Gymnosperms, with the name literally meaning ‘naked seeds’, are a parallel line of seed-bearing plants with each class likely to be representing a specific stage in evolutionary progression. There are altogether four types of gymnosperms, according to their order on the evolutionary tree: Cycadophyta, Ginkgophyta, Coniferophyta and Gnetophyta.
Cycadophyta, or the Cycads, first appeared in the Carboniferous period around 320 million years ago and were the dominant land plant at the time of the dinosaurs. They are subtropical to tropical palm like plants and usually have large, distinct trunks. They are considered to be plants that have true secondary growth, that is, they increase in width rather than in length.
The reproductive structures of the Cycads are in the form of cones and the plant is dioecious, meaning that there are male and female plants. Male cones are composed of microsporophylls, bearing numerous microsporangia, while the female cones are composed of megasporophylls, bearing a few ovules.
Fossils of this phylum dates back to the Permian period 280 million years ago. The entire phylum of Ginkgophyta is now represented by only one surviving species, which is Ginkgo Biloba, alternatively known as the ‘maidenhair tree’. There are no natural populations of this plant left and their survival is solely due to cultivation in temples in China and Japan.
The plant’s leaves are distinctively fan-shaped with dichotomous veins. Unlike most gymnosperms, the tree is deciduous, meaning that it loses its leaves according to the season. This happens in autumn after the leaves turn a beautiful golden colour. Like the Cycads, Ginkgo is dioecious and its male and female cones are very highly reduced. The male part is composed of stalked microsporangia while the female part is composed of paired ovules on short stalks. The seeds of the plant have a rotten smell due to the butyric acid in their fleshy coats and thus usually only male trees are cultivated on streets.
Also known as the Conifers, the phylum dates back to at least the Late Carboniferous period some 290 million years ago and is the most diverse of all gymnosperm groups with about 50 genera and 550 species. Tall vascular plants can be found in this group.
An example of the Conifers is the genus Pinus, better known simply as pine trees. There are some 90 species of pine trees and they have all leaves that are very narrow and are needle like. Their stems, however, are broad and show secondary growth. The plant’s xylem contains trachieds and phloem contains sieve cells, showing that it is a primitive plant.
Pines trees are bisexual, with small, clustered male cones on lower branches and large, solitary female cones on upper branches. One single female cone is similar in size to an entire cluster of male cones and the main axis of female cones can be considered equivalent to its branch. Other examples include firs and spruces and plants of this phylum are often of great economical value.
With only about 70 species, there are only three extant genera of the phylum and each genus differs greatly both structurally and reproductively. Their only common characteristics are that the pollen tube discharges sperms directly into the cytoplasm of the egg and the xylem of plants of this phylum contains vessel members rather than trachieds like other gymnosperms. The three genera are the Gnetum, the Ephedra and the Welwitschia.
The Gnetum is a group of 30 species of tropical shrubs or climbers that have leaves with a wide lamina and net-like veins, which feels leathery to the touch. The plant contains male and female strobilus. The Ephedra is a genus containing about 35 species of tropical shrubs living in arid areas. Their leaves are small, scale like and there are separate male and female plants. There is only one extant species of Welwitschia, the Welwitschia mirabilis, and it is found in arid areas in southwest Africa. The majority of the plant is buried in sandy soil, with the only part showing being two strap-like leaves. Plants in this phylum actually share many similar characteristics with flowering plants.