In this article we will discuss about:- 1. Definition of Gymnosperms 2. External Features of Gymnosperms 3. Classification 4. Internal Structure 5. Reproduction 6. Affinities and Relationship 7. Evolutionary Significance.
Definition of Gymnosperms:
The term gymnosperms (gymnos = naked; sperma = seed) was introduced by Theophrastus in 300 BC to describe plants with unprotected seeds. According to Goebal gymnosperms are phanerogams without ovary.
The phanerogams or Spermatophyta (sperm = seed; phyton = plant) or seed plants are those plants which reproduce by means of seeds, not spores. Gymnosperms are the vascular plants where seeds are not enclosed within an ovary (opposite to an angiosperm or flowering plants where seeds are enclosed by mature ovaries or fruits).
In these plants the ovules are borne naked or the surface of the megasporophylls, which are often arranged in the cones. Fossil records indicate that the gymnosperms must have evolved approximately 300 million years ago from non-seed producing ancestors of the extinct division of Progymnospermophyta which were fern like in appearance (form a bridge between pteridophytes and angiosperms).
Gymnosperms were dominant plants over the earth’s surface during the jurassic and cretaceous periods of mesozoic era. At present about 83 genera and approximately 790 species of living gymnosperms are distributed throughout temperate, tropical and arctic regions of the world.
External Features of Gymnosperms:
1. Gymnosperms are predominantly woody plants, represented by trees, shrubs or rarely climbers.
2. They are usually xerophytic, some of them are deciduous while others are evergreen. Sequoia sempervirens (California or Coast red wood) is probably the tallest living tree reaching a height nearly 112 m and attaining a growth of 15 m. Smallest gymnosperm is Zamia pygmaea.
It is 25 cm tall. Taxodium maxicanum has a trunk with the enormous diameter of 17 meter. The bristlecone pines (three species of pines i.e., P. aristata, P. longaeva and P. balfouriana) are thought to reach an age greater than that of any other single organism known, upto nearly 5000 years.
3. Plant body is sporophytic and can be differentiated into root, stem and leaves.
4. Generally the plants possess well developed tap root system. In some gymnosperms the roots show symbiotic relationship e.g., coralloid roots of Cycas with algae and mycorrhizal roots of Pinus with fungi.
5. Stem is erect, woody and branched (unbranched in Cycas and tuberous in Zamia). Presence of leaf scars on the stem is the characteristic feature of gymnosperms.
6. The arrangement of the leaves on the stem may be spiral or cyclic. They may be of one kind (monomorphic) or two kinds (dimorphic, foliage leaves and scale leaves). Foliage leaves are green, simple, may be small (microphyllous e.g., Pinus) or large (megaphyllous e.g., Cycas). Their main function is photosynthesis. Scale leaves are present around the reproductive structures and apex. They are mainly protective in nature.
Classification of Gymnosperms:
In older times gymnosperms were kept among angiosperms. It was Robert Brown (1827) who first of all recognised these plants due to presence of naked ovules and placed them in a distinct group called gymnosperms. Bentham and Hooker (1862-83) in their ‘Genera Planterum’ placed this group in between dicotyledonae and monocotyledonae.
The classification of gymnosperms is quite controversial because several genera and a few orders like the cordiatales and cycadeoidales are known only in fossil state. Attempts have, however, been made from time to time to classify them. Some of the important classifications are as follows:
Van Tieghem (1898) treated gymnosperms as one of the two divisions of Spermatophyta and further divided it as follows:
Afterward due to the discovery of large number of fossil genera three more orders viz. Cycadofilicales, Benettitales and Cordaitales were included in this group.
Eichier (1883) considered gymnosperms as one of the two divisions under Phnanerogamae. the second division being Angiospermae.
Eicher (1889) classified the seed plants as:
Coulter and Chamberlin (1910) divided the gymnosperms into seven order:
Engler and Prentl (1926), Rendle (1926) and others also divided the gymnosperms into 7 orders.
Seward (1919) on the basis of wood structure divided gymnosprms into two classes:
Class 1:
Manoxylic (wood is not compact).
Class 2:
Pycnoxylic (wood is compact).
Birbal Sahni (1920) divided the gymnosperms into two divisions:
Chamberlin (1934) divided the gymnosperms into two sub-classes: Cycadophyta and Coniferophyta.
Sub-class I. Cycadophyta:
(a) Gymnosperms with fern like pinnatified leaves.
(b) Unbranched stem, and large foliage leaves
(c) Large globose or columnar trunks.
(d) Only surviving representatives are modern cycades.
(e) Large pith and cortex
(f) Wood is relatively small, manoxylic.
It is divided into three orders:
Order 1Cycadofilicales:
Group of fossil plants, plants resembling with ferns, hence they are given the name pteridospermae (i.e., seed bearing ferns) e.g. Lyginopteris, Medullosa.
Order 2 Bennettitale or Cycadeoidales:
(a) Long, profusely branched stem.
(b) Simple, small leaves (needle, like, scale like or laminate)
(c) In stem the amount of wood is much more than cortex and pith, Pycnoxyhlicwood.
(d) Group includes extinct as well as extant order.
It is divided into four orders:
Order 1. Cordaitales:
All members are extinct e.g., Cordaites, Dadoxylon,
Order 2. Ginkgoales:
All members except one Ginkgo biloba are extinct.
Order 3. Coniferales:
The order includes both fossil and present day forms e.g., Pinus, Cedrus, Sequoia.
Order 4. Gnetales:
This order includes modern living forms. The order differs from other gymnosperms by the presence of vessels in the xylem e.g., Ephedra, Gnetum, Welwitschia.
Pant proposed a classification of gymnosperms in 1957. He divided the gymnosperms into three divisions Division cycadophyta and coniferophyta are subdivided into four classes each, while division Chlamydospermophyta is represented by a single class.
Division 1. Cycadophyta:
Manoxylic wood, unbranched or poorly branched trunks, leaves, large and pinnate, divided into four classes:
Class 1 Pteridospermopsida.
It is divided into six orders:
Order 1. Lygenopteridales
Order 2. Medullosales
Order 3. Glossopteridales
Order 4. Peltaspermales
Order 5. Coryostospermales
Order 6. Caytoniales.
Class 2. Cycadopsida, includes one order Cycadales.
Class 3. Pentoxylopsida, includes one order Pentoxylales.
Class 4.Cycadeoideopsida or Bennettiospida includes one order Bennettitales like angiosperms.
Division 2. Chlamydospermophyta:
Like angiosperms
Class 1. Gnetopsida – 2 orders – 1 order: Gnetales
Order 2. Welwitschiales
Division 3. Coniferophyta:
Large sized trees with profusely branched stems, pycnoxylic wood, simple leaves, divided into four classes:
Class 1. Coniferopsida – 3 orders : Order 1. Cordaitales, Order 2. Coniferales, Order 3. Ginkgoales.
Class 2.Ephedropsida.1 order. Ephedrales
Class 3. Czekanowskiopsida – 1 order . Czekanowskiales
Class 4.Taxopsida – 1 order.Taxales.
The main features of Pants classification are as follows:
(a) He strictly followed the recommendations of international code of Botanical Nomenclature, 1956.
(b) A new class Ephedropsida with a single order Ephedrales has been included under division Coniferophyta.
(c) A new class Czekanowskiopsida with a single order Czekanowskiales has been established within discovery of new genus Czekanowskia.
(d) The class Pentoxylopsida with order Pentoxylales has been included under division Cycadophyta.
(e) Orders like Glossopteridales, Peltaspermales, Coryostospermales, Caytoniales have been included under class Pteridospermopsida.
Raizada and Sahni (1960) divided the gymnosperms into 8 orders:
Following Pilger and Melchoior (1954), Sporne (1965) divided gymnosperms into three classes and nine orders:
Cronquist, A. Takhtajan and Zimmermann (1966) divided all embryo bearing plants (Embryophyta) into divisions. They placed gymnosperms in the division Pinophyta.
Bierhorst (1971) classified gymnosperms into 3 classes and 11 orders:
Taylor (1981) divided the gymnosperms into six divisions:
Division 1. Progymnospermophyta
Division 2. Pteridospermophyta
Division 3. Cycadophyta
Division 4. Cycadeoidophyta
Division 5. Ginkgophyta
Division 6. Coniferophyta
Gifford and Foster (1989) raised the important groups to the rank of division.
Recently in 2011, new classification and linear sequence of extant (still existing) gymnosperms based on previous molecular and morphological, phylogenetic and other studies was proposed by Maarten J.M., Christenhusz and co-workers.
It is as follows:
Sub-class I. Cycdidae:
order A. Cycadales
Family 1. Cycadaceae
1 genus, 107 species e.g. Cycas
Family 2. Zamiaceae
9 genera, 206 species e.g. Zamia, Microcycas ec.
Sub-class II. Ginkgoidae:
Order B. Ginkgoales
Family 3. Ginkgoaceae
1 genus, 1 extant species e.g., Ginkgo
Sub-class III. Gnetidae:
Order C. Welwitschiales
Family 4. Welwitschiaceae 1 genus, 1 species Welwitschia
Order D. Gnetales
Family 5. Gnetaceae
1 genus, 30 species e.g. Gnetum Order E. Ephedrales
Family 6. Ephedraceae
I genus, 40 species e.g., Ephedra
Sub-class IV Pinidae:
Order F. Pinales
Family 7. Pinaceae
II genera 225 species e.g. Cedrus, Pinus, Picea etc
Order G. Araucariales:
Family 8 Araucariaceae 3 genera, 41 species, e.g. Araucaria
Family 9.Podocarpaceae 19 genera 180 species e.g. Phyllocladus, Halocarpus etc.
Order H. Cupressales:
Family 10. Sciadopityaceae
1 genus, 1 species e.g. sciadopitys
Family 11.
Cupressaceae 29 genera
130 species e.g. Cunninghamia, Cupressus etc.
Family 12. Taxaceae
6 genera 28 species e.g., Austrotaxus, Pseudotoxus etc.
Internal Structure of Gymnosperms:
1. The roots are diarch to polyarch.
2. In stem a well-developed vascular system is present in the form of vascular bundles. The vascular bundles are open, endarch and are arranged in a ring.
3. Secondary growth takes place in the gymnosperms and there is the formation of annual rings.
4. The xylem is composed of tracheids with border pits and xylem parenchyma.
5. Secondary wood is of two types: manoxylic (porous and loose due to presence of large amount of parenchyma and broad medullary rays e.g., Cycas, useless commercially) and pycnoxylic (compact and hard due to less amount of parenchyma and narrow medullary rays e.g., Pinus). Bordered pits may be uniseriate or multiseriate.
6. Like pteridophytes, in the wood xylem vessels and fibres are absent (except in Gnetales).
7. Phloem is composed of sieve tubes and phloem parenchyma. Companion cells are completely absent.
8. Leaves are characterised by the presence of thick cuticle and sunken stomata.
9. Mesophyll tissue may be differentiated into palisade tissue and spongy parenchyma e.g., Cycas or remain undifferentiated e.g., Pinus, Cedrus etc.
10. Lateral veins are absent in most of the gymnosperms. So, the translocation of the nutrients takes place with the help of transfusion tissue.
Reproduction in Gymnosperms:
1. It is of two types: vegetative and sexual, reproduction.
2. Vegetative reproduction takes place by the formation of bulbils or adventitious buds which develop on the stem in the axil of the scale leaves e.g., Cycas.
3. Gymnosperms are heterosporous and sexual reproduction is oogamous type. In living gymnosperms the reproductive structures are arranged in the form of strobili or cones (except the ovulate structures of Cycas).
4. Plants may be monoecious (e.g., Pinus) or dioecious (e.g., Cycas).
5. Usually the cones are monosporangiate (unisexual) but in certain members e.g., Ephedra bisporangiate (bisexual) cones have been reported.
Male Cones:
6. Male cone consists of many microsporophylls (stamens) arranged on central axis.
7. Microsporophylls bear microsporangia (pollen sacs) on the abaxial (lower) side.
8. Sporangial development is eusporangiate (develop from a group of cells).
9. Meiosis occurs in microspore mother cells and haploid pollen grains (male gametophytes) are developed.
10. During the development of male gametophyte only one (e.g., Cycas) or two (e.g., Pinus) prothallial cells are formed.
11. Pollen tubes act as sperm carrier only in higher forms (e.g., Pinus, Ephedra) whereas in the lower forms it acts as haustorium (e.g., Cycas).
12. Pollen grains may be smooth (e.g., Cycas) or winged (e.g., Pinus).
Female Cones:
13. Female cone is the aggregation of megasporophylls (carpels) which bear megasporangia (ovules).
14. Ovules are naked or not enclosed inside the ovary because ovary wall is absent.
15. Ovules are orthotropus and are covered by a single integument (unitegmic e.g., Cycas) or two integuments (bitegmic e.g.. Ephedra). An additional covering in the form of aril or cupule is also present outside the integument.
16. The single integument is differentiated into three layers: outer fleshy (outer sarcotesta), middle stony sclerotesta) and inner fleshy layer (inner sarcotesta).
17 In young ovules meiosis occurs and 4 haploid megaspores are produced.
18. Three mgaspores degenerate and one functional megaspore forms female gametophyte (monosporic) or endosperm.
19.Endosperm is haploid because it develops before fertilization.
20. Female gametophytes bear archegonia.
21. The number of archegonia is variable. In Gnetum only one, in Pinus two and in Cycas more archegonia develop. The archegonium in Gnetum is represented by ovum only.
22. Each archegonium has a single egg and a ventral canal cell. Neck canal cells are absent. In some members e.g. Cycas venter canal cell is also absent.
Pollination:
23. The microspores are liberated in various stages of development of male gametopohyte e.g., they are liberated at 3-celled stage in Cycas. 4-celled stage in Pinus and 5-celled stage in Ephedra. Except in Cycas and Ephedra the male gametes are non-motile.
24. Pollination is anemophilous i.e., brought about by wind.
25. Pollen grains come in direct contact with the ovule. They are deposited in the pollen chamber where they germinate.
Fertilization:
26. It is siphonogamous i.e., takes place with the help of pollen tube, Water is not essential for fertilization.
27. An oospore is formed after fusion of male and female gametes.
Embryogeny:
28. The development of embryo is meroblastic i.e., only a part of the zygote (basal) develops into an embryo.
29. There are free nuclear divisions in the early stages of development of embryo (except Gnetum and Welwitschia). Later it gets differentiated into upper (haustorial), middle (supensorial) and basal (embryonal) cells.
30. Polarity is endoscopic with the shoot end directed away from the micropyle.
31. Polyembryony is observed in some members of gymnosperms e.g. Pinus. Several embryos develop due to fertilization of more than one egg or by the division of the zygote (cleavage polyembryony) but only one embryo attains maturity.
32. A true fruit like that of angiosperms is absent in gymnosperms. It is because of the absence of ovary.
33. The ovule after embryo formation turns into the seed. The integuments of the ovule act as seed coat.
34. Number of cotyledons varies in different members, they are two in Cycas and many in Pinus. These become green while still enclosed within the seed.
35. The seeds represent three generations:
(i) Integuments and nucellus represent the parent sporophytic phase.
(ii) Endosperm represents the gametophytic phase.
(iii) Embryo represents the new sporophytic phase.
36. Gymnosperms show distinct alternation of generation. The sporophytic or diploid phase is dominant, long lived and independent while the gametophytic or haploid phase is short lived and is dependent on gametophytic phase.
Affinities and Relationship of Gymnosperms:
Gymnosperms occupy a place in between pteridophytes and angiosperms in the plant kingdom. Therefore, gymnosperms bear close affinities with the pteridophytes on the one hand and the angiosperms on the other. In many other characters they differ from both.
Affinities and relationship of gymnosperms with other groups of plants are as follows:
Resemblances or Similarities with Pteridophytes:
1. Sporophytic, independent plant body is present in both the groups. It is differentiated into root, stem and leaves.
2. Sporophyte possess a well-developed vascular tissue.
3. Xylem lacks vessels and phloem companion cells.
4. Young leaves show circinate vernation.
5. Presence of megaphyllous leaves.
6. Gymnosperm and few pteridophytes e.g. Selaginella are heterosporous i.e. form micro- and megaspores in micro- and megosperangia, borne on the micro and megasporophylls respectively.
7. In Cycas, sporangia are grouped in sori like pteridophytes.
8. The female sex organ is archegonium in both the groups.
9. The male gametes of Cycas and Ginkgo are motile like the pteridophytes.
10. Permanent retention of megaspore within the megasporangium.
11. Gametophytes are endosporic and highly reduced.
12. Female prothallus develops before fertilization and there is free nuclear division.
13. Germination of spores is precocious in gymnosperms and hetrosporous pteridophytes.
14. Development of distinct embryo after fertilization.
15. Like the pteridophytes, gymnosperms show marked alternation of generation between gametophytic and sporophytic phase. Sporophytic generation or sporphytic phase is dominant, independent and large at maturity while the gemetophtic generation exhibits progressive reduction and dependence.
Difference between Gymnosperms and Pteridophytes:
Gymnosperms:
1. Occur in Xerophytic habitats.
2. Have taproot.
3. Eustelic organization.
4. Secondary growth present.
5. Stems are aerial.
6. Only microspores are shed from microsporangia.
7. Pollen tube is developed to carry male gametes to archegonium.
8. Archegonia lack neck canal and neck canal cell and archegonium is absent in Gnetum.
9. Megasporangium is protected by integument and is called ovule.
10. Water is not essential for fertilization.
11. Gametophyte is fully dependent on sporophyte.
12. Presence of free nuclear divisions in earlier stages of development.
13. Seeds are present.
Pteriodophytes:
1. Hygrophytes (grow in moist and shady places).
2. Possess adventitious roots.
3. Not found.
4. Absent.
5. Usually underground rhizomes.
6. Both micro- and megaspores are shed from sporangia.
7. Pollen tube is absent in all heterosporous pteridophytes.
8. In archegoniun, neck canal and neck canal cells are present.
9. Ovule is absent.
10. Water is essential for fertilization.
11. Gametophyte is green and autotrophic.
12. Absent.
13. Absent.
Similarities with Angiosperms:
1. Main plant body is sporophytic and is differentiated into root, stem and leaves.
2. Plants are trees or shrubs and may be erect or climbing.
3. Root system is well developed and the roots may be diarch, triarch, tetrach or polyarch.
4. The xylem is exarch in the roots.
5. Stem is eusteltic. Vascular bundles are conjoint, collateral, open and endarch.
6. Secondary growth takes place.
7. Wood may be monoxylic or polyxylic.
8. Vessels and companion cells also occur in some gymnosperms (Gnetales) like angiosperms.
9. Heterosporous and have reduced gametophytes.
10. Nucellus is surrounded by integument to form a structure called ovule.
11. Like gymnosperms many angiosperms are wind pollinated.
12. Megaspore permanently remains inside the megasporangium and develops into female gametophyte.
13. Pollen grains grow into pollen tube.
14. Male gametes are non-motile in majority of gymnosperms and angiosperms.
15. Fertilization is siphonogamous.
16. Suspensor is formed during development of embryo.
17. Formation of endosperm.
18. Formation of seeds from ovules.
19. As in gymnosperms, polyembryony is found in several angiosperms.
20. Embryogeny is endoscopic.
21. Life cycle is similar in both groups.
Difference between Gymnosperms and Angiosperms:
Gymnosperms:
1. Plants are mostly woody trees.
2. Unisexual, may be monoecious or dioecious.
3. Majority of the gymnosperms are perennial.
4. Rarely reproduce by vegetative means.
5. Vessels in xylem element and companion cells in phloem are completely absent.
6. Cones are present.
7. Beautifying devices like sepals and petals are absent.
8. Pollination is anemophilous.
9. Ovules are naked.
10. Presence of prothallius.
11. Archegonia present.
12. Endosperm is formed before fertilization.
13. Endosperm is haploid.
14. Double fertilization absent.
15. Cleavage polyembryony prevalent.
16. Zygote undergoes free nuclear divisions.
17. Fruit formation absent.
Angiosperms:
1. Plants may be herbs, shrubs or trees.
2. Bisexual as well as unisexual, monoecious or dioecious.
3. Angiosperms may be annual, biennial or perennial.
4. Vegetative reproduction is very common.
5. Present.
6. Absent.
7. Flowers consist of sepals and petals.
8. Pollination may be anemophilous, entomophilous, hydrophilous or zoophilous.
9. Ovules are enclosed within the ovary wall.
10. Absent.
11. Absent.
12. Formed after fertilization.
13. Endosperm is triploid.
14. Present.
15. Absent.
16. Zygote does not undergo nuclear divisions.
17. Present.
Evolutionary Significance of Gymnosperms:
Gymnosperms are the most ancient seed plants that originated during the late Paleozoic (290-540 million years ago) era but flourishing well during the Mesozoic age. Their long evolutionary history is full of diverse lines of evolution. One of these lines of evolution included the early fern-like seed bearing group of plants known as cycadofilicales or the Pteridospermae.
Neuropteris, Lygenopteris are some of the examples of such plants. They had fern like general appearance and foliage but possessed the primitive type of seed. These fossil seed ferns are evidence of the origin of the gymnosperms from an ancient stalk of fern like individuals.
During the course of evolution of the seed habit, a number of morphological adaptations were necessary e.g., secondary wood, stelar system, origin of leaf, development of pollen grains, ovule and seed. First of all seed plants are heterosporous i.e. produce two types of spores-megaspores and microspores (hence it is assumed that ancestors of male plants are heterosporous).
Development of pollen is of evolutionary significance because sperm is protected within pollen. Pollens are able to travel by wind. It allows plant to use several recombination’s over long distances. Development of seed is also of evolutionary significance because seed contains the plant embryo and enough energy to nurture the plant embryo through germination.
The evolutionary significance of the seed is related to the plant capability of distant colonization and to the protection of embryo. The seeds containing embryo can be carried by wind, water and animals and germinate in different environments.
This fact contributes to the exploitation of a variety of ecological niches and for the diversity of plant species. Seed in addition protects embryo against external aggressions and they also provide germination under more adequate conditions (inside the seed). These features contribute to the evolutiary significance of gymnosperms.