Gymnosperms - Cycadales Overview

Gymnosperms - Cycadales

• Gymnosperms belong to four phyla: Coniferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta.
• Cycadales is an order within the Cycadophyta phylum.
• Cycadales are commonly known as cycads.
• Cycads originated from seed ferns (Cycadofilicales) during the Carboniferous period and were dominant vegetation during the Triassic period of the Mesozoic era.
• The order includes eleven living genera and about 370 species.
• Most cycads are woody trees, except for Zamia pygmaea.
• Stems are mostly unbranched and covered with persistent leaf bases.
• Leaves are arranged in whorls at the stem apex and are pinnately compound.
• Wood is monoxylic.
• Micro- and megasporophylls typically develop on male and female strobili.
• Cycads are low and palm-like, typically 4-8 feet tall.
• Tallest species can reach up to 20 feet.
• Stems are unbranched, columnar, and covered with persistent leaf bases.
• Female reproductive structures (megasporophylls) are not aggregated in cones.
• Ovules (two or more) are on the lower margins in ascending order.
• Plant shows tuberous stems when young, progressing to a columnar and unbranched stem.
• Leaf-shoot apex is protected by a rosette of brown scale leaves.
• Plants grow slowly, adding a new crown of leaves every 1-2 years, alternating with scale leaves.
• Leaves are pinnately compound megaphyllous leaves with 80-100 pairs of leaflets on the rachis.
• Leaf bases are rhomboidal in shape, attaching the leaf transversely to the stem.
• Leaflets are thick, leathery in texture, ovate or lanceolate in shape, and photosynthetic.
• Scale leaves are very small, rough, and dry, triangular in shape, and brown in color.
• Roots are of two types: normal and apogeotropic coralloid.
• Normal taproots grow from the radicle deep into the soil, producing lateral branches.
• Some lateral roots grow apogeotropically towards the soil surface and branch dichotomously.
• These roots are short, thick, and swollen at the tips.
• Coralloid roots appear coral-like on the soil surface.
• They do not bear root caps.
• The cluster has lenticel-like apertures.
• Roots become infested by N2-fixing blue-green algae (cyanobacteria), bacteria, and diatoms.
• Examples include Nostoc punctiforme and Anabaena cycadacaerum.
• Symbiotic relationships are thus established.
• Vegetative reproduction occurs via bulbils developing in crevices of scale leaves and leaf bases at the basal part of an old stem.
• New plants are produced on detachment.
• Cycads are strictly dioecious (separate male and female plants).
• Male plants are rare.
• Male strobili (cones) are borne singly at the apex of the trunk.
• Cones are shortly stalked and large (up to 50 cm long or more).
• Microsporangia are confined to the abaxial (lower) surface.
• Microsporangia are present in sori, each with 2-6 sporangia containing haploid microspores (pollen grains).
• Female plants do not produce cones.
• A whorl of spirally arranged megasporophylls arises around the short apex.
• Megasporophylls resemble foliage leaves and are approximately 10-23 cm long.
• Lower petiole part bears naked ovules on margins.
• Largest ovules are (6 cm x 4 cm) seen in C. circinalis.
• Ovules are sessile, ovoid, or spherical in shape.
• Thick integument with three layers: outer and inner fleshy layers, and a middle stony layer.
• Ovules are well supplied with vascular bundles.
• There are many species, including Cycas circinalis, Cycas revoluta, Cycas rumphii, C. pectinata, C. siamensis, and Zamia sp.
• Characteristics of megasporophylls can be used to distinguish different Cycas species.

Tree Forms

• Different tree forms are depicted, including round, spreading, pyramidal, oval, conical, vase-shaped, columnar, open, weeping, and irregular.

Additional Information

• Cycad scale infestation is a severe issue.