of the past leave leaf scars that encircle the stem. Internally, stems are mostly made up of soft storage tissue rather hard than wood.
The roots of cycads look very unusual and are known as coralloid roots because they have a similar shape to coral. Their roots share an important relationship with the blue-green algae, cyanobacteria. The roots provide the cyanobacteria with protection while the cyanobacteria supplies the roots with nitrogen based nutrients.
Distribution of cycads
The distribution of cycads is naturally mostly limited to tropical regions. They are found naturally in every continent except Europe and Antarctica and are now also commonly planted garden plants.
Cycads are found in a range of habitats including tropical forests, seasonally dry forests and savannas. One species of the African cycads, genus Encephalartos, has evolved to withstand the stress of living in desert environments.
Typically, cycad plants are most successful in habitats with well-drained soils. They are found in a large number of places but are never the dominant plants of an ecosystem.
Diversity and taxonomy
There is currently around 250 species of cycads living on Earth. They were most diverse during the Jurassic Period when they made up around 20% of the world’s plant species.
The cycads are gymnosperm plants and hence sit in the class of plants known as Pinopsida. They form the sub-class Cycadidae and the order Cycadales which includes three families and 11 genera. The majority of species belong to the family Zamiaceae which contains around 130 species.
Evolution of cycads
Cycads are an extremely old group of plants. They are known to have been present at least 280 million years ago from the dating of fossils found in China. They were most abundant in the Jurassic Period between 201-145 million years ago, occasionally referred to as the ‘Age of Cycads’ by botanists.
Reproduction and dispersal
Cycads produce separate male and female plants with distinct cones. Cones are often brightly colored in reds, oranges and yellows. Pollination is believed to be mostly completed by weevils and small bees which are attracted to seeds by heat and scent.
They reproduce with seeds which have fleshy outer layers to attract animals. Animals are the main vector for dispersing seeds into new areas. Cycads often grow in open areas and new it is common for seeds to germinate following fires.
- Over half of the world’s cycads are threatened with extinction in the wild but are helped by the fact that many species are commonly planted household plants.
- A number of species from the genus Encephalartos are referred to as bread palms because the insides of their starchy trunks can be used to make bread.
- Cycads can produce dangerous toxins that can affect the nervous system of animals, cause vomiting, and lead to cancerous growths.
Last edited: 22 May 2015