The Cycadophytes


The Cycads are an ancient phylum, surviving through the age of the dinosaurs to modern times. Today, the cycads are divided into 3 families, 11 genera and 250 species. There is a great deal of variation within the phylum, but a few unique characteristics hold them together and differentiate them from other seed-bearing plants.


Characteristics of Cycadophyta:


Girdling Leaf Traces


Also found in some ferns, these leaf traces arise from the stele at a point opposite the point of leaf attachment and encircle the stem.



Coralloid roots


These secondary roots are found on all Cycads. They form masses of lightly colored, club shaped structures, at or near the surface of the soil. Usually containing cyanobacteria, they are important in nitrogen fixation.



Family Characteristics

CycadaceaeZamiaceaeStangeriaceae
female sporophylls not in conesfemale sporophylls in conesfemale sporophylls in cones
leaflets with a single unbranched midrib and no lateral veinsleaflets with multiple branching veinsleaflets with multiple branching veins
germination platyspermicgermination radiospermicgermination radiospermic
carpophylls multi-ovulatecarpophylls bi-ovulatecarpophylls bi-ovulate
ovules ascendingovules invertedovules inverted
stipules presentstipules presentstipules absent
leaflet vernation circinateleaflet vernation flatleaflet vernation circinate


Reproduction in Cycads

Cycads are dioecius plants (separate male and female plants) and eusporangiate. Development of micro- and megagametophytes takes place in cones that are apically placed on the male and female plants, respectively. Wind was previously thought to be the major pollination mechanism for cycads. That idea has now been replaced with insect pollination, typically weevils and other small insects.

"The cycad cone, male or female, is a fertile, determinate shoot, in which the sporophylls are, in effect, reduced fertile leaves"(Norstog and Nicholls, 1997).

Female Reproductive Structures


Female cones are usually singular, only one per plant, though there are a few exceptions. There are typically two ovules per megasporophyll which are attached to the inner part of the sporophyll end, or shield. A mature cone (picture on the right) splits open and dehisces its seeds. In the picture given here, you can see the shields breaking apart, exposing the bright red seeds.



Male Reproductive Structures


Male cones can be singular, but more often there are two or more per plant. The microsporophylls are bladelike and the microsporangia are borne on their lower surfaces. The microsporangia are arranged radially in sori. The sperm of Cycads are flagellated, a trait not found in most seed-bearing plants. The picture to the left shows a cluster a several pollen cones and various stages of maturity. The cones that are fully mature have attracted a swarm of weevils.




Sources


Hill, K.D. & Stevenson, D.W. 1998.The Cycad Pages.http://plantnet.rbgsyd.gov.au/PlantNet/cycad/


Jones, David L. Cycads of the World. Smithsonian Institution Press. 1993.


Norstog, K.J., Nicholls, T.J. The Biology of the Cycads. Cornell University. 1997.


The Virtual Cycad Encyclopedia Palm & Cycad Societies of Florida. 1999. http://www.plantapalm.com/vce/toc.htm



Researched and written by Erin MacDonald