Coralloid roots
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.
| Cycadaceae | Zamiaceae | Stangeriaceae |
|---|---|---|
| female sporophylls not in cones | female sporophylls in cones | female sporophylls in cones |
| leaflets with a single unbranched midrib and no lateral veins | leaflets with multiple branching veins | leaflets with multiple branching veins |
| germination platyspermic | germination radiospermic | germination radiospermic |
| carpophylls multi-ovulate | carpophylls bi-ovulate | carpophylls bi-ovulate |
| ovules ascending | ovules inverted | ovules inverted |
| stipules present | stipules present | stipules absent |
| leaflet vernation circinate | leaflet vernation flat | leaflet vernation circinate |
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