What is Gymnosperm?
The word “Gymnosperm” comes from the Greek words “gymnos”(naked) and “sperma”(seed), hence “Naked seeds.” Gymnosperms are the seed-producing plants, but unlike angiosperms, they produce seeds without fruits. These plants develop on the surface of scales or leaves, or at the end of stalks forming a cone-like structure.
Gymnosperms belong to kingdom ‘Plantae‘ and sub-kingdom ‘Embryophyta’. The fossil evidence suggested that they originated during the Paleozoic era, about 390 million years ago. They can be represented by 900 species.
Basically, Gymnosperms are the plants in which the ovules are not enclosed by the ovary wall, unlike the angiosperms. It remains exposed before and after fertilization, and before developing into a seed. The stem of a gymnosperm can be branched or unbranched. The thick cuticle, needle-like leaves, and sunken stomata on the leaves reduce the rate of water loss in these plants.
The gymnosperms consist of conifers, the cycads, the gnetophytes, and the species of Gynkgophyta division and Ginkgo biloba.
Characteristics of Gymnosperms
Following are the important characteristics of gymnosperms:
- They do not produce flowers.
- Seeds are not formed inside a fruit. They are naked.
- They are found in colder regions where snowfall occurs.
- They develop needle-like leaves.
- They are perennial or woody, forming trees or bushes.
- They are not differentiated into ovary, style and stigma.
- Since stigma is absent, they are pollinated directly by the wind.
- Two gametes are produced by the male gametophytes but only one of them is functional.
- They form cones with reproductive structures.
- The seeds contain endosperm that stores food for the growth and development of the plant.
- These plants have vascular tissues which help in transportation of nutrients and water.
- Xylem does not have vessels, and the phloem has no companion cells and sieve tubes.
Also Read: Angiosperms
Classification of Gymnosperms
Gymnosperms are classified into four types as given below –
Cycads are dioecious (meaning: individual plants are either all male or female). Cycads are seed-bearing plants where the majority of the members are now extinct. They had flourished during the Jurassic and late Triassic era. Nowadays, the plants are considered as relics from the past.
These plants usually have large compound leaves, thick trunks and small leaflets which are attached to a single central stem. They range in height anywhere between a few centimetres to several meters.
Cycads are usually found in the tropics and subtropics. There are also members that have adapted to dry arid conditions and some also have adapted to oxygen-poor swampy environments.
Another class of Gymnosperms, Ginkgophyta has only one living species. All other members of this class are now extinct.
The Ginkgo trees are characterised by their large size and their fan-like leaves. Also, Ginkgo trees have a large number of applications ranging from medicine to cooking. Ginkgo leaves are ingested as a remedy for memory-related disorders like Alzheimer’s.
Ginkgo trees are also very resistant to pollution and they are resilient against diseases and insect infestations. In fact, they are so resilient that after the nuclear bombs fell on Hiroshima, six Ginkgo trees were the only living things to survive within a kilometre or two of the blast radius.
Just like the other members of gymnosperms, Gnetophytes are also relics from the past. Today, only three members of this genus exist.
Gnetophytes usually consist of tropical plants, trees, and shrubs. They are characterized by flowery leaves that have a soft coating. This coating reveals an ancestral connection with the angiosperms.
Gnetophytes differ from other members of this class as they possess vessel elements in their xylem.
These are the most commonly known species among the gymnosperms. They are evergreen, hence they don’t shed their leaves in the winter. These are mainly characterized by male and female cones which form needle-like structures.
Coniferous trees are usually found in temperate zones where the average temperature is 10 ℃. Giant sequoia, pines, cedar and redwood are one of the many examples of Conifers.
Following are some of the examples of gymnosperms:
Gymnosperm Life Cycle
The gymnosperm life cycle is both haploid and diploid, i.e., they reproduce through the alternation of generations. They have a sporophyte dominant cycle.
The gametophyte phase is relatively short. The reproductive organs are usually cones.
Male Cones– These have microsporophylls that contain microsporangia. Microsporangia produces haploid microspores. Few microspores develop into male gametes called pollen grains, and the rest degenerate.
Female Cones– The megasporophylls cluster together to form female cones. They possess ovules containing megasporangium. It produces haploid megaspores and a megaspore mother cell.
The pollen reaches the egg through wind or any other pollinating agent and the pollen grain releases a sperm. The nuclei of male and female gametophytes fuse together to form a zygote. This is known as fertilization.
The seed appears as scales which can be seen on the cones of the gymnosperm.
Also Read: Alternation of Generations
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