The following points highlight the top five gymnosperms found in the world. The gymnosperms are: 1. Cycadales 2. Coniferales 3. Taxales 4. Ginkgoales 5. Gnetales.
Gymnosperm # 1. Cycadales:
Cycadales include only 9 genera and about 100 species. They are distributed only in the subtropical regions of the world. Out of these nine genera of Cycadales, four are distributed in the western hemisphere. The remaining five Cycadales are distributed in the eastern hemisphere.
The Cycadales of the western hemisphere (i.e. Zamia. Ceratozamia, Dioon and Microcycas) are represented mainly in the northern region of the globe while that of the eastern hemisphere (i.e. Encephalartos, Bowenia, Stangeria, Macrozamia and Cycas) are represented in the southern region of the globe.
Zamia of the western region and Cycas of the eastern region are the two genera which cross the equator towards other side.
Zamia reaches even up to South America, and Cycas is well-distributed in China, South Asia, Myanmar and Australia. Johnson (1959) has recognized a 10th genus of Cycadales, under the name Lepidozamia. As far as the distribution of the individual living genera of Cycadales are concerned, Cycas is distributed from Japan to Queensland and occurs widely in India and Madagascar.
Macrozamia, represented by 9 species and the single species of Bowenia are restricted to north-east Australia and Queensland in their distribution. About 14 species of Encephalartos are widely distributed in South America while the single species of Stangeria is restricted only to Natal.
Two species of Ceratozamia and 3 species of Dioon are restricted to Mexico, whereas single species of Microcycas is found only in Cuba. Zamia is represented extensively by its approximately 30 species from Florida to Chile.
Gymnosperm # 2. Coniferales:
The Conifers are noteworthy in their distribution. They are widely distributed in countries in and around Pacific Ocean, Australia, New Zealand, China, Japan and several other parts of the world. These are the dominant forest makers of the world, and are represented by 54 living genera and over 500 species.
Foster and Gifford (1959), however, mentioned the presence of only 50 genera and about 500 species of conifers in the present-day world, and these occur widely in both northern and southern hemispheres of the globe. Conifers are actually the tallest trees in the world and these are the existing oldest trees too.
Sequoia sempervirens attains a height of over 112 metres and some plants of Pinus aristata are still existing in the mountains of California since last 4000 years. 9 genera of the northern region are represented only in China. In a broader sense the family-wise distribution of Coniferales is also very significant.
Abietaceae, Taxaceae, Cephalotaxaceae and Taxodiaceae are the families of northern hemisphere, whereas Araucariaceae and Cupressaceae are represented chiefly in southern hemisphere. The members of Pinaceae are distributed in both the hemispheres. Libocedrus is distributed on all the four quarters of the Pacific Ocean, whereas Pinus is the only northern genus which crosses the equator.
Pinaceae (Abies, Cedrus, Larix, Picea, Pinus, Pseudotsuga and Tsuga) is essentially a family of northern regions of the world. Taxodiaceae, at one time, played an important role in the forests of the northern hemisphere, but now shows only a relict distribution.
Two species of Taxodium grow in lowland swampy regions of southern USA while its remaining one species grows luxuriantly in upland region from Texas to Guatemala. Taxodium was of widespread occurrence in the past in Western Asia, Europe and Canada.
Metasequoia was widely distributed in the past in North America, Greenland, Asia and several Arctic regions but now occurs only, in a very restricted area of Central China by its only one species (M. glyptostroboides). Sequoia sempervirens, the only living species of the genus, occurs naturally in a narrow coastal belt in the Californian region.
Sequoiadendron, the another genus of Taxodiaceae represented by only one species these days (s. giganteum) is found only in central California.
In the past, it grew widely in Europe and Greenland. Cryptomeria japonica is restricted only to the parts of China and Japan while the only living species of Sciadopitys (S. verticillata) is today restricted to Japan. In the southern region of the world Taxodiaceae is represented only by three species of Athrotaxis, confined mainly to Tasmania.
Cupressaceae is a family of about 22 genera and 150 living species. Eleven of its genera are monotypic. About half of its genera are found in northern hemisphere and the remaining half are distributed in southern hemisphere. Common genera of northern half of the globe include Cupressus, Chamaecyparis, Retinospora, Thuja and Juniperus while that of southern region are Callitris, Libocedrus and Papuacedrus.
Among the northern genera, Juniperus with its approximately 70 species is widely distributed. It occupies a continuous broad belt round the northern hemisphere. Papuacedrus occurs in New Guinea but extends into the northern hemisphere across the equator. Libocedrus is confined to New Caledonia and New Zealand while Callitris occurs only in New Caledonia, Tasmania and Australia.
Podocarpaceae is mainly a family of southern hemisphere, and only a small representation is observed north of the equator.
Podocarpus is found commonly from Patagonia to West Indies in the western hemisphere and from New Zealand to Japan in the eastern hemisphere. Dacrydium occurs naturally in New Zealand, Tasmania, Malaysia, Myanmar, southern parts of China and Chile. Phylloclade’s, another genus of the family, is found chiefly in New Zealand.
It also occurs casually in Philippines, New Guinea and Tasmania. Acmopyle is found in Fiji and New Caledonia, while Microstrobos occurs in Tasmania and eastern Australia. Microcachrys and Saxegothaea, the two monotypic genera of Podocarpaceae, are restricted to Tasmania and Chile, respectively.
Cephalotaxaceae, a family containing only one genus (Cephalotaxus) occurs today only in some regions of eastern Asia. Araucariaceae is today represented only by two genera viz. Araucaria and Agathis. Araucaria occurs widely in south America, Australia, New Guinea and New Caledonia while Agathis occurs chiefly in eastern region extending from Philippines to New Zealand and from Malaysia to Fiji.
Gymnosperm # 3. Taxales:
Taxales consisting of only one family (Taxaceae), are represented by five living genera viz. Taxus, Austrotaxus, Pseudotaxus, Torreya and Amentotaxus. Taxus, the most common genus of this order, is found widely in North America, Europe and Asia and extends even into Malaysian regions.
Austrotaxus is restricted to New Caledonia, while Pseudotaxus is found only in a small region of eastern China. Torreya occurs in eastern Asia, Floridian and California, while Amentotaxus is today confined to eastern parts of Asia with its only four species.
Gymnosperm # 4. Ginkgoales:
Ginkgoales, an order with a world-wide distribution in the past, is now represented by only one surviving species (Ginkgo biloba) with only a very limited distribution in the world. In the natural condition, if G. biloba at all occurs anywhere in the world it now occurs only in a small region of southern China. However, it is widely cultivated in China and Japan for its edible seeds.
Gymnosperm # 5. Gnetales:
Gnetales are represented these days by only three genera (Gnetum, Welwitschia and Ephedra). Gnetum occurs naturally in the moist tropical rain forests of Malaysia, southern China, India, West Africa, South America, Amazon basin and in some Pacific Islands, while Welwitschia mirabilis, the lone living species of the genus, is found only in a narrow coastal belt of about 1000 km long in south-west Africa. Ephedra occurs in arid regions of North America, South America and in a broad belt from the Mediterranean to China.