The name gymnosperm literally translates to 'naked seeds' and gymnosperms are generally understood to be seed-bearing plants that do not flower. There are four classes of living representatives of gymnosperms: Cycadinae, Ginkgoinae, Coniferinae and Gnetinae. Each class is thought to represent a certain stage in evolutionary progression by different groups of ferns.
Fertilization of gymnosperms is different from that of most lower vascular plants considering that it is accomplished by wind. Pollen grains from male gametophytes are caught in the movement of air, then transferred to female gametophytes. A pollen tube then germinates from the pollen grain, which enters through the micropyle in the ovule to convey the sperm to the egg.The majority of female gametophytes of gymnosperms produce more than one archegonium, or egg producing organ, making it possible to have multiple embryos in the same ovule. However, usually only one embryo survives in each ovule.
The four classes of gymnosperms
Also known as the Cycads, the group of plants first appeared in the Carboniferous period around 320 million years ago, and they were in fact the dominant land plants when dinosaurs roamed the earth. Living in subtropical and tropical areas, they are palm like and usually have large distinct trunks and can reach 18 meters or more in height. At the moment, there are 9 genera and about 100 species of Cycadinae. This group of plants is considered to have true secondary growth, which basically means they can increase in width as well as in length.
Pollen and seed cones of Cycads are borne on different plants, which mean there are respectively male and female plants. Male cones are microsporophylls with numerous microsporangia while female cones are megasporophylls with more than one ovule. After deposition of pollen grains near the micropyle in ovules, germination occurs and pollen tube develops which absorbs nutrients as it grows allowing it to near the archegonium. The tube then burst and one single motile sperm is released. Therefore, in the case of Cycadinae the pollen grain does not bring the sperm directly to the egg.
Fossils have shown that Ginkgoinae dates back to the Permian period 280 million ago. However, the sole living survivor of this evolutionary line today is Ginkgo biloba, aka the maidenhair tree, easily recognizable due to its fan-shaped leaves with dichotomous pattern of veins. Different to most gymnosperms, Ginkgo is deciduous and its leaves fall in autumn after turning a beautiful golden color. There are actually no wild populations of this tree left in the world but they have been cultivated in temples in China and Japan for a long time. Seeds of this plant however, often have a rotten smell due to the butyric acid in their fleshy coats and therefore usually only male trees are found on the streets.
Like the Cycads, Ginkgo is also dioecious but its male and female cones are highly reduced and grow on short stalks. Fertilization might not occur until the ovules are shed from the trees. The micropyle of the ovule would secrete a sticky fluid when mature which allows pollen grains to stick to it and as the sticky fluid dries up, pollen grains are drawn in. In this case, two motile sperms are released from the pollen tube, which also only conveys part of the way towards the egg.
Also known as the Conifers, it is known to have survived since the Late Carboniferous period around 290 million years ago. Being the largest and most diverse of all gymnosperms, the group comprises of about 50 genera and 550 species. Plants in this group are usually very tall and can reach more than 100 meters in height. Examples include pine trees, firs and spruces, all of great economic value.
Pinus, or pine trees, is probably the most familiar to us among the group. With some 90 species, pine trees have broad stems which show secondary growth and leaves that are narrow and needle like, one of its few drought-resistant features. The plant is bisexual with small, clustered male cones on lower branches and large, solitary female cones on upper branches. The pollen grains are highly reduced with large air sacs, giving it low density and thus enhancing the dispersal by wind. The pollen tube of Conifers discharges the sperm directly into the cytoplasm where although more than one egg is fertilized, usually one develops.
There are only three extant genera and about 70 species of Gnetinae. The three genera are called Gnetum, Ephedra and Welwitschia and each differs greatly both in structure and the way they reproduce. In the case of Gnetophyta, the pollen tube also dischrages the sperms directly into the cytoplasm of the eggs.
Gentum is a group of about 30 species of tropical trees and vines with large leathery leaves that have a wide lamina and net-like veins. The male strobilus is basically layers of microsporangia and aborted ovules while the female strobilus is made up of whorls of ovules. No archegonium is formed with these plants.
The some 35 species of Ephera are tropical shrubs with small scale like leaves and apparently jointed stems. They consist of separate male and female plants with strobili appearing as series of bracts and archegonium is formed in the ovule.
Welwitschia mirabilis, the only extant species of Welwitschia, is found in arid areas of southwest Africa and it is a most bizarre plant. The majority of the plant in buried in sandy soil with the only exposed part being a large woody concave disk with two strap-like leaves. Male cones of this plant are tiny and are found short stalks while female cones are evidently much larger and no archegonium is formed in this case either. Plants in this group are actually surprisingly similar to flowering plants when compared to the other gymnosperms.