When we look at a coniferous tree, or admire the age of a Great Redwood, we subconsciously recognise a botanic group known as the gymnosperms. The group includes the largest tree and the oldest tree and the slowest growing plant on earth. It includes the enigmatic Ginko tree which is a sole species survivor from the Jurassic age when dinosaurs roamed the Earth and shared this group. It also includes the cycad ferns that can be found in tropical and subtropical climates.
Despite their wide variety of forms, the gymnosperms share a common feature. Rather than produce flowers, these plants produce naked seeds. The seeds can be exposed in a wide variety of structures including the pine cone with which we are all familiar. The group name Gymnosperm is actually derived from a Greek word that means naked seed.
The gymnosperms evolved some time between the arrival of primitive plants such as ferns and the mosses and the flowering plants that we know and love. Historically the gymnosperms are first seen in the fossil record some 370 million years ago during the Devonian era. Although scientists are still debating whether the gymnosperms and flowering plants share a common ancestor, what they have found is intriguing. Despite their wide variety of forms, and the individual success of some species such as the conifers that have spread in large numbers, the gymnosperms are not represented by a great many species. Less than 1,000 species of gymnosperms are known, and of those over 50% are conifers, 3% Cycads and only one is a Ginko. The gymnosperms, particularly the conifers, are remarkable because they have been so successful, having spread in vast coniferous forests across the globe with comparatively little evolutionary change. The conifers that we recognise today are believed to have evolved in the Triassic era some 240 million years ago when conditions were much drier than in previous times.
The conifers might be regarded as the highest form of development among the gymnosperms. Biology and geography text books often discuss how conifers have adapted to the severe drought conditions that are found within the taiga of northern Canada and Siberia. During winter when the ground is frozen the conifer has little access to liquid water and is exposed to drying winds. The conifer has developed tough narrow leathery leaves to endure the drying winds.
Less well known are the water conservation features than are common to all gymnosperms. Although the gymnosperms are found in many climatic zones including temperate forests and subtropical and tropical zones where water is plentiful these features help explain how it is that the gymnosperms have been able to colonise two of the most arid conditions on earth, the taiga and the Namibian desert.
Crucially, the gynosperms do not require water for fertilization and they produce seeds. The male plant releases pollen which is transported by wind or insect. When the pollen lands on the female pine cone fertilization occurs when the sperm penetrates the egg. A seed may take up to three years to mature on the female plant. The seed is packed with foodstuffs which will support a young plant in harsh conditions. It also enables the plant to postpone germination if conditions are harsh.
The gymnosperms also have a very advanced system of vascular bundles that allow water and food transport across the plant. These systems are markedly advanced compared with their predecessors among the ferns and mosses. The gymnosperms deposit lignin within the cell walls. This protects against water loss from individual cells and provides strength to the extent that large tree structures are possible. Those conifers that endure very harsh conditions, such as the larch which is able to resist very harsh conditions in Siberia are also able to deposit an anti-freeze into the bundles to enable fluids to continue to flow during the winter.
Perhaps the most remarkable gymnosperm is the Welwitschia Mirabilis from the division Gnetophyta. These are extremely long lived and slow growing plants that survive in the extremely arid conditions of the Namib desert in which the rainfall is less than 10mm (0.4 inches) per year. The plant produces just two leaves which continue to develop for a thousand years and has a deep tap root. The leaves contain special cells that can extract moisture from the coastal sea fogs. The remarkable existence of the Welwitschia is such that it celebrated on the Namibia coat of arms.
