Gymnosperms are a group of plants which produce seeds that are not contained within an ovary or fruit. The seeds are open to the air and are directly fertilized by pollination.
“Gymnosperm”, from the Greek, gymnos, “naked” and sperma, “seed”, develop their seeds on the surface of scales and leaves, which often grow to form cone or stalk shapes, contrasting in characteristics from the angiosperms, flowering plants which enclose their seeds within an ovary.
The gymnosperms consist of the conifers, the cycads, the gnetophytes and the sole extant species of the Gynkgophyta division, the Gingko biloba.
Examples of Gymnosperm
Conifers, in the division Pinophyta or Coniferophyta, are the most numerous of the gymnosperms; woody and with vascular tissue, these are cone bearing trees and shrubs.
Conifers can be found growing in all parts of the world, although they most notably dominate the boreal forests of the northern hemisphere. Many are adapted to cold climatic conditions, with downward facing branches, which help to shed snow, and specific biochemical properties that provide resistance to freezing.
Examples of conifers include pines, yews, redwoods, spruces, firs and cedars.
The conifer forests of the world cover huge areas of land and provide the largest terrestrial carbon sink. Conifers are also valued economically; their softwood is used for the production of paper and timber, they are used to cultivate pine nuts, and the berries of the juniper bush are used to flavor gin.
The appearance of the cycads (division Cycadophyta) typically constitutes a single, stout, cylindrical, woody trunk and a crown of large, hard and stiff, evergreen compound leaves, which grow directly from the trunk in a rosette formation. The cycads are dioecious, meaning that each individual plant is either all male or all female.
The cycads are partly xerophyte, which means they are adapted to survive in areas with very little liquid water, although their distribution largely centers around the subtropical Tropic of Cancer and the Tropic of Capricorn, as well as tropical regions such as Central and South America, China and South East Asia, India and Sri Lanka, Madagascar, and Tropical and Southern Africa.
The cycads were much more numerous in the past than today, peaking in ‘the age of the cycads’ – the Jurassic period. There are only three extant families within the cycads today: the Cycadaceae, Stangeriaceae and Zamiaceae.
The gnetophyta are distinguishable within the gymnosperms because they have vessel elements, a system of channels mostly found in the angiosperms, which transport water within the plant.
Covering 70 species over three genera, the gnetophyta are morphologically variable, including trees, shrubs, stumps, vines and creepers with leaf shapes ranging from opposite, to whorled, scale-like and straplike.
The distribution is determined by the genus: Welwitschia are unique to the Namib Desert and surrounding areas in South West Africa; Gnetum are found in tropical forests; and Ephedra are found mostly in arid or desert areas of South West America, North Africa, Southern Europe and Central Asia.
The closest relatives to the cycads, Gingko is a genus of gymnosperm of which Gingko biloba is the sole extant species.
Gingkos are large, slender, shade-intolerant trees, growing up to 160ft with distinctive fan-shaped leaves. They are deep rooted and resistant to damage from wind and snow. They are also resistant to disease and insect damage, owing to an exceptionally large genome, which enables antibacterial and chemical defense mechanisms.
Gingko first appears within the fossil record in the Permian period, 270 million years ago, and the Gingko biloba remains largely unchanged today, earning it a classification as a ‘living fossil’.
Native only to China, Ginkgo biloba is cultivated globally for use in various traditional medicines and as a food source.
Gymnosperm Life Cycle
As in all other vascular plants, gymnosperms have a sporophyte dominant life cycle (the sporophyte is the diploid multicellular stage, which comprises the body of the plant, i.e., a leafy tree). The gametophyte phase is relatively short, and sees gametes produced on the reproductive organs, which are usually cones.
The female ovulate cone, or megasporophyll, bear the megasporangium, diploid cells, which undergo meiosis to produce four haploid spores. Of these haploid spores, only one survives as the megaspore. The surviving megaspore then, through mitosis, develops into the female gametophyte. Within the female gametophyte there is an egg and an endosperm mother cell; the endosperm mother cell creates endosperm, which eventually ‘feeds’ the embryo.
The male cone, called the microsporophyll, is a small, spongy, leaf-like organ which bears the microsporangium. The microsporangium contains the male microspores, which undergo meiosis to generate the male gametophyte, pollen. The pollen grain contains the pollen tube cell and the generative cell (which contains two sperm, although one dies).
When the pollen reaches the egg cell, either by wind or by animal through pollination, the pollen grain releases the single sperm. The nuclei of the female and the male gametophytes then fuse to create a diploid zygote. The endosperm, a haploid nutritional tissue, is released from the endosperm mother cell, and surrounds the zygote to form a seed. The seeds appear as the ‘scales’, which are visible on the cones of gymnosperms; these scales are then dispersed to form a new sapling sporophyte, which grows into a mature sporophyte, and the cycle continues.
Female cones are larger and woodier than male cones and are usually positioned higher up on the tree, although in dioecious species, such as the cycads, the male and female cones are borne on separate trees.