Geological time scale, Distribution of living and fossil Gymnosperm in India
Geological time scale:-
> Earth’s history is approximately 350 million years old.
> Geologists have used two major units for subdividing the geological history of the earth these are- time and strata.
> On the basis of time the geological history of earth has been divided into five Eras namely Archaeozoic, Proterozoic, Palaeozoic, Mesozoic and Coenozoic.
> Eras have been divided into periods and periods into epochs.
> Similarly on the basis of strata, the geological history of the Earth is divided into system,series, stage and zone.
> The various eras and their age along with plant groups evolved during that period have been described in the given figure.
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1. Precambrian Era:- Aarchaeozoic and Proterozoic eras are together known as Precambrian. It is said that life originated on the earth in Precambrian. The Rocks of this era have only a few fossils because either the environment was not suitable for fossilization or the organisms present during that time had so soft tissues that it was not fit for fossilization. Therefore, the absence of any fossil records makes it very difficult to tell something about the environment of Precambrian era.
2. Palaeozoic Era:-
> This era had vegetation including both aquatic and land plants. The aquatic plants included species such as Nematothallus and Prototaxites. These two species show similarities with thallophytes. The first fossils of land plants were found in the Silurian and Devonian periods of this era (225-350 million years ago). Manograptus, Sporogonites, Zosterophyllum, Cooksonia, Rhynia, Horneophyton and Psilophyton were important fossil genera discovered.
> In the Carboniferous and Permian periods, Lycopsids, Sphenopsids and Pteropsids formed dense forests. Seed ferns (Pteridospermatophytes) were also abundant in this period (age of seed ferns). Presence of annual rings in certain fossil plants of this period indicates seasonal changes.
> Significant changes in environment occurred in late carboniferous resulting in the development of swampy regions. This type of habitat was unstable for woody vegetation and in the late Devonian period the vegetation present on the earth was converted into coal due to geological events. The coal beds of Pennsylvania and West Virginia we are formed due to coalification of Lycopsids, Sphenopsids, seed ferns and gymnosperms of that period. Fortunately few Psilopsids, Lycopsids and Sphenopsids escaped destruction and eventually gave rise to the present forms.
3. Mesozoic Era:-
> The environment for the Triassic period of this era was very arid (dry) and thus unsuitable for the growth of plants requiring in humid climate. Fossil records however indicate the presence of many ferns, Cycadophytes and conifers in the Triassic period. This period is also known as the age of Cycads. Although flowering plants also originated in the period, their number was negligible.
> Significant changes occurred in the vegetation of the Cretaceous. Of the 300 cretaceous species collected from North America 200 species belong to angiosperms. It shows that angiosperm formed a significant part of the cretaceous vegetation.
4. Coenozoic Era:-
> This era is known as the “age of angiosperms”. High mountain ranges such as Himalayas in Asia and Alps in Europe were formed during this era. As a result, the effect of hot winds got reduced and ICE AGE started. Gradually the ice shifted towards the poles.
> Simultaneously the development of tropical and temperate regions occured. These changes had major effect on vegetation. The seed plants gradually migrated towards tropical regions and those which could not migrate became extinct. As most plants of the late coenozoic era were deciduous, it shows that there was seasonal variations too.
> The vegetation of the temperate and the Polar Regions mostly consisted of annual and biennial herbs which could withstand low temperature.
Distribution of living and fossil Gymnosperm in India:-
1. Cycadales:-
> Indian Cycadales are represented by only 5 species of Cycas and occur mainly in South India. These are Cycas beddomei (Madras and dry hills of Cudapah in Andhra Pradesh), C. circinalis (Andaman and Nicobar Islands and some dry deciduous forests of South India), C. rumphii (Andaman and Nicobar Islands), C griffithi (Manipur and Nega Hills) and C. pectinata (Assam, Bihar, Sikkim and several parts or Eastern India).
> C. revoluta, a Japanese species, is cultivated commonly in Indian gardens. It does not occur in wild state.
2. Gnetales:-
> Indian Gnetales include species of Gnetum and Ephedra. Welwitschia, the third genus of the order, has not been reported from India. Five species of Gnetum (viz. G. ula, G. contractum, G. gnemon, G. montanum and G. latifolium) occur in various parts of the country.
> According to Bhardwaj (1957) various Indian species of Gnetum along with their places of distribution in parenthesis include G. ula (Western Ghats near Khandala, forests of Kerala, Nilgiris, Andhra Pradesh and Orissa), G. contractum (Nilgiri Hills, Conoor and hills of Kerala), G. gnemon (eastern parts of the country, particularly Assam), G. montanum (Assam, Sikkim and parts of Orissa) and G. latifolium (Andaman and Nicobar Islands).
> Ephedra, in India, is represented by only 6 species. These are Ephedra foliata, E. gerardiana, E. intermedia, E. nebrodensis, E. regeliana and E. saxatilis. These are distributed widely in dry parts of Haryana, Punjab, Rajasthan, and parts of Sikkim, Kashmir, and also at high altitudes in Himalayas.
3. Coniferales:-
> Coniferales are the dominant forest-makers of the world. Out of 54 living genera of Coniferales in the world, ten have been reported from different parts of India. These are Abies, Cedrus, Cephalotaxus, Cupressus, Juniperus, Larix, Picea, Pinus, Podocarpus and Tsuga. The distribution of the majority of these members is restricted mainly in the Himalayas, and governed chiefly by altitudes.
Brief description of their distribution:-
- Abies is represented in the country by only 4 species, of which A. delavayi and A. densa grow in eastern Himalayas, whereas A. pindrow and A. spectabilis are restricted to western Himalayas. A. pindrow grows luxuriantly at about 2500 metres above sea level, whereas A. delavayi occurs commonly at an altitude of about 2750-3350 metres above sea level. A. densa grows luxuriantly in Darjeeling and adjacent hills.
- Cedrus, in India, is represented by only one species i.e. Cedrus deodara (vern. Deodar). This beautiful tree is famous for its wood, and grows in western Himalayas between 1200-3300 metres. Cephalotaxus grows in eastern Himalayas, and is represented by only two species (C. griffithi and C. mannii).
- Cupressus tortulosa grows throughout in the outer and middle ranges of Himalayas from Chamba hills in Himachal Pradesh to Aka Hills in Assam at an altitudes from 1800 to 2800 metres C. funiberus and C. sempervirens are cultivated as ornamental plants in Indian gardens.
- Six species of Juniperus have been reported from higher altitudes of eastern as well as western Himalayan regions of India and Bhutan. J. communis occurs between 2900-4250 metres in Garhwal Himalayas, whereas J. coxii grows both in eastern as well as Western Himalayas.
- J. macropoda grows between 2500-4300 metres in Laddakh, Kanawar and Alaknanda valley, whereas J. recurva and J. squamalata grow between 3000-5000 metres in the eastern Himalayas. J. wallichiana grows at an altitude between 3000 to 4200 metres in the Himalayan ranges.
- Larix griffithiana occurs generally mixed with Abies, Pinus and Tsuga in Sikkim, Chumbi valley of Tibet, Bhutan and Mishmi Hills in Assam. Picea smithiana grows in western Himalayas and attains a height of about 60 metres. It grows from Afghanistan to Kumaon.
- Pinus is represented by 6 species in India. These are P. roxburghii, P. wallichiana, P. insular is, P. gerardiana, P.armandi and P.merkusii. They are distributed throughout in Himalayas. Pinus roxburghii (vern. Chir) grows at lower altitudes between 2000-5000 feet, whereas P. wallichiana grows in north-west Himalayas ranging from an elevation of 5000-12000 feet.
- Pinus gerardiana (vern. Chilgoza) occurs on the Himalayan flanks extending from Punjab to Afghanistan and Baluchistan ranging within the altitudes of 5000 to 12000 feet. Pinus insularis grows in Khasia and Chittagong hills.
- Two species of Podocarpus (P.neerifolia and P. wallichianus) occur in India. The former grows in Andaman Islands and eastern Himalayas, whereas latter grows in Nilgiri hills and Assam. Tsuga dumosa grows luxuriently in Darjeeling and adjacent regions, along with Abies densa, above an altitude of 2700 metres.
- Besides the above-mentioned 10 genera, which grow naturally in different parts of India, some Coniferales are cultivated as ornamentals in Indian gardens.
- There include Araucaria cooki and A. cunninghamii of Araucariaceae, Thuja occidentalis and Cupressus cashmeriana of Cupressaceae. Callitris cupressi of Cupressaceae is grown as a hedge plant in Nilgiris, whereas Cryptomeria japonica of Taxodiaceae is grown in Darjeeling and other areas.