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WHAT ARE CYCADS?

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Here below you will find the original version of experts Moretti and Osborne

What are cycads?

The present-day group of plants known as cycads, technically members of the order Cycadales, are the diverse and modified progeny of a much larger group of cone-bearing ancestors that flourished in the Mesozoic Era, reaching a zenith of global radiation in the Jurassic Period, some 160 million years ago. These enigmatic plants have motile sperm cells like those of the ancient seed-bearing ferns but bear embryo-containing seeds as do modern-day flowering plants. Cycads are a key component in biology’s evolutionary jigsaw puzzle, but how closely the present-day plants are related to their Jurassic ancestors is uncertain. A popular view is that the extant cycads are unchanged survivors from prehistoric times—the quaint terms “living fossils” or “coelacanths of the plant world” are used romantically but not strictly correctly in this sense. A better-informed opinion is that these plants constitute a small but robust group that has continually evolved over time to exploit a variety of niche positions in an increasingly competitive floral society.

Although for a major plant group, the number of cycad species—now about 330—is relatively small, there is considerable diversity within the order. Growth habits range from tall, unbranched, palm-like forms, to dwarf plants with reduced leaves and small subterranean trunks. Cycad habitats, too, vary from tropical rainforest to temperate alpine environments, as well as seasonally wet woodlands and desert cliffs. In morphological terms, persistent leaf bases and girdling leaf traces provide mechanical support to a stem that does not have true woody tissue. The group has a suite of unique phytotoxins, and has symbiotic associations with specific nitrogen-fixing cyanobacteria that inhabit apogeotropic (upwardly-growing) coralloid root clusters. Some cycads also have contractile roots that draw the caudex downward as the stem extends, an adaptation that protects the growing point from fire, predators and climatic extremes. In addition to these unusual vegetative features, theCycadales have many derived reproductive structures and strategies. These include being dioecious (having separate male and female plants), bearing brightly colored seeds, and having cones that undergo thermogenesis (a curious pattern of diurnal heating) that releases pheromone-like compounds that target specific co-dependent insect pollinators. In contrast, the free-swimming spermatozoids, liberated during the fertilization process, represent an ancestral condition found in much earlier plant forms. The sporadic cases of sex changes (in both directions) add further interest and complexity to these plants.

Many of the cycad species occupy very specific habitats over small areas, and these plant populations are often low in number. At present, more that 50 cycad species are now on the IUCN Red List as critically endangered. Therefore the efforts of several international botanical gardens in maintaining ex-situ breeding colonies is vital. Gardeners and landscape architects find that the shape and symmetry of cycad foliage and trunks is visually appealing, and that the curious and often colourful reproductive structures are an added bonus. An increasing horticultural interest will surely promote commercially-viable nursery cultivation and further help to save these plants from extinction.

(Aldo Moretti e Roy Osborne)