||Photo Ken Hill|
Introduction to Cycads
The cycads are a small group of plants with many unique features,
an ancient origin and a very long history.
Cycads are known
to have lived in the Permian era, over 200 million years ago - even
before the dinosaurs roamed the earth. Although once abundant
across the globe, the cycads are now greatly reduced in both
numbers and distribution.
There are now about 250 species in 11 genera, compared to possibly 300,000
species of flowering plants, the group that now dominates the world vegetation.
All cycads are tropical or subtropical and each genus has a restricted
The very large divided leaves means that cycad
plants resemble palms or tree-ferns in overall appearance.
Cycads, however, differ greatly in almost all aspects of
detailed structure and reproductive behaviour. Cycad plants
are dioecious (i.e. male and female reproductive structures
are borne on separate plants), and reproduction is by
seeds, which are produced on open carpophylls or seed-bearing leaves.
Although technically woody plants,
unlike other woody plants,
cycads possess a pachycaul stem.
This is a thick, soft stem or trunk
made up of mostly storage tissue with very little true wood.
Within the trunk, leaf traces or veins leading to leaves
arise at a point opposite the attachment of the leaf, and
circle the trunk within the storage tissue. These are known
as with girdling leaf traces, and occur in some ferns but
no other seed plants.
The coralloid roots of cycads are also characteristics not seen in other seed
plants, and the cycads lack the axillary buds seen in other seed plants.
The living cycads (division Cycadophyta) include about 250 species,
with 11 genera in 3 families. The cycads have been classified
as "gymnosperms" in the past, although recent
studies have shown that this is not a natural group, some
members being closer to the flowering plants than to other "gymnosperms".
The "gymnosperms" are all ancient seed plants, many
now extinct, but with four major groups living today. These are the
cycads (Cycadophyta), the Welwitschia group (Gnetophyta), the
conifers (Pinophyta) and Ginkgo, the Maiden Hair Tree
(Ginkgophyta). They are now regarded as quite separate and
distinct classes of equivalent status to the flowering
plants (Angiosperms or Magnoliophyta).
Although the four groups of "gymnosperms" are very different from
one another in appearance
they all produce naked ovules in contrast to the flowering plants in which
the ovules are enclosed in an ovary.