What Is a Cycad? Palm-Like Plants That Are Not Palms At All

Cycads are seed plants belonging to a lineage separate from the conifers, the ginkgo and the flowering plants, and their resemblance to palms is superficial, since palms are flowering plants and the similarity comes from a shared growth habit rather than from relationship. They have a thick unbranched trunk, sometimes largely underground, topped by a crown of tough divided leaves that are produced in flushes rather than continuously. Male and female plants are separate, and reproductive structures form as large cones at the centre of the crown, which in some species are among the largest cones of any plant. Growth is extremely slow, with some individuals taking decades to produce a trunk of any height, and lifespans running to centuries. Their roots host nitrogen-fixing bacteria in specialised structures, which lets them grow on poor soils.

Several features are ancient or otherwise uncommon among living plants:

• •Motile sperm that swim, shared with ginkgo and with ferns and lost in most seed plants
• •Beetle pollination, which is the ancestral mechanism and which most species rely on with specific beetle partners
• •Thermogenesis in the cones of some species, which heat up to disperse scent and attract pollinators
• •Coralloid roots hosting cyanobacteria, which fix nitrogen and are the only such arrangement among seed plants
• •Potent toxins throughout the plant, which have been studied in connection with a neurological disease cluster
• •Very slow growth and long life, which make populations slow to recover from any loss

The Mesozoic is frequently described as the age of cycads and the description needs qualifying. Cycad-like plants were genuinely abundant and are common as fossils from that period, and a substantial proportion of those fossils belong to a separate extinct group that resembled cycads closely and is not closely related to them, which was recognised only after the fossils had been catalogued together for decades. Actual cycad diversity in the Mesozoic was more modest than the popular picture, and molecular work indicates that most living species result from a comparatively recent radiation within the last fifteen million years rather than being survivors from the deep past unchanged. The lineage is genuinely ancient and the living species are not, which is a pattern found in several groups described as living fossils and which the label obscures.

The relationship with beetles is specific enough to be worth describing, since it is among the oldest pollination arrangements known and is far from the passive process it might sound. Male cones of several species heat themselves substantially above the surrounding air, driven by rapid metabolism in the cone tissue, and that heat volatilises scent compounds that attract the beetles. The beetles arrive, feed and breed in the male cone, becoming covered in pollen. In some species the cone then produces the same compounds at a much higher concentration, which repels the beetles rather than attracting them, driving them out and towards female cones which emit an attractive concentration. That push and pull mechanism was worked out comparatively recently and is a striking piece of manipulation by a plant, and it means the pollinator relationship is highly specific and correspondingly fragile.

Cycads are the most threatened major group of plants assessed, with a large majority of species at some level of risk and several extinct in the wild. Habitat loss accounts for much of it. Collection accounts for a great deal more, since mature plants are valuable to collectors and a specimen that took a century to grow can be removed in an afternoon, and illegal collecting from the wild has driven several species to the edge with documented cases of populations reduced to single figures. Their biology compounds the problem, since slow growth, separate sexes and specific pollinators mean that a reduced population may be unable to reproduce at all even if the remaining plants are protected. International trade is regulated and enforcement is difficult. Botanic gardens hold substantial collections and several species now exist only in cultivation.

Not palms but a separate seed plant lineage with swimming sperm, beetle pollination, heat-producing cones and nitrogen-fixing bacteria in specialised roots. Much of the Mesozoic fossil abundance belongs to a separate extinct look-alike group, and most living species come from a radiation within the last fifteen million years. They are the most threatened major plant group, largely through collection.