Gymnosperms: Definition, Examples, and Reproduction
Gymnosperms which is also referred to as Acro gymnospermae are a group of flowerless plants that produce seeds and cones. The term gymnosperm literally translates to “naked seed” as they are flowerless. This is due to the fact that gymnosperm seeds are not encapsulated within the ovaries. Instead, they are exposed on the surface of leaf-like structures where they ‘sit’ and they are called bracts. Gymnosperms are vascular plants belonging to Embryophyta which is a subkingdom and it includes a vast variety such as cycads, gnetophytes, conifers, and ginkgoes. There are other recognizable examples of these trees and shrubs which include spruces, pines, ginkgoes, and firs. These classes of gymnosperms are readily available in biomes such as temperate forests and boreal forests. The species thrive in moist or dry weather conditions. As mentioned earlier, gymnosperms do not contain the ability to produce any fruits or flowers, which is unlike angiosperms. According to biologists, gymnosperms are believed to be the very first vascular plants. They started to appear and inhabit the lands in the Triassic Period, which is estimated to be around 208-245 million years ago. The vascular system allows for the transportation of water throughout the plant which in turn made it possible for the colonization of gymnosperm land.
Types of Gymnosperms
The Coniferophyta sub-group as its name suggests consists of conifers, which contain pines, cypresses, and other close relatives. Coniferophyta encompasses an enormous variety of species in the gymnosperms. Most of the conifers are evergreen, which means that they hold their leaves all throughout the different seasons in the year. They include a fraction of the largest and oldest trees on Earth. A broader list of examples of conifers incorporate sequoias, hemlock, pines, firs, and spruces. Conifers are a vital economic source for the lumber industry and products, which include paper that is constructed through the use of wood. Angiosperm wood is considered a ‘hardwood’ meanwhile gymnosperms offer softwood. The term conifer translates to “cone-bearer,” which is common to conifers. The cones are the reproductive structures categorized into male and female. Conifers are monoecious which means that both the sexes can be seen in the same tree. One other trait that is distinctive for conifers is their needle-like leaves. The pines have a single needle leave or clutter of needle leaf found on the stem. The Cypresses have a very flat and scale leaves on the stems. Some of the other conifers of the Agathis have very thick leaves, and conifers of the Nageia have flat and broad leaves. Conifers are a part of taiga forest biome. They have adapted for life in the cold conditions of the boreal forests. Needle leaves also contain a coat of wax on the surface which helps to prevent any water loss in the dry climatic conditions.
The next division of gymnosperms is Cycadophyta. This includes cycads which are found in tropical forests and other subtropical regions. These also tend to be evergreen plants which have a feather-like leaf structure and long stems which spread the large leaves over the trunk. At first sight, cycads tend to resemble palm trees, but in turn, are not related to the palms. The plants can live for years and years since they have a very slow growth process. It takes the King Sago palm up to 50 years to reach a height of 10 feet. Unlike the conifers, the cycad trees can only either produce male cones (known as pollen) or only female cones (known as ovules). The female ovule-producing cycads if and only if produce seeds if a male is within close proximity. Cycads tend to rely on insects for pollination to carry pollen from the male cones to female cones. The animal’s aid in the mass dispersal of the seeds which are large and colorful. Roots of the cycads are conquered by the bacteria which are photosynthetic and are called cyanobacteria. The microbes production consists of certain neurotoxins and poisons which can accumulate in the plant’s seeds. These toxins are not harmful and they actually provide protection against bacterial and fungal parasites but the reason cycad seeds are called toxins is that they can be dangerous to humans if injected.
The only surviving plants of the ginkgophyte division of gymnosperm are the ginkgo biloba. These plants are very much exclusive to China in a naturally growing environment. Ginkgoes can live for an extremely long time at an estimation of thousands of years. These are characterized by their fan-shaped leaves which commonly change their colors in the season of autumn. Ginkgo biloba is also very large and with the tallest trees ranging to about 160 feet. The older the tree, the thicker the trunks and the deeper are the roots. These thrive very well in sunlit areas around the world and where they can receive plenty of water and have access to lots of soil drainage. Just like the cycads, these ginkgo plants can either consist of male or female cones. The sperm cells use a hair like a feature called flagella allowing it to swim towards the egg in the ovule. These trees are very durable and also surprisingly fire resistant. On top of that, they are resistant to pests and diseases.
The Gnetophyta part of gymnosperms has very less number of species present within the 3 genera: Ephedra, Gnetum, and Welwitschia. A lot of species which are from the Ephedra are shrubs that are mostly found in desert areas of the American highlands or the cool areas of Himalayan Mountains in India.
Some Ephedra species contain medicinal proprieties and the main source of the decongestant drug ephedrine. Ephedra species has slim stems and scale-like leaves.
Gnetum species have some shrubs and trees, from which most of them are woody vines that climb around the other plants. They live in tropical rain forests and have wide, flat leaves that represent the leaves of flowering plants.
The male and female reproductive cones are kept on separate trees and often represent flowers, although they are not. The vascular tissues and structure of these plants are similar to that of flowering plants.
Economic Importance of Gymnosperms:
The Major Economic Importance of Gymnosperms are given below:
- Edible Seeds:
Seeds of Pinus gerardiana are safe to eat. They are eaten after roasting.
Gymnosperms have softwood. The same is used in making of light furniture, plywood, packing cases, match sticks, railway sleepers, etc.
Many gymnosperm kinds of wood are used in the manufacturing of paper. They include Picea, Pinus, Larix, and Abies.
- Fiber Boards:
Needles of Pinus and other conifers are used in the production of fiberboards that are used in making packing cases.
Sawdust is engaged in making linoleum and plastics.
The resin is a semi-fluid produced by special tubes which contain terpenes, resin acids, and esters. It solidifies when exposed to air. Hence, it plugs the places of injury. It helps in securing female cones after pollination, scale leaves around leaf bases and apical buds. Resin holds water. It is antiseptic and toxic to pests.
Hence, it stops microbial and insect attack. The resin is commercially extracted and distilled to attain turpentine and rosin. Rosin is used in waterproofing, sealing joints and manufacturing of writing paper. Turpentine is used to make paints, polishes, and wax. It is engaged medicinally in removing pains, curing bronchitis and expelling worms.
Drug ephedrine is extracted from Ephedra, a gymnosperm. The drug is used for preventing respiratory ailments, including asthma.
Features of Gymnosperms
- Gymnosperms have great importance and show some unique features. Their basic feature is the absence of flowers and the presence of naked, open seeds. Since these plants do not have flowers, the fruits are also not present in these groups of plants.
- The major source of pollination and dispersal is wind. These are usually medium to tall trees, with some shrub species also present. One of the tallest tree species of a gymnosperm is known as Sequoia.
- The plant body shows segregation and is divided into leaves, stem, and roots. The leaves are normally needle-like with a thick cuticle and sunken stomata, commonly seen in conifers. This particular feature helps in the reduction of water loss due to transpiration.
- Taproot system is the root system present in the gymnosperms. In some plants, these roots have involvement with fungi and form mycorrhiza, e.g. Pinus. In some species like the Cycas plant, are linked with nitrogen-fixing cyanobacteria and are in the form of specialized roots called coralloid roots. Such plants are also vascular, with both xylem and phloem being present.
Characteristics of Gymnosperms:
- Gymnosperms are a small cluster of seed plants which are represented by only 900 living species.
- Gymnosperms are more ancient than angiosperms. They formed leading vegetation on earth around 200 million years back in the Mesozoic era. Today they are present only in cold areas, where instead of rain; snow is the source of water.
- Angiosperms have replaced the gymnosperms in other warmer areas. Only a few gymnosperms can be seen in the warmer areas, Such as; Cycads, Araucaria.
- All gymnosperms are perennial and woody in nature, forming either bushes or trees. Some of these are huge and live for thousands of years. e.g., Sequoia is the tallest measuring 111.6 meters high and Zamia pygmaea is the smallest measuring 26cm.
- Flowers are not present. Two types of sporophylls, microsporophylls and megasporophylls are generally aggregated to form separate cones or strobilus, seed cones (female cones) and pollen cones (male cones) respectively.
- Seeds do not take place inside a fruit. They are naked.
- A distinction of the ovary, style, and stigma is not present.
- Ovules are sessile and orthotropous. Each ovule is bordered by a 3-layered integument.
- Female gametophyte consists of archegonia.
- Pollination straight as a stigma is not present and the pollen grains directly reach the micropylar ends of ovules. Pollination is regularly accomplished by wind (anemophily).
- Male gametophyte produces 2 male gametes or sperms. Usually one of them is functional.
- An external water source is not necessary for the transportation of male gametes. Alternately, a pollen tube is formed for effecting fertilization (siphonogamy) by the male gametophyte.
- Seeds have a food-laden tissue or endosperm for future growth of an embryo into a seedling. The female gametophyte is represented by these tissues.
- Like Pteridophytes, xylem does not have vessels except in some geophytes. Phloems do not contain companion cells and sieve tubes. Sieve cells are not arranged in an end to end row order.
- Vascular tissues are arranged into vascular bundles in the same way the angiosperms are arranged. Vascular bundles of the stems are open so that there is secondary growth.
Reproduction and Fertilization
In gymnosperms, Strobilus is known as its reproductive system. The male and female Strobilus both can be present on the
same tree (seen in Pinus) or on different trees (seen in Cycas). Gymnosperm plants are heterosporous. They generate different spores, which are the haploid microspores and megaspores.
Male Cones – The Male Strobilus or male cones consists of microsporophylls, which have the microsporangia that produce the haploid microspores. Some of these microspores grow into male gametes called the pollen grains, while the rest collapse.
Female Cones – The megasporophylls form a group and are known as the female Strobilus or cones. They contain the ovules having the megasporangium. Hence, produces the haploid megaspores and a megaspore mother cell.
The megaspore mother cell undergoes meiotic division and produces 4 megaspores. One of these megaspores grows into the female gametophyte which is multi-cellular. The female gametophyte also consists of two or more archegonia, which are the female sex organs.
Fertilization – When the pollen grains are let out from the microsporangium, they get detached through wind and reach the female cones. The pollen grain grows a pollen tube, which develops towards the archegonium.
The male gametes get discharged close to the mouth of the archegonium. A mixture of the male and female gametes takes place. After fertilization, a zygote develops to create the embryo and the ovule forms the seed.