Zamiaceae in Australia

From Pacsoa
Jump to: navigation, search
Welcome to the PACSOA Palms and Cycads wiki !

If you have any information about this topic, please help by updating this article. Once you are registered you can contribute, change, or correct the text, and even add photos on this page. Click on the edit tab above and play around. Any mistake can be easily corrected, so don't be afraid.

Zamiaceae in Australia

Australia'a indigenous cycads are distributed throughout the continent but are absent from Victoria, South Australia and Tasmania. The best known fossil deposits of these ancient plants are in Victoria. One genus of the Australian Zamiaceae, Bowenia has a fern-like appearance and indeed is known hera as 'Byfield fern'. To describe cycads to the uninitiated, they are a group of woody stemmed plants, some with thick columnar caudices, others with the trunk completely subterranean. The usually unbranched trunk is covered with rows of persistent leaf-bases from ground level to apex. The fronds in a few species emerge circinate from the apex as do the ferns. The crown of fronds is the primary attraction of these plants. In infancy the young fronds are very tender, delicate and susceptible to injury. However in maturity they become tough and strong with a glossy surface in most species.

Contemporary cycads are remnants of an almost extinct flora which was once plentiful and dominated the vegetation of the earth. They are gymnosperms, plants that differ from the angiosperms (flowering plants) by reason that their ovules are not enclosed in an ovary.

It has been established that the cycads are descendant from an ancient and now extinct order of plants known as the Bennittitales which flourished in the Mesozoic Era, 230-65 million years ago. The Bennettitales became extinct during the Cretaceous and only one branch of its companion Cycadales survived; these being the ancestors of the contemporary cycads.

Charles Chamberlain was so convinced that cycads were directly linked with the ancient ferns that in his book, 'The Living Cycads' (1919), he went to great lengths to illustrate these beliefs explaining the similarities between fern and cycad. As with the ferns, sexual reproduction is carried out by means of ovules and motile male sperm, but however unlike ferns, cycads produce seeds, The following illustrates one of his theories; "... An extremely idealistic view, which might pass for a Devonian or Carboniferous heterosporous fern ancestor of the Cycadofilicales, or for one of the Cycadofilicales themselves (is as such). The large outer leaves are strictly vegetative. Just within the crown of vegetative leaves is a crown of smaller leaves bearing microsporangia on the under surface; and in the centre is another crown of small leaves beadng megasporangia upon their margins. The inner leaves, bearing megasporengia, have been modified, the modification consisting in a reduction in size and a simplification of the outline. The microsporophylls, the leaves bearing microsporangia, represent the first stage in the evolution of the cone. The inner leaves, the megasporophylls, show a distinct advance toward a structure which can be recognized as a cone. This is as far as the Paleozoic predecessors of the cycads have advanced in the evolution of the cone ..."

It seems that because of similarities between cycads and the fact that their world number is so few, most early botanists linked them together and often gave the same generic name to plants discovered even if in a different country. This happaned in Australia to plants of Macrozamia which at first were named Encephalartos. Now it can be accepted that the Cycadales is divided into three families Cycadaceae, with the sole genus of Cycas, Stangeriaceae, with the sole genus Stangeria and Zamiaceae which contains all of the remaining genera.

According to L.A.S. Johnson, Macrozamia consists of six species in section Macrozamia and eight species in section Parazamia. The genus Lepidozamia consists of two species, L. peroffskyana and L. hopei. There appear clear indications that the fossil cycads of Victoria are related to the north Queensland L. hopei, the fossils being named L. hopeites.

Study by taxonomists into Macrozamia reveals that section Parazamia is of comparative recent evolutionary origin. Mostly, they are smaller than those of section Macrozamia, the juvenile characteristics carry into the adult plant. Small subterranean caudices and toothed shiny pinnae are two things that are typical and appear in adulthood. Furthermore, the pinnaee on the rachis do not noticeably reduce in size at the base of the fronds. Section Parazamia occurs often with several of section Macrozamia Reports reach us that hybrids do occur between species from both sections, at least in Queensland. Botanical information suggests that section Parazamia in New South Wales could cross with Macrozamia communis, but this has yet to be seen.

Contributed by:

Len Butt (from Palms & Cycads No. 31).

External Links:

Google, Google Images, PACSOA Forums