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Consortium for Educational Communication Module on General Characters of Gymnosperms By Syed Akhlaq Geelani Lecturer in Botany Mobile No.9622841493 E-mail:[email protected] Consortium for Educational Communication Text The word gymnosperm has been derived from a Greek words gymnos meaning naked and sperma meaning seeds. Thus the gymnosperms are group a of plants with naked seeds. The word gymnosperm was first used by Theophrastus in 300 B.C. in his book “Enquiry into Plants”. Theophrastus included all the plants with naked seeds under gymnosperms. They have been generally placed in the division Spermatophyta, along with angiosperms. Gymnosperms show close affinity with pteridophytes on one hand and with the angiosperms on the other. Thus the gymnosperms act as a bridge between pteridophytes and angiosperms. The gymnosperms originated during the late Paleozoic era but flourished well during Mesozoic era. Hence the Mesozoic era is called as the age of gymnosperms. However, most of the members of this group have now become extinct and less than a hundred genera are known today. Some gymnosperms like Ginkgo biloba, and species of Cycas and Metasequoia are called living fossils, as these plants are found both in living and fossil forms, and the number of fossil forms is much more than the living forms. There are about 70 genera with 725 living species of gymnosperms in the world today. In India, there are 16 genera with about 53 species. These genera are Cycas from Cycadales; Abies, Cedrus, Cephalotaxus, Cupressus, Juniperus, Larix, Picea, Pinus, Podocarpus, Taxus and Tsuga from Coniferales; Ephedra from Ephedrales; and Gnetum from Gnetales. In addition, Ginkgo biloba, a species from ChinaJapan, is also grown in certain parts of India. Occurrence and Distribution 1.Gymnosperms are found both in tropical and temperate regions; some species occur in arctic regions. The living gymnosperms comprise 70 genera and 725 species. Of these, majority belong to conifers or cone- bearing plants. The conifers are mostly distributed in the cooler regions of Europe, Asia and North America. They are dominant in upper timber-line zone of mountain ranges including the Himalayas. Consortium for Educational Communication 2.The living gymnosperms are widely distributed in the cold climates where snow, rather than rain, is the source of water. The only native gymnosperms which thrive in warm areas are the cycads. 3. In India, mostly the conifers thrive well in hilly areas. Very few gymnosperms, except some cycads, grow in plains. Cycas circinalis grows wild in some southern States. Some gymnosperms are grown as ornamental plants in gardens, e.g. species of Araucaria. Habit 1.Living gymnosperms are mostly perennial, xerophytic, mostly 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. 2.Gymnosperms vary in their size from small herbs, e.g. (Zamia pygmaea) to large gigantic trees. Sequoia sempervirens, a gymnosperm, is the tallest living plant in the world, reaching a height of about 125 meters, and girth of about 30 meters. It is also known as coast red-wood of California. Taxodium mucronatum and Sequoiadendron giganticum with a girth of 125 feet and 80 feet, respectively, are the other gigantic gymnosperms. On the contrary Zamia pygmaea, with an underground tuberous stem, is the smallest gymnosperm, about 25 cm in height. External morphology 1. Most of the gymnosperms are tall, evergreen trees or shrubs. There are no herbs. Certain species of Gnetum are twiners. 2.The plant body is sporophyte and differentiated into root, stem and leaves. 3.Plants posses well-developed tap root system. Root often shows symbiotic association with algae, e.g. Cycas, or with fungi e.g. Pinus (Fig.1). In Cycas, coralloid roots are Consortium for Educational Communication present in addition to the normal roots. 4.The stem is aerial, erects, woody, unbranched, as in Cycas, or branched as in Pinus, Cedrus, etc. In some genera like Pinus, the branches are of two types. i). Long shoots or branches of unlimited growth ii). Dwarf shoots or branches of limited growth. 5.The leaves are mostly xerophytic, evergreen, simple or compound, and vary in size from small scaly leaf to more than six feet long leaves as in Cycas. The leaves of Pinus are of two types, i.e. scale leaves and foliage leaves. Scale leaves are minute, membranous, brown and deciduous. Foliage leaves are present on dwarf shoots only, and they are known as foliage spurs (Fig. 2). They are green, simple, needle-like or pinnately compound. The leaves of Cycas exhibit circinate vernation, a characteristic feature of ferns. The arrangement of leaves may be opposite and decussate as in Gnetum, whorled as in Cedrus, or spirally arranged as in Taxus. Internal structure Stem 1.Anatomically the stem shows an epidermis, cortex and stele. The epidermis is single-layered and very soon replaced by periderm. Cortex is parenchymatous. Based on the relative quantity of soft tissues and mechanical tissues, gymnosperms are classified into manoxylic and pycnoxylic types. 2.In manoxylic forms, as in Cycads, the wood is not compact, and has parenchyma. In pycnoxylic forms wood is compact, and relatively less amount of soft tissue is present. The stele is surrounded by a typical epidermis followed by a layer of pericycle. 3.The vascular system consists of a number of vascular bundles arranged in the form of a ring, surrounding the central pith. Consortium for Educational Communication 4.The vascular bundles are collateral, endarch and open. This type of stele is called eustele. 5.Phloem consists of only sieve tubes and phloem parenchyma. Companion cells are absent. 6.Xylem consists of trachieds and xylem parenchyma only. Vessels are absent except in Gnetales. The xylem is either endarch or mesarch. 7.Secondary growth is seen in all the members. It is usually normal. However, anomalous secondary growth may be found in Cycas and Gnetum. Normal secondary growth takes place due to the activity of fascicular cambium, which produces the typical annual rings. 8.Parenchymatous pith is present in the central region of the stem. In some cases mucilage ducts or resin canals are found both in the pith and cortex (Fig.3). Internal structure of Leaf 1.Anatomically, the leaf exhibits xerophytic characters. The epidermis is always covered by a cuticle. The stomata are sunken, and often present on pits. 2.The mesophyll may be differentiated into palisade and spongy parenchyma as in Cycas or undifferentiated with arm parenchyma as in Pinus (Fig.4). 3.In some members, as in Cycas with a single-veined leaf, a transfusion tissue is present to help in lateral conduction, necessitated by the absence of side veins. 4.The stomata are sunken and often present on pits. The stomata are either syndetocheilic or haplochelic. 5.In syndetocheilic type, guard cell and subsidary cells originate from the same mother cell. In haplochelic type, the guard cells and subsidiary cells originate from the different mother cells. Haplocheilic stomata are considered to be primitive compared to syndetocheilic. Consortium for Educational Communication 6.In Gnetum both types of stomata can be seen. Stomata may be distributed on both sides of leaf as in Pinus and Ginkgo, or confined to the lower side as in Cycas. Internal structure of root 1.Most of the gymnosperm roots show a typical dicotyledonous root structure; the xylem is either diarch or triarch. Cortex is mostly parenchymatous. As in the stem, secondary growth is seen both in the cortex and the stele. Reproduction 1.Vegetative reproduction is very rare, and is seen in some cycads where it is brought about by the formation of bulbils. 2.Spore formation is the typical mode of reproduction in gymnosperms. 3.All the gymnosperms are heterosporous, i.e. they produce two different kinds of spores, the male microspores and the female megaspores. The spores are borne inside the sporangia. 4.The two types of sporangia are born on special leaflike structures, called sporophylls. The microsporangia or pollen sacs are born on microsporophylls or stamens, and the megasporangia or ovules are born on megasporophylls or carpels (Fig.5). 5.The sporophylls are usually aggregated in the form of compact structures, called cones or strobili. Plants may be monoecious or diocious with reference to strobili. Very rarely bisporangiate strobili may be found. 6.Male cones are generally short-lived and smaller than female cones. Microsporophylls may be flat, as in Cycas, or peltate as in Pinus. 7.Female cones are usually long-lived. They sometimes Consortium for Educational Communication remain on the plants for several years till maturity and ripening of the seeds. Megasporophylls may be foliar or leaf-like and not aggregated in the form of cones as in Cycas (Fig. 6), or cauline and aggregated in the form of cones as in Pinus. 8.Microsporangia are born on the abaxial or morphologically lower surface of microsporophylls. They may be numerous and grouped into sori as in Cycas or reduced to two as in Pinus. 9.In Cycads, the male cones are the largest among the gymnosperms, and arise singly at the top of the plant. Female cones are not formed, but megasporophylls are loosely arranged around the stem apex. 10. In Pinus, the male cones arise in clusters, at the end of the long shoots, in the axil of scale leaves. Female cones are large and are arranged in two or three pairs on the long shoots (Fig.7). 11. In Gnetum the reproductive structures are known as inflorescence. The male inflorescence or male cone consist a long axis, on which cup-shaped structure, called cupules, are formed by the fusion of brackets. In the axil of these cups, male flowers are born. The female inflorescence or female cone also contains a long axis with cupules. In the axil of these cupules, 4-10 female flowers or ovules are arranged in a ring. Bisporangiate strobili are also noticed in some species of Gnetum. Ovule and female gametophyte 1.In gymnosperms, ovules are naked, and born on megasporophylls. The ovules of Cycas are largest among the plant kingdom. The ovule consists of central mass of parenchymatous tissue called nucellus. The nucellus is surrounded by integuments, which covers the whole nucellus except at the top, where it leaves a small passage called microphyle (Fig. 8). Consortium for Educational Communication Consortium for Educational Communication 2.The integument is differentiated into an outer fleshy layer or sarcotesta, middle stony layer sclerotesta and the inner fleshy layer or sarcotesta. The apex of the nucellus develops into pollen chamber, where pollen grains undergo period of rest after pollination. 3.The development of female gametophyte begins with the differentiation of megaspore mother cell in the nucellus. Its development is either monosporic or tetrasporic. Megaspore mother cell undergoes meiosis to form linear tetrad of four megaspores, out of which, upper three cells degenerate, and the lower most remains as functional megaspore. 4.The functional megaspore then develops into female gametophyte. Archegonia are formed towards the micropylar end of the female gametophyte. In Gnetum archegonia are not formed. A few nuclei at the microphylar end, of the female gametophyte act as egg, which is an angiospermic character. The archegonia of gymnosperms do not have neck canal cells and Venter canal cells. Microspore and male gametophyte 1.Pollen grains or microspores are unicellular. In Pinus pollen grains are winged (Fig.9). Microspores develop into male gametophyte. During the germination of microspores, one or two prothelial cells are cut off at one end, leaving large anthredial cell, which divides into generative cell and larger tube cell. The generative cell divides into stalk cell and body cell. Body cell produces two male sperms. The tube cell forms the pollen tube, which acts as carrier of sperms (Fig.10). Formation of male gametophyte Pollination 1.In gymnosperms, pollination is anemophilous. The ovule secretes mucilaginous substance, which oozes out from the micropyle of the ovule and forms a pollination drop, which traps the pollen grains. The pollen grains are drawn on due to drying of pollination drop. Further changes take place in the pollen chamber. The pollen tube grows through the tissue of the nucellus, until it reaches the neck of the archegonia. Fertilization 1.Gymnosperms do not require water for fertilization. 2.At the time of fertilization, the tip of the pollen tube bursts and releases the sperms. The sperms swim to the neck of the archegonia with the help of cilia, and one of the sperms fuses with the egg to form the zygote. In Pinus and Gnetum the sperms are non-motile, which is an advanced character. 3.This type of fertilization is called siphonogamy. The young sporophyte Consortium for Educational Communication 1.After fertilization the zygote is formed. In gymnosperms the development of zygote is meroblastic, in which only the basal (embryonal) part develops into an embryo or embryos, while the upper (houstorial) and middle (suspensorial) parts do not take part in the formation of embryo. Embryo is straight with the exception Bennettitales. Embryogeny 1.In gymnosperms, except in Gnetum, the diploid nucleus undergoes free nuclear division. In Cycas, as many as 1000, or more, nuclei are formed as a result of free nuclear division. In Pinus, only four nuclei are formed, after that the wall formation begins, resulting in the formation of 16-celled proembryo. Polyembryony 1.Polyembryony is the characteristic feature of gymnosperms. In cycads, more than one archegonia are fertilized, resulting in the formation of more than one embryo. This phenomenon is known as simple polyembryony. In conifers, polyembryony is due to the separation of four embryonal cells, and each of them develops into an embryo. This phenomenon is called cleavage polyembryony. It occurs in Pinus, but only one embryo attains maturity and the rest degenerate. 2.In gymnosperms, the embryo is differentiated into root, stem and leaf. It is endoscopic, which means plumule is directed away from micropyle and radical towards the microphyle (Fig.11). Seed 1.After fertilization, the ovule develops into seed. The inner fleshy layer of ovule turns into tegmen, and middle stony layer forms testa or the seed coat, which protect inner embryo. 2.Embryo is developed at the end of long suspensor which is pushed down into the food ladden endosperm that develops before fertilization. Female gametophyte, Consortium for Educational Communication soon after it has absorbed the food from the nucellus, becomes endosperm. 3.As there is no double fertilization or triple fusion, the endosperm is a haploid tissue. 4.The whole of the nucellus, except a small portion, is crushed. The remaining small part of the nucellus persists in the seed in the form a narrow film, or cap, known as perisperm. 5.Since there is no closed ovary, true fruits, like that of angiosperms are not found in the gymnosperms. The developed ovules remain naked. 6.Middle stony layer of the integument becomes extremely hard and protects the female gametophyte and embryo. 7.Cotyledons may be one or two, as in Cycas, or a whorl of many, as in Pinus. They become green while still enclosed within the seeds. 8.The seed generally undergoes a period of dormancy. However, the seed of Ginkgo and Cycas germinates without undergoing any period of rest. 9.During germination, the seed coat ruptures, radical grows downwards and develops into primary tap root, while the plumule grows upright and develops into stem, whose growth is mostly unlimited except in Welwitschia. 10. Sporophytic generation is diploid and is dominant and independent, while the gametophytic generation is much reduced and dependent. Affinities Gymnosperms form a heterogeneous plant group with several evolutionary trends which have been operative since past. Evolutionary trends reveal that gymnosperms form a bridge between pteridophytes and angiosperms. Keeping in view their resemblance with pteridophytes and angiosperms on one hand, and certain difference with these Consortium for Educational Communication groups on the other hand, they have been segregated as an independent group of plants. Resemblance with Pteridophytes Gymnosperms resemble with pteridophytes in the following features. 1.Both the plant groups show heterologous alternation of generation. Sporophytic generation is dominant, independent and large at maturity, while the gametophytic generation shows progressive reduction and dependence. 2.Sporophytes of both the plant groups are distinguished into root, stem and leave, and possess well-marked vascular system. 3.Many cycads resemble ferns in having large compound leaves and circinate vernation. 4.Both heterosporous pteridophytes and gymnosperms possess endosporic gametophytes. 5.Male gametes of cycads (Cycas) and Ginkgo are motile like that of pteridophytes. 6.Female sex organs are archegonia in both pteridophytes and gymnosperms. 7.Male gametophytes of heterosporous pteridophytes and majority of gymnosperms are highly reduced and posses only two prothalial cells. Consortium for Educational Communication pteridophytes Gymnosperms differ from pteridophytes in the following features: 1.Gymnosperms are commonly large-sized trees, shrubs or climbers. Pteridophytes on the other hand, are smaller in size. 2.Gymnosperms posses tap roots whereas roots of pteridophytes are adventitious. 3.Gymnosperms have eustelic organisation, whereas in pteridophytes eustele is not found. 4.Gymnosperms undergo secondary growth, whereas pteridophytes lack secondary growth. 5.In gymnosperms male and female gametophytes are simple, non-green and dependent on sporophytes. They are very much reduced in comparison to gametophytes of pteridophytes. 6.In gymnosperms megaspore remains in megasporangium, whereas in pteridophytes they are shed from the sporangia and develop into independent female prothallus. 7.In gymnosperms, neck canal cells and venter canal cells are absent, whereas archegonia of pteridophytes are provided with neck canal cells and venter canal cells. 8.Seed formation is not common in pteridophytes except in Selaginella, whereas in gymnosperms seeds are produced. 8.Endosperm develops before fertilization, as a result of free nuclear division in the tissue of female gametophyte in both gymnosperms and pteridophytes. Affinities with angiosperms 9.Embryogeny is endoscopic in many pteridophytes and majority in gymnosperms Gymnosperms resemble with angiosperms in the following features. 1.Plants of both the groups are woody and perennial. Differences between gymnosperms and 2.The stele is eustelic both in gymnosperms and angiosperms. Consortium for Educational Communication 3.Both show secondary growth, except monocots. 4.Both are heterosporic and endosporic. 5.Megaspore is retained in megasporangium, resulting in the formation of seed. 6.The male gametophytes are highly reduced. 7.Pollen tube acts as carrier of sperms. 8.The sperms are non-motile except in Cycas. 9.The plants show distinct alternation of generations. Differences between gymnosperms and angiosperms Gymnosperms differ from angiosperms in the following features: 1.Majority of angiosperms are herbaceous, whereas gymnosperms are mostly woody. 2.Angiosperms may be annuals, biennials or perennials, whereas gymnosperms are mostly perennials. 3.In angiosperms, xylem possesses vessels and phloem bears companion cells, whereas in gymnosperms (except in Gnetales) xylem is devoid of vessels and phloem lacks companion cells. 4.In gymnosperms cones are unisexual, whereas in angiosperms the flowers are usually bisexual. 5.In gymnosperms ovules are naked, whereas in angiosperms they are enclosed in the ovary of the carpels. 6.Gymnosperms do not show double fertilization or triple fusion, whereas in angiosperms it is a common phenomenon. Consortium for Educational Communication 7.Angiospermic seeds are enclosed within the ovaries that develop into fruits, whereas in gymnosperms seeds are exposed. Classification of gymnosperms Chamberlain (1934) divided the gymnosperms into two large groups: A.Cycadophyta: - In this group sporophylls are in cones. It includes three groups; Cycadofilicales, Bennettitales and Cycadales. B.Coniferophyta:- This group included four orders; Ginkgoales, Cordaitales, Coniferales and Gnetales. Of these Cycadofilicales, Bennettitales and Cordaitales are totally extinct. Cycadales, Ginkgoales, Coniferales and Gnetales are living. 1.Arnold (1948) divided gymnosperms into three phyla. a.Cycadophyta – including pteridospermales, Cycadeoidales and Cycadales. b.Coniferophyta – including Cordaitales, Ginkgoales, Taxales and Coniferales. c. Chlamydospermophyta- including Ephedrales and Gnetales.