Gymnosperms are divided into four groups:
(1) Ginkgophyta:
Fossils as well as living plant. Only one plant is living e.g. Ginkgo biloba.
(2) Gnetophyta:
ADVERTISEMENTS:
Small trees or shrubs. All plants are living. Reproductive structures are unisexual, secondary xylem has vessels: e.g. Gnetum, Ephedra, Welwitschia.
(3) Cycadophyta:
Plants are small with unbranched stem. Large compound leaves. In T.S. of stem, pith and cortex are large and vascular tissue less. Manoxylic wood.
Cycadophyta has been divided into following 3 orders.
ADVERTISEMENTS:
Order 1:
Cycadofilicales:
(Pteridosperms) fossils only, Plants showed some characters of ferns and other characters of gymnosperms e.g. Medullosa.
Order 2:
ADVERTISEMENTS:
Bennettitales:
(Cycadeoidales) fossils only, Leaf base permanently attached to stem. e.g. Williamsonia.
Order 3:
Cycadales:
ADVERTISEMENTS:
Some fossils and some living plants. Palm like plants e.g. Cycas. Cycas has approximately 20 species found in Australia, New- Zealand, Japan, China, India, Burma, Indonesia and Pacific Islands. In India, cycas species are common in Orissa, Bengal, Assam, Madras, Karftataka and Andmans. Four species are found in India: Cycas revoluta, C. circinalis, C. rumphi and C. beddomei.
(4) Coniferophyta:
Large trees with unbranched stem. Simple foliage leaves. T.S. of stem shows small pith and cortex and large vascular tissue. It has Pycnoxylic wood. The plants are evergreen with dense and massive vascular tissue and non-motile gametes. They are monoecious (Both types of cones are present on the same plant).
Cones are compact and woody. Araucaria (Monkey’s puzzle) (A tall conifer of South America, New-Zealand and East Australia) grows in warm weather in plains as well as foot hills. Other conifers form vast and dominant wood lands in mostly north temperate regions of Europe, Asia, North America etc. e.g. Pinus, Cedrus, Picea, Abies (Fir), Thuja, Larix. Conifers are dominant constituents of north temperate flora due to following reasons:
ADVERTISEMENTS:
(i) In the temperate areas, conifers have an advantage over angiospermic trees. While the angiosperm trees shed their leaves during autumn winter period, the conifers remain evergreen.
(ii) Conifers have a number of xerophytic characteristics which help in conserving water. They are thus able to tide over the winter period when the soil becomes frozen and water availability is very little.
(iii) Enzymes of conifers are functional even at – 35°C at which temperature they become inactivated in other plants.
Coniferophyta has been divided into following four orders:
ADVERTISEMENTS:
Order 1:
Cordaitales: Fossils only. Leaves were spirally arranged on the stem with parallel venation, e.g. Cordaites.
Order 2:
Ginkgoales: Fossils as well as living plant. Only one plant is living e.g. Ginkgo biloba.
Order 3:
Coniferales:
Fossils as well as living plants. Usually dwarf and long branches. Foliage leaves on dwarf branches only e.g. Pinus. In India about six species of Pinus are common. The plants grow in North-East and North-West Himalayas. In the hills of South India, the plants grow under cultivation.
Pinus gerardiana (Chilgoza pine) is common in dry valley of North-West Himalayas-Kashmir and quite common in Kinnaur district of Himachal Pradesh. Plants prefer scantly rainfall and Leany snow-fall.
Trees may attain a height of 11-20 meters. Pinus wallichiana (P. excelsa) or Blue Pine are commonly called “Kail”. They grow in the hills of Kashmir, Himachal Pradesh and Punjab. Trees may attain a height of upto 50 meters.
Pinus roxburghi or P. longifolia (Chir Pine) commonly occur in the outer Himalaya regions ranging from Indus to Bhutan. It grows in Kashmir, Punjab, Himachal Pradesh and Uttar. Pradesh. Trees may attain a height of about 35 to 53 meters.
Pinus merkusii (Teenasserim pine) commonly grow on hillocks in Burma and East India-Bengal etc. Trees are usually very small reaching a height of hardly 3 meters.
Pinus insularis or P. khasya (Khasi Pine) commonly occur in the hill ranges of Assam. It is abundant in Khasia hills but also occurs in Naga Hills and Chittagong.
P. armandi:
(Armand’s Pine) plants are very common in Nefa.
In addition to these species, several exotic pines like P. montana, P. laricia and P. sylvestris, have been introduced in India.
Order 4:
Gnetales:
Small trees or shrubs. All plants living. Reproductive structures are unisexual secondary xylem has vessels e.g. Gnetum.