Current Affairs 11th Class

Category : 11th Class

(Gk. Gymnos = naked; sperma = seed)

The term gymnosperm was introduced by Theophrastus. Gymnosperm (Gk. Gymnos = naked ; sperma = seed) are the plants with exposed or naked seeds or ovules. These plants represent the most ancient group of seed plants. Gymnosperms are called fruitless phanerogames or flowering plants without ovary Robert Brown (1827) separated them from angiosperms and placed under a distinct group due to presence of unprotected ovules in them. The gymnosperms originated much earlier then angiosperms. However, most of the members of this group have now become extinct and only few living forms are known today.

Distribution

Plants of gymnosperms occur throughout the world. The group is presently represented by only 900 living species. Of these, about 500 species belong to 'Conifers' or cone bearing plants. Several species of conifer occur in north-west America and eastern and central China. In India several members are found in Himalayas, Podocarpus and Cupressus in the central and Larix, Tsuga, Cephalotaxus in the eastern.

World's tallest gymnospermous tree is Sequoia sempervirens (368.5 ft.) in height. Largest gymnospermous tree is Sequoiadendron (13-16 meters) in diameter. The longest gymnospermous tree (in age) is Pinus longavaea (4900 years old). The smallest gymnospermous tree is Zamia pygma.

Habit

Living gymnosperms are mostly perennials, xerophytic, evergreen, arboreal and woody plants. They grow as woody trees, bushy shrubs or rarely as climbers (e.g., Gnetales). None of them are herbs or annuals.

External features

(1) The plant body is sporophyte and differentiated into root, stem and leaves.

(2) The plant possess well developed tap root system. In some cases the roots are symbiotically associated with algae (e.g., Coralloid roots of Cycas) or with fungi (e.g., Mycorrhizal roots of Pinus).

(3) The stem is erect, aerial, solid, woody and branched (unbranched in Cycadales) but almost tuberous in Zamia.

(4) The leaves may be microphyllous or megaphyllous.

(5)          In gymnosperm xylem is generally made up of tracheids but vessels have been observed in Gnetum, Ephedra and Welwitschia.

(6) The primitive haplochelic type of stomata are found in Cycas, Pinus, Ginkgo, Ephedra etc.

(7) Dwarfism is seen in many gymnosperms perhaps due to wind velocity. eg., Picea engelmannali (15 cm).

Gymnospermous wood

Manoxylic wood : The wood formed may be in one ring due to persistent cambium. Such a wood is called as monoxylic e.g., Pinus.

Cambial activity is short lived, cortex and pith are broad, parenchymatous rays are broad, wood is soft and commercially useless. e.g., Cycas.

Pycnoxylic wood : The wood is formed in more than one ring due to ephimeral nature of cambium. Such a wood is called as polyxylic.

Cambial activity is long lived, cortex and pith are reduced, parenchymatous rays are few, wood is hard and compact, wood is commercially most important and used as good quality timber. e.g., Pinus.

Reproduction

Gymnosperms are heterosporous, i.e., produce two different kinds of spores – the male microspores and the female megaspores. The spores are borne inside the sporangia. The two types of sporangia are borne on special leaf-like structures, called sporophylls. The microsporangia (pollen sacs) are born on microsporophylls (= stamens) and the megasporangia (ovules) are borne on megasporophylls (= carpels).

The sporophylls are usually aggregated in the form of compact structures called cones or strobili. The cones are generally unisexual, i.e., the male cones are microsporangiate (pollen cones) and the female cones are megasporangiate (seed cones). The male cones are short lived whereas the female cones are long lived. The female cones remain attached on the plants for several years till the maturity or ripening of the seeds.

Pollination

The microsporangium (Pollen sac) produces numerous light pollen grains. Pollination is anemophilous (wind pollination). The ovules are orthotropous and remain exposed on the megasporophyll. Each ovule is surrounded by integuments. It encloses the nucellus and a female gametophyte formed from the haploid megaspore. The female gametophyte contains archegonia. The pollen grains are captured by the pollination drop secreted by the micropyle of the ovule. When it dries, the grains are sucked in the pollen chamber. The pollen grains now germinate. A pollen tube is formed due to elongation of the tube cell. In Cycas and Ginkgo the pollen tube is haustorial in nature. The lower end of the tube bursts and releases the male gametes which fuse with the egg to form the zygote.

Fertilization

Fertilization occurs by siphonogamy, i.e., the male gametes are carried to the archegonia through pollen tube (except in Cycas where pollen tube functions as haustorium and fertilization occurs by zoodiogamy). Fertilization thus takes place in the absence of external water.

Embryogeny

The zygote undergoes free-nuclear divisions in Cycas followed by wall formation. There are no free-nuclear divisions in Sequoia and Gnetum. The embryo is soon differentiated into an upper haustorial, middle suspensor and lower embryonal regions. In Pinus, on the other hand, the zygote gets differentiated into four tiers of four cells each, designated as open tier, rosette tier, suspensor tier and embryonal tier. Cleavage polyembryony is seen in Pinus. The embryonal part shows differentiation of radicle, hypocotyl, cotyledons and plumule.

Seed

As a result of fertilization the ovule develops into a seed. The integument forms the seed coat. The middle stony layer of the integument forms the testa whereas the inner fleshy layer gives rise to tegmen. The nucellus persists as a cap like perisperm. In Taxus a fleshy aril is also present at the base as a cup like structure. The seeds of gymnosperms comprise tissue of three generations namely parent sporophytic (integument and nucellus), gametophytic (endosperm) and second sporophytic (embryo).

Living fossils

When a group of plants is represented by a single genus or species while rest of the other representatives of the group have become extinct and fossilized the long surviving individual is called a living fossil e.g., Ginkgo biloba. However, Cycas is also regarded as a living fossil because most of the cycad species are confined to tropical and subtropical regions and the group is becoming endangered. Therefore, cycads have been referred as reptiles of plant kingdom or panda of vegetable kingdom.

Economic importance

(1) Ornamentals : Some of the gymnosperms are grown in the gardens in different parts of the world e.g., Cycas revoluta, Ginkgo biloba, Araucaria cookii, A. bidwilli, Biota orientalis, Cupressus sp., Juniperus sp., Thuja sp., Taxus baccata, Cryptomeria japonica etc.

(2) Wood : Several plants of this group yield useful timber. The wood of Cedrus deodara is used for making railway sleepers. It is also used as a structural timber and making bridges. The wood of Callitris verrucosa, Pinus roxburghii, P. wallichiana, P. pinaster, P. lambertiana etc. is used for making furniture. Juniperus virginiana wood is used for making pencils. The gymnosperm Agathis australis is perhaps the largest timber producing tree of the world. Soft wood of many gymnosperms is used for making toys.

(3) Resins : Several conifers yield resin which is obtained by tapping. By distilling the oleoresin is obtained from pines.

The resins are of three types namely hard resins, oleoresins and gum-resins. Several hard resins are obtained from living and fossil conifers which are as under :

Copals : Kauri copal is obtained from Agathis australis and manila copal from Agathis alba.

Amber : It is obtained from the fossil conifer Pinites succinifera.

Sondarac : This pale-yellow resin comes from Callitris quadrivalis and Tetraclinis articulata. The turpentines are oleoresin which are also contributed by conifers.

An important source of turpentine is Pinus australis, P. ponderosa and P. caribeae. Besides, some of the following products of turpentine nature are also obtained from conifers :

Canada balsam : It is obtained from Abies balsamea.

Spruce gum : It is obtained from Picea rubens.

Bordeaux turpentine : It is obtained from Pinus pinaster.

Venetian turpentine : It is obtained from Larix decidua.

(4) Essential oils : They are obtained from several plants. These oils are used mainly in perfumery, soap industry etc. The important oil yielding plants are Tsuga canadensis, Picea glauca, Abies siberica and Cedrus deodara.

The oil obtained from Juniperus virginiana (cedar wood oil) is also used in microscopic work.

(5) Paper industry : The wood of several gymnosperms, particularly those of conifers is used in paper industry e.g., Abies pindrow, Picea smithiana, Cryptomeria japonica, Pinus roxburghii, Tsuga canadensis etc.

(6) Edible : The seeds of Pinus gerardiana (chilgoza) and P. roxburghii are edible. Sago is obtained from Cycas revoluta. The seeds of Cycas sp. are ground and used in the preparation of many edible products.

(7) Medicinal use : Species of Ephedra yield an alkaloid called ephedrine. It is used in the preparation of medicines for the treatment of cough, asthma and bronchitis.