Botany

What are Gymnosperms



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The dark brooding spruce trees of the taiga, windswept cypress trees on the California coast, giant Norfolk pines dotting the skylines of beach towns along the Australian coast: what do they all have in common? They are gymnosperms, cone bearing plants which never flower and never lose their leaves. Once they were the dominant plant forms on earth. Now they compete with the Angiosperms, the flowering plants, and in most ecosystems they have been supplanted. But still the gymnosperms remain and in some places dominate: the giant Sequoias or redwoods in Northern California, the Ponderosa Pines of Eastern Oregon, the few remaining cedars of Lebanon and the vast subarctic spruce forests of Russia, Alaska and Canada. In other places humans have come to their aid for mutual benefit. Much of the native forests of the North Island of New Zealand have been replaced by introduced pine plantations and there are pine plantations in many parts of Australia too. These trees are softwoods and are relatively fast-growing, producing timber for the housing and paper industries in relatively short periods of time. Pinus radiata is the cow of the plant world, domesticated both for its own gain and that of its masters, trading long lives for numerous, cosseted offspring.

The word Gymnosperm means naked seed. Unlike the later Angiosperms, gymnosperms do not cover their seeds in a fleshy ovary. Instead they are attached to the surfaces of a modified leaf, which is held in a strobilus or cone. Microspores or pollen grains are produced by the male plants while the females produce megaspores, which when fertilized, develop into seeds.

They evolved some several hundred million years ago, when the earth was covered only in ferns and mosses. They are descendants of the first ferns to produce seeds, the so-called seed ferns or Pteridosperms, which were widespread by the end of the Paleozoic Era but died out in the Mesozoic. Externally the pteridosperms resembled fern trees but internally their structures resemble the most primitive gymnosperms, the Cycads. The cycads perfected the vascular systems that allowed for great size in trees and they perfected sexual reproduction, producing copious quantities of pollen, born by the winds to cones bearing female organs in which seeds were formed in large quantities, spreading their kind around the world and providing new food sources for the dominant life forms of the time, the dinosaurs. The cycads that exist today in subtropical ares of the world are living fossils, having evolved in the Permian Age about 250 million years ago and survived all this time essentially unchanged. Cycads look superficially like palm trees but palms are relative newcomers, flowering plants that evolved in the Cretaceous a mere hundred million years ago.

The other living fossil Gymnosperm is the Ginko tree, Ginko biloba. Ginkos were once widespread and they are common in the fossil record but were thought to be extinct. Then they were discovered growing in Buddhist monasteries in southeastern China. They were cultivated and due to the efforts of humans are now widespread again, being a popular street and garden tree.

The main group of gymnosperms, the conifers (Pinophyta), appear first in the fossil record from the late Carboniferous period, almost three hundred million years ago. Modern families appear in the Triassic Period, the first age of dinosaurs, including the ancestors of the modern redwoods and the Auracaria genus of pines found in Australia, New Zealand and Norfolk Island.

A common characteristic of conifers is to have leaves shaped like needles. This is a water saving device as the needles are compact. They can also be easily dropped and replaced when old and so the floors of pine forests are often covered in pine needles. Many pines and other gymnosperms have very acidic needles that when shed prevent the germination of other plants around the bases of the pine trees, thus preventing competition. Gymnosperms living in extreme climatic conditions also coat their needles in wax to prevent water loss. Conifers are especially cold-adapted and can live in subarctic conditions that would kill most broad leafed trees. For this reason, as one journeys north, one finds more and more conifers, and in the arboreal regions of the world they are the dominant tree species.

The biggest trees in the world belong to the conifers and so do the oldest. The biggest are the redwoods or sequoias, Unfortunately the largest ones were cut down by loggers, but a few huge examples still remain to awe us with their size. The oldest trees in the world also live in California, the bristlecone pines of the desert regions, some of which are as much as five thousand years old.

The living conifers are divided into seven major families, but three are quite small so I will just discuss the 'big four'. The first of these are the Araucariaceae, with three genera and forty species. These impressive pine-like trees live in the southern Hemisphere. The Wollomi Pine, an Araucaria, gained world-wide attention when it was discovered only a few years ago in a secret valley in Australia, where it had been growing for millions of years.

The second family, the Cupressaceae are the cypresses, with thirty genera and 142 living species. They are characterised by leaves in ranks of four or whorled and the sweet smelling Junipers are the best known. Redwoods are members of the cypress group as well. This is the most widespread of the conifer groups, being present on all continents except Antarctica.

The largest family is the Pinaceae with 232 species. This great family includes all the pines (genus Pinus) plus spruce trees (Picea), hemlocks, (Tsuga), Douglas Fir trees (Pseudotsuga), true fir trees (Abies) and the larches (Larix). These are also the most economically important of the conifers, with many species cultivated or managed for their valuable wood products.

The second largest family are the podocarps (Podocarpaceae) with 173 species. This family is less well known as it is mostly found in the Australasian region. Visitors to New Zealand will see the impressive Rimu and Totara trees although unfortunately many have been cut down and replaced with Pinus radiata for the timber industry.

The gymnosperms represent an important stage in the evolution of higher plants. Although they have been overtaken by the more diverse flowering plants they are still important in many ecosystems, especially in colder climates and high in mountainous regions. They are also very important economically and they are loved by many people for their beauty and their grandeur. Long may the gymnosperms flourish.

References: http://www.geocities.com/Heartland/Plains/6761/ecycads.htm http://hcs.osu.edu/hcs300/gymno.htm http://www.conifers.org/

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