Cycadales and Their Important Features

Cycadales are woody plants with a stocky cylindrical stem, bearing a crown of very large coarse leaves. They are not related to any other living plant group and represent a transitional stage between ferns and other gymnosperms. They also have naked seeds on modified leaves (sporophylls) and are true gymnosperms.

Cycads were more common during the mid-Mesozoic era and looked very similar to palms. They first appeared in Triassic times. There are 12 genera in 3 families and approximately 300 species distributed across tropical and subtropical regions of North, South, and Central America, Africa, a few Caribbean islands, eastern and southeastern Asia and Australia. 

Hierarchy Classification

• Kingdom: Plantae
• Division: Cycadophyta
• Class: Cycadopsida
• Order: Cycadales

Origin of Cycadales

Based on the anatomical studies by Worsdell in 1906, it is believed that Cycadales have originated from some members of Medullosaceae, a family of pteridosperms, popularly known as seed ferns. He concluded that cycads were descendants of Medullosa stellate, a medullated vascular cylinder.

Mamay, in 1969, concluded about the Palaeozoic origin of Cycadales based on his discovery that some cycas look like megasporophylls from the lower Permian period in the Palaeozoic era.

General Characteristics

• Such plants are dioecious. They have a sporophytic plant body, where the sporophyte is well-differentiated into roots, columnar unbranched stem, and pinnately compound leaves
• Young leaves display circinate vernation, and an extremely massive apical meristem is present in all cycads. Its members exhibit xerophytic characters
• The pith and cortex have mucilage canals
• Reproductive organs are in the form of cones located in a terminal or lateral position
• The abaxial side of microsporophyll consists of microsporangia. The microsporophyll forms a compact structure and arranges itself on the cone axis in male cones
• Sperm flagella are arranged in spiral bands, and the male gametes are large
• Megasporophylls have several ovules arranged in the middle region, and the tip portion is the sterile part. The ovules are orthotropous

Cycads Reproduction Mechanism

• Vegetative method- cycads reproduce vegetatively by the formation of bulbils or buds. These buds develop on the stem, and upon detachment from the stem, they develop into a mature plant
• Sexual Reproduction method-  In cycads, sexual reproduction takes by the formation of 2 different spores: the female spore that is large and non-motile and the male spore that is motile. The female and male spores fuse to form a new plant in the case of cycads, and this process is known as oogamous mode of sexual reproduction

Family of Cycads

There is no true female cone in cycads. The megasporophyll arises at the female plant’s apex and is loosely arranged in a rosette manner. The male cone is fusiform in shape and bears some spirally arranged microsporophyll.

Cycads are grouped into 3 distinct families known as Cycadaceae, Stangeriaceae, and Zamiaceae. In the Cycadaceae family, seeds are formed on the periphery of megasporophyll cells that are further aggregated in dense masses at the tip of the trunk. On the other hand, members of the Zamiaceae consist of seed cones, also known as strobila.

Cycadaceae Family

• It consists of deciduous trees and perennial herbs

• Vesicular-arbuscular mycorrhizal roots are present

• The unbranched or dichotomously branched stem is present

• Spiral, petiolate, pinnately compound leaves are present and have circinate vernation

• The strobili are large, terminal, with numerous microsporophylls, which bear many spherical microsporangia abaxially

Stangeriaceae Family

• The smallest family of Cycads is found in South Africa, Eastern Cape regions, and Australia

• The leaves are in the shape of ferns, with pinnae having a midrib and branching lateral veins
• The female cones have 2 ovules

Zamiaceae Family

• It consists of dioecious trees and perennial herbs

• Vesicular-arbuscular mycorrhizal roots are present

• Unbranched or irregularly branched stem with an aerial trunk

• The leaves are usually pinnately compound spiral, petiolate, stipulate and show circinate vernation
• The pollen cones consist of numerous microsporophyll, having many spherical microsporangia abaxially

Economic uses of cycads

• Cycads can be used as a source of starch collected from the trunk
• They have medicinal uses on a small scale. Young seeds of cycads are used to remedy bowel issues in India, Fiji, Sri Lanka, and some other areas
• The gum obtained from injured petioles and stems of some cycas is used as an adhesive, antidote for snake bites and insect bites, and on the malignant ulcers
• Leaves of certain cycas species act as a source of fibre
• The Ryukyu Islands and other areas in Japan use special methods to cut and dry the leaves for decorative purposes
• In Goa, Andamans, the Philippines, and Australia, fresh or dried leaves of Cycads are used in religious ceremonies
• Cycads fix nitrogen associated with cyanobacteria found in the roots that produce a neurotoxin called BMAA found in the seeds of cycads

Conclusion

Cycads are gymnosperms, seed plants with a stout and woody trunk with a crown of large, hard, stiff, evergreen, and pinnate leaves. They are dioecious; individual plants of a species are either male or female. Cycads trunks vary from only a few centimetres to several metres tall and grow very slowly and live very long. Due to their resemblance with palms, they are often mistaken for palms or ferns, angiosperms. Cycads are also of great economic importance and are used widely throughout.