Classification of gymnosperms-Characters,Life cycle,Imp..
Introduction:classification of gymnosperms
The division spermatophyte (sperma=seed, phyton=plant) as the name suggest include all the seed bearing plants. It has been divided into two sub-divisions
The sub-division Gymnosperms (gymnos=naked, sperma=seed) includes simpler and primitive plants of the division spermatophyta. They are characterized by the presence of naked ovules, borne unprotected(with out seed coat) on the surface of the megasporophylls.
Hence, unlike angiosperms, seeds of the gymnosperms are not enclosed in ovary. Gymnosperm are named so because they bear naked seeds, i.e. the ovules and the seeds are not enclosed within the fruit wall.
Goebel referred them as “Phanerogams without ovary”. Thus these are all seed plants without fruits. In comparison to angiosperms, it is smaller group having only 900 species. Even then they are distributed throughout the whole world.
Most of the Gymnosperms are making the dominant vegetation in the temperate coniferous forest.
They are more advanced than pteridophytes. Many gymnosperms are fossils.
Earlier gymnosperms were not regarded as a separate group and the members now included in this group were treated with angiosperms.
But Robert Brown (1827) for the first time recognised them as a group distinct from angiosperms due to the presence of naked ovules. On the basis of wood structure, Seward (1919) divided gymnosperm into two classes-
Class1- Manoxylic- Wood is not compact due to the presence of well -developed pith and cortex and broad medullary rays.
Class2- Pycnoxylic- Wood is compact,Because pith and cortex are reduced and medullary rays are narrow. Three orders (Cycadales, Cycadeoidales and Cycadofilicales) were recognised in Manoxylic and Four orders (Cordaitales, Ginkgoales, Coniferales and Gnetales) in Pycnoxylic.
Gymnosperms are the most ancient group of seed plants, originated in the Palaeozoic era. The geological records show that they were dominant plants over the earth’s surface during the Jurassic and cretaceous periods of Mesozoic era.
There are about 70 genera and 725 species of living gymnosperms, distributed throughout the temperate and tropical regions of the world. They even occur in arctic zones.
General Characters Classification of gymnosperms
- Gymnosperms are mostly woody trees, but angiosperms have a variety of habit-trees, shrubs or herbs.
- The ovules of angiosperm are enclosed within the ovary wall but the ovule of gymnosperm are naked.
- The structures like ovary, style and stigma are not found in gymnosperm.
- In the female gametophyte of the gymnosperms archegonia are present, but they are not found in angiosperm.
- In gymnosperm the endosperm is formed before fertilization and thus it is a haploid tissue, whereas in angiosperm it is formed after fertilization as a result of triple fusion and thus is a triploid tissue.
- Free nuclear division occur in gymnosperm but they do not occur in angiosperm.
Classification of Gymnosperm
Earlier, gymnosperms are not Accepted as a separate group and now included in the group that were treated with angiosperms. But Robert Brown (1827) for the first time approved them as a group distinct from angiosperms due to the presence/availability of naked(with out seed coat) ovules.
Bentham and Hooker (1862-83) considered them equivalent/same value to dicotyledons and monocotyledons and placed them between these two groups of angiosperms(having seeds with seed coats). They recognised three classes of gymnosperms.
Later, Engler (1889) created another group Ginkgoales to adapt or accomodate the genus Ginkgo. Van Tieghem (1898) treated gymnosperms as one of the two subdivisions of Spermatophyta.
To accommodate the fossil members, subsequently three more classes – Pteridospermae, Cordaitales and Benettitales were created.
Coulter with Chamberlain (1910), Engler with Prantl (1926), Rendle (1926) and others considered/think gymnosperm as a division of Spermatophyta, Phanerogamia or Embryophyta and they further divided them into seven orders :
Bierhost (1971) recognised three classes and 11 orders in gymnosperms;
Gymnosperms———3 classes •
- Cycadopsida •
- Coniferopsida •
Each class is divided into orders
First class of gymnosperms (Cycadopsida) have 4 orders
Second class of gymnosperm(Coniferopsida) is also divided into 4 orders
Third class of gymnosperm (Gnetopsida) is divided into three orders
Important Characters of various orders of gymnosperms
Order Cycadofilicales (Pteridospermales) Classification of gymnosperms
- All the members of Cycadofilicales are fossils(Remains of extinct sp or organisms) and commonly known as seed ferns. They form/made a connecting link/link between cycads and ferns.
- They originated within the Devonian of the Palaeozoic era and were the dominant a part of the vegetation within the Carboniferous .
- The leaves were large, frond like and pinnately compound.
- The leaf traces were large and with one or more strands.
- Seeds were formed on normal or specialised leaves. Example- Lyginopteris, Medullosa.
Order Benettitales (Cycadeoidales)
- The member of this order also occur only in fossilized form.
- They were originated in the Mesozoic era and were dominant in the Jurassic period and declined in the Upper Cretaceous period.
- The stem was erect, cylindrical and branched or unbranched. It was surrounded by persistant leaf bases.
- The vascular system was siphonostelic and the xylem was endarch.
- Growth rings were absent in the wood.
- The leaves were arranged/orderd in whorls at the tip/crown of the stem.
- The embryo was dicotyledonous. Example- Willamsonia, Benettites, Cycadeoidea.
- The Members of this order were commonly known as Cycads.
- They Arise/develop from the seed ferns at the end of the Carboniferous period and flourished well in the Jurassic period of the Mesozoic era.
- The order includes 11 living genera and about 100 species.
- They are woody trees except Zamia pygmaea.
- The stem is usually unbranched and is covered by persistant leaf bases.
- The leaf traces are direct as well as girdle type.
- The wood is manoxylic.
- The pollen tube mainly function as haustorium. Example- Cycas, Zamia etc.
Order Cordaitales classification-of-gymnosperms
- The order consists of extinct plants which were originated in the Devonian period.
- In the Carboniferous period they flourished well in the form of thick forests of the world and ultimately vanished in the Triassic period.
- The plants were tall and formed dense forest.
- The stem showed secondary growth and distinct growth rings were present in some species.
- The leaves show much diversity in size ranging from 5cm to 1m.
- The male and female strobili/cone present on the same plant (monoecious).
- There were only few ovules in a strobilus. Example- Callixylon, Mesoxylon.
- Coniferales is that the largest order of gymnosperms, represented by 52 genera and about 550 species.
- They originated in the Carboniferous period. They include three fourth of the living gymnosperms.
- The plants are usually long/tall, branched(having branching stem) and evergreen trees.
- The branches are usually dimorphic i.e. with long and dwarf shoot.
- The wood is Pycnoxylic.
- The plants are monoecious and dioecious.
- The fertilization is siphonogamous.
- The male gametes are non- motile. Example-Pinus, Taxus etc.
- This order include one monotypic family, Ginkgoaceae, represented only by one living member, Ginkgo biloba.
- The members originated in the Permian period of the Palaeozoic era.
- The members (living and fossils) of the order are trees.
- With the exception of some fossil taxa, they possess dimorphic branches.
- The leaves are branched and fan shaped.
- The veins are dichotomously branched.
- The plants are dioecious and the male and female structures are axillary in position. Example – Ginkgo biloba is the only living member.
- This is the most highly evolved group of gymnosperms and is regarded a connecting link between gymnosperm and angiosperms.
- The fossil history of the order Gnetales is not known and thus a definite conclusion can not be drawn about the time of their origin, but pollen grains of Ephedra were recorded in the Permian and Eocene period.
- The plants grow in xerphytic and mesophytic habitats.
- They are lianas, shrubs or trees.
- The branches are dimorphic with long and short shoots.
- The leaves also are dimorphic scale and foliage leaves.
- Fertilization takes place by pollen tube.
The life cycle of gymnosperms is consist of both haploid phase and diploid phase, i.e., they reproduce by the method of alternation of generations. They have a sporophyte or plant body is dominant in their life cycle.
The gametophyte phase(time when gametes are produced) is relatively short. The reproductive organsare usually cones(Strobli like Structure).
Male cones contain microsporophylls that contain/consist of microsporangia. Microsporangia produces haploid(With half number of chromosomes) microspores. Few microspores develop or produces male gametes called pollen grains, and therefore the remaining(rest) become degenerated.
The many megasporophylls are in the form of clusters together form the female cones. They have possess ovules that containing megasporangium within them.Megasporangium produces haploid megaspores and a megaspore mother cell .
The pollen grains reaches the egg with the help or source of wind and the other pollinating agents, After reaching at the female flower, the pollen grain releases a sperm. The nuclei of male and female gametophytes become fused together and form the zygote. This process is known as fertilization.
The seeds are appears as scales which may be seen with the naked eye on the cones of the gymnosperm.
Distribution of Gymnosperms in India Classification of gymnospermss
Gymnosperm are very poorly represented within the Indian flora. In the vast peninsular India they’re represented only by a couple of species of Cycas, Gnetum.
However, within the extra peninsular Himalayas and to some extent within the connected ranges of Kashmir, Assam and Arunachal Pradesh gymnosperms, essentially represented by conifers, enjoy a somewhat appreciable distribution, covering extensive tracts of forest land.
A total of 16 genera and 53 species of Gymnosperms occur in India.
[I] Cycads- Among cycads, only Cycas occur in India, and the genus is represented by four species, viz., C. circinalis, C. beddomei, C.pectinata is chiefly found in dry deciduous forest of Mysore, Tamil nadu and Orissa. Some species are occasionally cultivated in Indian gardens.
[II] Ginkgoales- Among Ginkgoales, only a few plants of Ginkgo biloba, a native of China, occur in India under cultivation in gardens.
[III] Conifers- Conifers are found predominantly in the Himalayas and are particularly rich in the north-west Himalayas. Their distribution is mainly governed by altitudes. In India they are represented by Abies, cedrus, Larix, Picea, Pinus, Tsuga etc. These are widely cultivated in the Indian gardens.
[IV] Gnetales- Gnetales are represented in India by a number of species of Ephedra and Gnetum. Eight species of Ephedra are known from India; of those seven are confined to north-west. Himalayan region and just one occur within the plains of Rajasthan and Punjab. Six species of Gnetum occur in India.
Economic importance of gymnosperm
1.) Ornamental value:
Different plant of gymnosperm have ornamental value. Cycas plants appear palm like. Foliage leaves of cycas remain fresh for long period so these are used for adornment purpose. Cycas, Araucaria and Thuja are commonly grown as ornamentals in house and gardens.
Conifers produced about 75% of the world timber. P. wallichiana (kali) are used to make sleepers, light furnitures, etc. Red woods is also important as it contains a substance that inhibit the growth of fungi and bacteria.
Some conifers like spruce are important source of pulpwood which is used in paper manufacturing.
4.) Industrial use:
[i] Resin: It is an important wood product of conifers. Resin aromatic and antiseptic substance that prevent the development of fungi and also deter insect attacks.
It is a mixture of liquid solvent turpentine and a waxy substance called rosin. Rosin is also a useful product. Ballerinas dip their shoes in rosin to enhance their grip on the stage.
[ii]Wood: The wood is juniperus is employed to creating pencils and cigar boxes. Wood of eastern spruce is particularly important for music industry. The wood resonance so it is ideal for use as sound board in violins and related musical instrument.Classification of gymnosperms
[iii]Canada Balsam: Canada Balsam extracted from balsam fir was used in the past for cementing optical lenses and still used in making permanent mounts or microscope slides.
5.) Food value:classification-of-gymnosperms
One species of gnetum is cultivated in java for shoots which are cooked in the coconut oil and eaten. Seeds of some species of cycas are roasted and brought as food. Seeds of Ginkgo biloba are nutritious.
Taxol is a drug obtained from the bark of Taxus brevfolia (Yew plant). Unlike most other cancer drugs, which keep cancer cell from reproducing by damaging their DNA, taxol “freezes” the cancer cell early in the process of cell division as a result the cells die.
Unfortunately, there’s for fewer taxol available than required to satisfy the necessity . Today taxol is chemically synthesized.
The drug ‘ephedrine’ widely utilized in the treatment of asthma and other respiratory problems, still is extracted from the Chinese species Ephedra, however, most of the drug is now in use in synthe-tically produced.
Frequently Asked Questions
What are gymnosperms?
Gymnosperm is a seed-producing plants that includes conifers, cycads, gnetophytes and ginkgos. They can not produce flowers or fruits and have seed without seed coats(Naked seeds).
Do gymnosperms have an embryo? Classification of gymnosperms
Yes, embryo is present in gymnosperms. In gymnosperms, the ovule form the seed encasing the embryo and endosperm in a seed coat, but it does not develop into a fruit after fertilization only seed is produced.
Give a few examples of gymnosperms.
Some of the examples of gymnosperms are as follows.
Cycas, pinus, Thuja, Cedrus, Abies and Larix etc.
How are gymnosperms different from an angiosperm?
Gymnosperms are called non-flowering plants while Angiosperms are called flowering plants. Seeds of angiosperms are enclosed in the ovary (a fruit) whereas gymnosperms don’t form flowers or fruits and have naked seeds present on the surface of leaves.
How are the gymnosperms classified?
The gymnosperms are classified as follows:-
Which stage dominates the life cycle of gymnosperms?
The dominant stage in the life cycle of the gymnosperms is a haploid stage . In this stage, a many celled haploid gametophyte produced from the spore and produces haploid(with haploid number of chromosomes) gametes. The gametophyte when matures(or become adult) produces male and female gametes.these gametes join to form a diploid zygote.
Where are gymnosperms found? Classification of gymnosperms
Gymnosperms are usually found in colder regions or in hilly areas where temperature is low ,i.e.like Muree,Swaat and in hilly areas, when snowfall occurs. However, cycads are found in dry and tropical regions of the world.
What are gymnosperms give two characteristics?
Key Points about Gymnosperms
- Gymnosperms are the plants that cannot produce flowers and belonging to the sub-kingdom Embophyta.
- They cannot form fruits and their seeds aren’t enclosed in an ovary or fruit. They (seeds) are present on the surface of the leaf-like structures(Scale leaves).
- They are classified as
- Gymnosperms are present or seen in the boreal and temperate regions forests.