Introduction
Vascular plants are a class of numerous plants which consist of a vascular system or a well-developed phloem and xylem. Phloem transports organic nutrients such as sucrose while the xylem system transports mainly minerals and water. These plants are also known as tracheophyte. Plants with well-developed vascular systems include the ferns, angiosperms, and gymnosperms. Amongst these groups, the oldest tracheophytes (Cooksonia) were discovered about 439 million years ago during the Silurian period ( Biswas & Johri, 2013) . The now extinct Cooksonia had unique characteristics such as they rose a few centimeters from the soil. They also had branching stems with no roots and leaves. The presence of a dark stripe in one of the five species of Cooksonia suggested the fragments of the vascular system. Later after the discovery of Cooksonia that is during the Devonian period, Psilophyton were discovered ( Wang & Ran, 2014) . These plants were similar in structure to Cooksonia, but they had a different branching pattern. Also, some of the species of Psilophyton grew up to two feet of the soil. In the late Devonian period another group of plants belonging to the tracheophytes was discovered. These were the Clubmosses and Ferns. These plants differed from the Cooksonia and Psilophyton due to the presence of the leaves and root systems. Ferns developed the first leaves with full developed branching veins. These early groups of tracheophytes did not disappear entirely for example about 20,000 species of the ferns and 1200 species of clubmosses are present today ( Wang & Ran, 2014) . Amongst the three broad groups of vascular plants: gymnosperms, angiosperms and the seedless vascular treess, our discussion shall focus on Gymnosperms.
The Life Cycle of Gymnosperms
Gymnosperms, just like many other plants, undergo the alteration of generations, meaning that their life cycles encompass both the haploid and diploid stages ( Biswas & Johri, 2013) . In the diploid stage, the cells consist of two groups of chromosomes while in the haploid stage only one chromosome is present. The process of reproduction in these plants takes relatively longer periods than other plants. For instance, the plants usually take approximately one year from the time of pollination until the whole process of fertilization is complete. Also, after the seeds have been formed some species hold them until explicit environments are met and even after the unique conditions are met, the seeds may lie dormant for several additional years before germination occurs ( Biswas & Johri, 2013) . The following diagram illustrates the steps of the gymnosperm life cycle.
Delegate your assignment to our experts and they will do the rest.
Discussion
Distribution of Gymnosperm around the World
In the world, there are about seven hundred and twenty-five species of gymnosperms. These species are distributed throughout the tropical and template areas of the world. They are however more dominant in the cold regions and account for approximately a third of the ecosphere’s forest. The figure 5 above shows the distribution of the gymnosperms across the world.
The Cycadales, which are mainly found in the subtropical regions of the earth consist of nine genera and approximately one hundred species. Out of these genera, four (Dioon, Microcycas, Zamia, and Ceratozamia) are mainly distributed in the Western Hemisphere while the remaining (Encephalartos, Bowenia, Macrozamia, Stangeria and Cycas) are highly found in the Eastern region ( Fragniere et al., 2015) .
.
The Conferales are widely distributed in countries in and around Pacific Ocean, New Zealand, Australia, Japan, China and many other parts of the world. They are the key forest markers of the earth consisting of fifty-four genera and more than five hundred species.
The Taxales consist of one family (Taxaceae) made up of five genera: Taxus, Pseudotaxus, Austrotaxus, Amentotaxus, and Torreya. Taxus is majorly found in North America, Asia, and Europe and spreads into the Malaysian regions. Austrotaxus is specifically restricted to New Caledonia while Pseudotaxus class are only found in a little area of eastern China. Torreya is found in East Asia, California, and Florida. The last group (Amentotaxus) is only found in East Asia ( Fragniere et al., 2015) .
The Ginkgoales consist of only one living species (Ginkgo biloba) found only in a small part of southern China. However, the plant is extensively cultivated in Japan and China for its eatable seeds ( Fragniere et al., 2015) .
The last group the Gnetales consist of Gnetum, Ephedra and Welwitschia genera. Gnetum occurs in the rainforests of Southern Amazon basin, China, Malaysian, South America, West Africa, India, and some Pacific Islands ( Fragniere et al., 2015) . Ephedra occurs in dry regions of North and South America and the broad belt from China to the Mediterranean. The last genera (Welwitschia) is found in South West Africa.
The Economically Important Gymnosperms
The Cycadales
These are seed plants that have a stout and woody stem with a large, stiff and hard, always green leaves. These plants can be used:
As food: seeds of Cycas are edible, stems of Cycas revoluta are suitable bases of Sago starch, the seeds and stems of Cycas revoluta are used to manufacture wine, and Zamia is rich in starch ( Wiersema & Leon, 2016) .
As medicine: Leaves of Cycas circinalis and the pollen grains of some Cycas species, which contain a numbing effect.
As ornaments: Some species of Cycas are used correctly for decoration purposes
The Cycadales are mainly found in the Eastern and Western hemispheres.
The Ginkgoales
The Ginkgoales is a seed producing plant made up of only one species (Ginkgo biloba). The plant is mainly used as a medicine for curing Alzheimer’s disease ( Wiersema & Leon, 2016) . Additionally, the plant is used as ornaments due to its beautiful ornamental leaves. The seeds of this plant are also edible, and therefore it is a food source just like Cycadales. The plant is a highly adaptable plant and can grow in both the template and the Mediterranean climates.
The Taxales
These plants are often included in the order of Coniferales but they differ from confers due to their solitary ovules. They consist of one family (Taxaceae) made up of five genera: Taxus, Pseudotaxus, Austrotaxus, Amentotaxus, and Torreya. The Taxus leaves are used as medicines, and its park is used to manufacture drugs known as taxols (anti-cancer drugs) ( Wiersema & Leon, 2016) . These plants can be grown in Asia, North America, Europe, Calfornia, Florida and eastern China.
Plant Diseases that Affect Gymnosperms
Cankers
These usually form on the woody stems. They include the Nectaria Canker and Cytospora Canker. Necteria Canker causes the death of twigs and branches of most gymnosperms such as the taxus, Ginkgo biloba and Cycas species ( Van der Plank, 2013) .
Verticillium Wilt
Verticillium species are fungi that are living in the soil, which attack roots of several woody and herbaceous plants. The fungus attacks the heart and can travel up the xylem or release spores which travel up quickly ( Van der Plank, 2013) . Given that these fungi interfere with the xylem that is essential for the transportation of water and mineral salts means that the plant dies more quickly.
Conclusion
As discussed above, gymnosperms are some of the old plants that form most of the earth’s forests. These plants are important as they can serve as food, ornamentals and also be used as medicines in curing some of the challenging diseases such as cancer and Alzheimer’s disease. Therefore, conserving these plants is extremely essential.
References
Biswas, C., & Johri, B. M. (2013). The gymnosperms . Springer Science & Business Media.
Fragniere, Y., Bétrisey, S., Cardinaux, L., Stoffel, M., & Kozlowski, G. (2015). Fighting their last stand? A global analysis of the distribution and conservation status of gymnosperms. Journal of Biogeography , 42 (5), 809-820.
Van der Plank, J. E. (2013). Plant diseases: epidemics and control . Elsevier.
Wiersema, J. H., & Leon, B. (2016). World economic plants: a standard reference . CRC press.
Wang, X. Q., & Ran, J. H. (2014). Evolution and biogeography of gymnosperms. Molecular phylogenetics and evolution , 75 , 24-40.
