GYMNOSPERMS

GYMNOSPERMS

“Gymnosperms are a group of plants that produce seeds not enclosed within the ovary or fruit. “

The word “Gymnosperm” comes from the Greek words “gymnos”(naked) and “sperma”(seed), hence known as “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.

Characteristics of Gymnosperms

Following are the important characteristics of gymnosperms:

1. They do not produce flowers.
2. Seeds are not formed inside a fruit. They are naked.
3. They are found in colder regions where snowfall occurs.
4. They develop needle-like leaves.
5. They are perennial or woody, forming trees or bushes.
6. They are not differentiated into ovary, style and stigma.
7. Since stigma is absent, they are pollinated directly by the wind.
8. The male gametophytes produce two gametes, but only one of them is functional.
9. They form cones with reproductive structures.
10. The seeds contain endosperm that stores food for the growth and development of the plant.
11. These plants have vascular tissues which help in transportation of nutrients and water.
12. Xylem does not have vessels, and the phloem has no companion cells and sieve tubes.

Classification:-

• Gymnosperms consist of four main phyla: the Coniferophyta, Cycadophyta, Gingkophyta and Gnetophyta.
•  Conifers are the dominant plant of the gymnosperms, having needle-like leaves and living in areas where the weather is cold and dry.
•  Cycads live in warm climates, have large, compound leaves, and are unusual in that they are pollinated by beetles rather than wind.
• Ginkgo biloba is the only remaining species of the Gingkophyta and is usually resistant to pollution.
• Gnetophytes are the gymnosperms believed to be most closely related to the angiosperms because of the presence of vessel elements within their stems.

Evolution of Gymnosperms

• Gymnosperms are believed to have evolved from the paleozoic to the mesozoic eras.
• There are 3 groups of extinct plants that played important roles in the evolution of modern gymnosperms
• They are progymnosperms, aneurophytales, and a groups of primitive gymnosperms: archaeopteridales.
• In middle devonian period, progymnosperms arose from the trimerophytes which were extant until the lower carboniferous period.
• The cordaitales were trees and shrubs during the carboniferous and permian periods both in swamp and dry land which had slender leaves.
•  They also had vascular cambium tissues and ovulate cones.
•  The cordaitales apparently gave rise to the phylum ginkgophyta, which persists to present day and others which have relatives to cycadophyta, gnetophyta and coniferophyta.
•  Voltziales is an extinct order of trees that gave rise species related to modern conifers.
• Pteridosperms were the first seed plants, with integuments protecting ovules to various degrees
• Another extinct group of Pteridosperms are the Bennettitales, which resemble present cycads.
• The Archaeopteridales may have given rise to the Cordaitales and the Voltziales.
• The Archaeopteridales arose from the Aneurophytales.
• Progymnosperms gave rise to aneurophytales that gave rise to pteridosperms and archaepteridales.
•  Aneurophytales were homosporous, producing many identical spores and had three dimensional branching
• Unlike the progymnosperms, the pteridosperms produced seeds appearing in the late devonian period.

Diversity of Gymnosperms

Modern gymnosperms are classified into four phyla. The first three (the Coniferophyta, Cycadophyta, and Gingkophyta) are similar in their production of secondary cambium (cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation) and their pattern of seed development. However, these three phyla are not closely related phylogenetically to each other. The fourth phylum (the Gnetophyta) are considered the closest group to angiosperms because they produce true xylem tissue.

Coniferophyta (conifers - pine, spruce, redwood)

These are the most commonly known species among the gymnosperms family.They are evergreen hence they do not shed their leaves in the winter. These are mainly characterised 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.

Cycadophyta ( cycads)

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. Some members have adapted to dry arid conditions, and some also have adapted to oxygen-poor swampy environments.

Ginkgophyta ( ginko bolona)

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.

Gnetophyta (Ephedra)

Just like any other member 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 characterised 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.

Origin of seed habit

Seed habit is known to be the most successful method in the evolution of sexual reproduction in plants.

The first seed was found to be Gymnosperms that was first appeared during the Devonian period in the time line. Soon it started appear on the land plants. As like the Gymnosperms seed habit has been found to be emerged during the Devonian period and now dominating every land plants.

Seeds are specialized structure resultant of gamete fusion. It has several advantages including late rejuvenation, safety besides others.

This means that primitive plants do not have the capacity to produce seed. Hence evolution has taken a direction and all advanced plant now bear seeds.

So seed habit is the ability to produce seed like propagules. This has many intermediary steps including heterospory (which has started in advanced pteridophytes), integument development (started in progymnosperms) and development of seed coat (started in gymnosperms).

GEOLOGICAL TIME SCALE

FOSSILISATION AND FOSSIL GYMNOSPERMS

Gymnosperms means (Greek gymnos = naked; sperma = seed. i.e., the plants with naked seeds. Gymnosperms are phanerogams or spermatophytes without ovary and fruit. Their seeds or ovules are naked or exposed, without a fruit wall. They are therefore considered as fruitless flowering plants and are referred to as "Phanerogams without ovary. Gymnosperm seeds develop either on the surface of scale or leaf-like appendages of cones, or at the end of short stalks (Ginkgo). The word gymnosperm is coined by Theophrastus in 300 B.C. and called them "plants with naked seeds". Palaeobotany is the study of plant fossils preserved in rocks.

The word "Fossil" has been defined as "any evidence of prehistoric life. The first mention of a fossil plant was made by a German scholar Albertus Magnus in the thirteenth century.

Fossils (from Latin fossus, literally having been dug up) are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record.

The process of preservation of living beings or their parts in form of fossils is called fossilization. 'Birbal Sahni' is known as father of Indian Palaeobotany.

(His main contribution is Pentoxifylline of Jurassic gymnosperms from Raj Mahal hills in Bihar).

BirbalSahni Institute of Paleobotany is situated on Trans-Gomati River bank, Lucknow (India).

The essential conditions for fossilization is that whole organism be buried alive soon after death without decay by bacteria, fungi etc. This is the reason why only small number of plants get fossilized.

GYMNOSPERMS LIFE CYCLE

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