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Somewhere
in the complex evolutionary pattern between the ferns and seed-plants
came the cycads, which are tropical and relatives of other cone-bearing
plants.
 Cycads are the
survivors of a plant group that was abundant in the Mesozoic flora and
reached its zenith in the Jurassic Period, some 160 million years ago.
Consequently, cycads are often referred to as "living fossils" and
"coelacanths of the plant world."
Cycads once had an
almost worldwide distribution. Fossils have been found as far north as
Alaska, Britain, Greenland, and Siberia and as far south as Antarctica.
Today, these plants are confined in nature to limited areas in tropical
and subtropical regions around the globe. The present distribution
pattern of the eleven genera may be due in part to continental drift
between 50 and 250 million years ago. The pattern exemplifies the way a
group of plants, presumably descendent from one common ancestor,
underwent a series of genetic changes as a result of geographical
isolation and consequential ecological differentiation to give rise to
several genera.
Botanists have long been
fascinated by this group of plants because cycads represent a living
link to a prehistoric time long before the dominance of flowering
plants. They are intrigued by the unusual combination of ancestral,
derived, and unique features. The layman's, landscape designer's, and
horticulturist's fascination with cycads are founded only partially on
the antiquity of this plant group. The size, shape, and symmetry of the
trunks and foliage provide significant decorative touches to the
landscape, while the curious and often colorful cones are
an added bonus.
In spite of the
scientific and popular interest they engender, cycads remain among the
least understood with respect to their evolutionary relationships when
compared to other seed plants. Only during the last decade have
intensive systematic investigations been undertaken by researchers.
Based on reports from our colleagues, there are still many yet
undescribed and unpublished cycad taxa. Also, debate continues on the
taxonomic circumscriptions of the presently described species. Today,
the group Cycadales includes eleven genera of cycads. Ceratozamia.
Chigua, Dioon, Microcycas, and Zamia are confined
to the Americas. Stangeria and Encephalartos are found
in Africa. Three genera are endemic to Australia: Bowenia, Macrozamia,
and Lepidozamia. Australia, Asia, and the South Pacific and Indian
Ocean islands contain species within Cycas. Approximately 300
species are recognized within the Cycadales, with new species being
found and described on a regular basis as field work intensifies in the
unexplored tropical regions of the globe.
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