Gymnosperms & their classification
Gymnosperms are plants with naked seeds that are often in cones.
They have no flowers.
They developed hundreds of millions of years ago and many are known only by fossil records.
The largest known trees, and the longest living plants, are gymnosperms.
Most of the living gymnosperms are evergreen trees or shrubs with distinct roots, stems and leaves.
In most the stem is woody and branched; cycads are less woody and unbranched.
A characteristic feature is the presence of leaf scars on the stem.
Plants often have 2 different types of leaves – foliage leaves which are simple, needle-like or pinnately
compound and minute scaly leaves.
Gymnosperms reproduce by spores which, after fertilisation, form seeds.
Individual plants may be monoecious (both male and female) or dioecious ( either male or female).
Male cones are smaller than the female and shorter lived.
Cones, or strobili, consist of a central stem surrounded by closely packed, modified leaves or sporophylls
which hold the spore producing sporangia.
The sporophylls are arranged in a spiral or in ranks. The cone scales of conifers are modified stems.
Male or staminate cones produce microspores in microsporangia on the lower surface of microsporophylls
and these develop into pollen grains – usually in vast amounts.
Female or ovulate cones produce ovules in megasporangia attached to the upper surface of megasporophylls.
After fertilisation a seed forms which is attached to the megasporophyll but has no protective ovary
covering them thus the description of a naked seed.
When the seed cone is mature the seed scales break apart releasing the seeds which are often winged.
The whole cycle may take up to 3 years to complete.
Classification of the Gymnosperms.
There are a number of current classifications of the gymnosperms with variations in ranks and families.
I have used:
A. Division Cycadophyta > Cycadaceae, Strangeriaceae and Zamiaceae.
B. Division Ginkgophyta with 1 family, Ginkgoaceae – Ginkgo or maidenhair tree.
C. Division Gnetophyta with Gnetum, Ephedra and Welwitschia.
D. Division Pinophyta or Coniferae with all living conifers.
- Podocarpaceae including Phyllocladaceae.
- Pinaceae with pines, cedar, larch, spruce plus some smaller genera.
- Cupressaceae includes cedars, cypress, redwood, sequoia and a few others
- Taxaceae, the yew family which often now includes the closely related Cephalotaxaceae.
- Sciadopityaceae the umbrella pine.
plus most genera previously in Taxodiaceae.
Conifers are the most common gymnosperms seen and include the tallest and most massive trees in the world –
the Sequoias and Redwoods in North America.
There are 22 species native to Australia in families including:
Cupressaceae – Tasmanian cedar (was in Taxodiaceae) and cypress pine.
Podocarpaceae – the Huon Pine, plum pines and Celery-topped Pine.
Many non-native species are cultivated as ornamental plants and a few, including, Pinus radiata are widely
used in plantations or as windbreaks.