Classification of gymnosperms-Characters,Life cycle,Imp..

Introduction:

As the name suggests, the division spermatophyte (sperma=seed, phyton=plant) includes all the seed-bearing plants. It has been divided into two sub-divisions

• Gymnosperms
• Angiosperms.

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(without seed coat) on the surface of the megasporophylls.

Hence, unlike angiosperms, the seeds of gymnosperms are not enclosed in the ovary. Gymnosperm is named so because they bear naked seeds, i.e. the ovules, and the seeds are not enclosed within the fruit wall.

Goebel referred to them as “Phanerogams without ovary”. Thus these are all seed plants without fruits. In comparison to angiosperms, it is a 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 recognized 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-

Class 1– Manoxylic- Wood is not compact due to the presence of a well-developed pith and cortex and broad medullary rays.

Class 2– Pycnoxylic- Wood is compact Because the pith and cortex are reduced and medullary rays are narrow. Three orders (Cycadales, Cycadeoidales, and Cycadofilicales) were recognized in Manoxylic and Four orders  (Cordaitales, Ginkgoales, Coniferales, and Gnetales) in Pycnoxylic.

Gymnosperms are the most ancient group of seed plants, originating 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 the 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 the ovary, style, and stigma are not found in a gymnosperm.
• In the female gametophyte of the gymnosperms, archegonia are present, but they are not found in the angiosperm.
• In gymnosperm, the endosperm is formed before fertilization. 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 occurs in gymnosperm but they do not happen in angiosperm.

Classification of Gymnosperm

Earlier, gymnosperms are not Accepted as a separate group and are now included in the group that was treated with angiosperms. But Robert Brown (1827) approved them as a group distinct from angiosperms for the first time due to the presence/availability of naked(without seed coat) ovules.

Bentham and Hooker (1862-83) considered them equivalent/same value as dicotyledons and monocotyledons and placed them between these two groups of angiosperms(having seeds with seed coats). They recognized three classes of gymnosperms.

• Cycadaceae
• Coniferae
• Gnetaceae

Later, Engler (1889) created another group of Ginkgoales to adapt or accommodate 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 of gymnosperm as a division of Spermatophyta, Phanerogamia, or Embryophyta and they further divided them into seven orders :

• Cycadofilicales
• Cycadales
• Benettitales
• Ginkgoales
• Coniferales
• Cordaitales
• Gnetales

Bierhost (1971) recognized three classes and 11 orders in gymnosperms;

Gymnosperms———3 classes •

• Cycadopsida
• Coniferopsida
• Gnetopsida

Each class is divided into orders

The first class of gymnosperms (Cycadopsida) has 4 orders

• Pteridospermales
• Cycadales
• Cycadeoidales
• Caytoniales

The second class of gymnosperm(Coniferopsida) is also divided into 4 orders

• Cordaitales
• Coniferales
• Taxales
• Ginkgoales

The third class of gymnosperm (Gnetopsida) is divided into three orders

• Ephedrales
• Gnetales
• Welwitschiales

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 are 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 part of the vegetation within the Carboniferous.
• The leaves were large, frond, and pinnately compound.
• The leaf traces were large and with one or more strands.
• Seeds were formed on normal or specialized leaves. Example- Lyginopteris, Medullosa.

Order Benettitales (Cycadeoidales)

• The member of this order also occurs only in fossilized form.
• They 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 persistent leaf bases.
• The vascular system was siphonostelic and the xylem was endarch.
• Growth rings were absent in the wood.
• The leaves were arranged/ordered in whorls at the tip/crown of the stem.
• The embryo was dicotyledonous. Examples are Willamsonia, Benettites, and Cycadeoidea.

Order Cycadales

• 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 for Zamia pygmaea.
• The stem is usually unbranched and is covered by persistent leaf bases.
• The leaf traces are direct as well as girdle type.
• The wood is manoxylic.
• The pollen tube mainly functions as a haustorium. Examples- 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 a 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 are on the same plant (monoecious).
• There were only a few ovules in a strobilus. Examples- Callixylon, Mesoxylon.

Order Coniferales

• Coniferales is 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 stems), and have evergreen trees.
• The branches are usually dimorphic i.e. with long and dwarf shoots.
• The wood is Pycnoxylic.
• The plants are monoecious and dioecious.
• The fertilization is siphonogamous.
• The male gametes are non-motile. Example-Pinus, Taxus, etc.

Order Ginkgoales

• This order includes 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.

Order Gnetales

• 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 was recorded in the Permian and Eocene period.
• The plants grow in xerophytic 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

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 that is dominant in their life cycle.

The gametophyte phase(the time when gametes are produced) is relatively short. The reproductive organs are usually cones(Strobli-like Structure).

Male Cones

Male cones contain microsporophylls that contain/consist of microsporangia. Microsporangia produces haploid(With half number of chromosomes) microspores. Few microspores develop or produce male gametes called pollen grains, and therefore the remaining(rest) become degenerated.

Female Cones

The many megasporophylls are in the form of clusters together form the female cones. They possess ovules that contain megasporangium within them. Megasporangium produces haploid megaspores and a megaspore mother cell.

Fertilization

The pollen grains reach the egg with the help or source of wind and other pollinating agents, After reaching the female flower, the pollen grain releases sperm. The nuclei of male and female gametophytes become fused together and form the zygote. This process is known as fertilization.

The seeds appear as scales which may be seen with the naked eye on the cones of the gymnosperm.

Distribution of Gymnosperms in India Classification of

Gymnosperm is very poorly represented within the Indian flora. In 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 the 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 northwest 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 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 the northwest. Himalayan region and just one occurs within the plains of Rajasthan and Punjab. Six species of Gnetum occur in India.

Economic importance of gymnosperm

Ornamental value:

Different plants of gymnosperm have ornamental value. Cycas plants appear palm-like. Foliage leaves of cycas remain fresh for long periods so these are used for adornment purposes. Cycas, Araucaria, and Thuja are commonly grown as ornamentals in houses and gardens.

Timber:

Conifers produced about 75% of the world’s timber. P. wallichiana (kali) is used to make sleepers, light furniture, etc. Redwood is also important as it contains a substance that inhibits the growth of fungi and bacteria.

Paper:

Some conifers like spruce are an important source of pulpwood which is used in paper manufacturing.

Industrial use:

• Resin: It is an important wood product of conifers. Resin aromatic and antiseptic substances that prevents the development of fungi and also deters 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.
• Wood: The wood Juniperus is employed to create pencils and cigar boxes. The wood of eastern spruce is essential for the music industry. The wood resonance so it is ideal for use as a soundboard in violins and related musical instruments.Classification of gymnosperms
• Canada Balsam: Canada Balsam extracted from balsam fir was used in the past for cementing optical lenses and is still used in making permanent mounts or microscope slides.

5.) Food value:

One species of gnetum is cultivated in java for shoots which are cooked in coconut oil and eaten. Seeds of some species of cycas are roasted and brought as food. The seeds of Ginkgo biloba are nutritious.

6.) Taxol 

Taxol is a drug obtained from the bark of Taxus brevfolia (Yew plant). Unlike most other cancer drugs, which keep cancer cells 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 is far 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 synthetically produced.

Key Points about Gymnosperms

• Gymnosperms are plants that cannot produce flowers and belong 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
• Coniferophyta
• Cycadophyta
• Ginkgophyta
• Gnetophyta
• Gymnosperms are present or seen in the boreal and temperate region’s forests.

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