Essay on Gymnosperms | Plants

In this essay we will discuss about Gymnosperms. After reading this essay you will learn about: 1. Meaning of Gymnosperms 2. Origin of Gymnosperms 3. General Characters 4. Systems of Classification 5. Vegetative Organs 6. Reproductive Organs 7. Ovules 8. Pollen Grains 9. Pollination and Fertilization 10. Embryogency 11. Life Cycle 12. Evolutionary Trends.

Contents:

1. Essay on the Meaning of Gymnosperms
2. Essay on the Origin of Gymnosperms
3. Essay on the General Characters of Gymnosperms
4. Essay on the Systems of Classification of Gymnosperms
5. Essay on the Vegetative Organs of Gymnosperms
6. Essay on the Reproductive Organs of Gymnosperms
7. Essay on the Ovules in Gymnosperms
8. Essay on the Pollen Grains in Gymnosperms
9. Essay on the Pollination and Fertilization in Gymnosperms
10. Essay on the Embryogency of Gymnosperms
11. Essay on the Life Cycle of Gymnosperms 
12. Essay on the Evolutionary Trends of Gymnosperms

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1. Essay on the Meaning of Gymnosperms:

Gymnosperms (Gr. gymnos=naked, sperma=seed) and angiosperms (Gr. angios=closed; sperma=seed) are the two sub-divisions of division Spermatophyta (Gr. sperma=seed; phyton=plant) of plant kingdom. Spermatophyta includes all those plants which bear seeds.

Gymnosperms literally mean naked seed, and, therefore, this sub-division is characterized by having the ovules borne unprotected on the surface of megasporophylls.

In angiosperms, on the other hand, the ovules are borne within a closed cavity, the ovary, formed by the megasporophyll, and after fertilization the ovary becomes a fruit which encloses one or more seeds. The word gymnosperm was first used by Theophrastus, one of Aristotle’s pupils.

The lower gymnosperms, such as members of Cycadales, show resemblances with the higher cryptogams (i.e. pteridophyta) whereas the higher gymnosperms, such as Gnetales and Coniferales, resemble members of angiosperms to some extent. Gymnosperms thus form a bridge between the pteridophytes and angiosperms, and have, therefore, been referred as “Phanerogams without ovary” by Goebel.

Gymnosperms form a small group of plant kingdom, and are represented by only about 70 genera and 725 species. In-spite of being represented by such a small number, the representatives of this group are widely distributed throughout the world, and on many mountains they form a dominant part of the vegetation.

Gymnosperms are most ancient and a history of the fossil records indicates that they once formed a predominant part of the earth’s vegetation. An idea of their history may be gained from Fig. 1.1, which shows the relative abundance of different orders during successive geological periods.

It becomes clear from this figure that both Pteridospermales and Cordaitales first appeared in the Upper Devonian or Lower Carboniferous, i.e. more than 300 million years ago. In addition to Pteridospermales and Cordaitales, two other groups of fossil gymnosperms, i.e. Bennettitales and Pentoxylales have been recognized.

Bennettitales were distributed during Mesozoic period from Middle Cretaceous to Upper Triassic while the Pentoxylales were distributed during Jurassic period of Mesozoic era. The living members of the gymnosperms are represented by Cycadales, Ginkgoales. Coniferales and Gnetales.

Precisely, gymnosperms are defined as those seed plants in which the ovules are not enclosed in ovary and the pollen typically germinate on the surface of the ovule. There is no double fertilization in gymnosperms, and the xylem in these plants lacks vessels (except in Gnetales). Theophrastus, a pupil of Aristotle, was the first to use the word “gymnosperms” in 300 B.C. in his book “Enquiry Into Plants”.

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2. Essay on the Origin of Gymnosperms:

The major question, whether gymnosperms had one, two, three or more ancestral origins, or whether they represent a monophyletic, di-phyletic, tri-phyletic or polyphyletic group, remains still to be solved.

A brief analysis of this question is under-mentioned.

Monophyletic Origin of Gymnosperms:

Several botanists, including Beck (1960), believe that gymnosperms are of monophyletic origin. Plants grouped under Progymnospermopsida (e.g. Aneurophyton, Tetraxylopteris, Protopitys, Archaeopteris, Callixylon) are of definite significance in the monophyletic origin. Several characteristics of Cycadophyta are combined by these plants with those of Coniferophyta.

The Progymnospermous plants also show intermediate steps in the evolution of megaphylls from branch systems. Progymnosperms, of course, show certain anatomical characters associated commonly with gymnosperms, and a full gymnospermous status may be granted to them.

In-spite of some definite evidence the confirmation of the theory of monophyletic origin of gymnosperms depends upon the discovery of some more fossil genera from Devonian or Silunan penods and reinvestigation of some already available fossil plants of these periods.

Sporne (1965) has also mentioned that “the evidence which is available at the moment seems to favour the view that gymnosperms are monophyletic. Confirmation must, however, await the discovery of further fossils from Devonian, or even Silurian deposits.”

Di-Phyletic Origin of Gymnosperms:

Some definite correlation between the anatomy of wood, form of leaf and structure of seed of two major groups of gymnosperms (i.e. Cycadophyta and Coniferophyta) have lead some botanists to conclude that gymnosperms are of di-phyletic origin. In the members of Cycadophyta. the wood is manoxylic, and it remains associated with the large-sized leaves(megaphylls) and radial symmetry in the seeds.

On the other hand, in the members of Coniferophyta, the wood is pycnoxylic, and it remains associated with the small-sized leaves (microphylls) and bilateral symmetry in the seeds. Due to these peculiarities of these two major groups, gymnosperms appear to be of di-phyletic origin.

Tri-Phyletic Origin of Gymnosperms:

Greguss (1972), in the latest edition of his book entitled “Identification of Living Gymnosperms on the Basis of Xylotomy”, pleaded for a tri-phyletic origin of gymnosperms. He opined that on the basis of Xylotomy (wood-anatomy) three well-defined evolutionary series may be traced among the existing gymnospermous taxa.

All these three series among existing gymnosperms have definite correlations with three types of pteridophytes, another tri-phyletic group:

1. Cycadales (Cycas), Ginkgoales (Ginkgo), Araucariaceae, Podocarpaceae and probably Taxales showing correlations with Pteropsida of Pteridophytes;

2. Cupressaceae showing correlations with Sphenopsida of Pteridophytes and;

3. Pinaceae and Taxodiaceae showing correlations with Lycopsida of Pteridophytes.

3. Essay on the General Characters of Gymnosperms:

1. The living gymnosperms are mostly woody, evergreen, perennials with shrubby or tree-like habit. The plants show xerophytic characters. They form the major components of any temperate forest of the world.

2. Except a few deciduous genera (e.g. Larix and Taxodium), most of the gymnosperms are evergreen.

3. Some of the gymnosperms attain a height up to 100 metres, e.g. Sequoia sempervirens (nearly 112 metre, the tallest living tree). The smallest gymnosperms is Zamia pygmaea, a cycad. Its fronds are only 4 or 5 cm long.

4. From the economic point of view, gymnosperms are highly important in forestry and horticulture. They yield timber, resins, essential oils, drugs and edible nuts. The newspaper industry is almost completely dependent on the wood of conifers for its paper requirements.

5. The radicle forms the tap root. The tap root system is exarch and diarch to polyarch.

6. The stem is branched but generally un-branched in Cycas. In Cycas revolute (Pant, 1973) the stem sometimes shows branching. Branching in the stem of Cycas circinalis has also been observed in a plant growing in the botanical garden of Meerut College, Meerut (U P.).

7. In stem the vascular bundles are conjoint, collateral, open and endarch. They are usually arranged in a ring.

8. Cambium is present, and the plants show secondary growth.

9. The xylem is composed of xylem parenchyma and tracheids with bordered pits. Vessels are absent (except in Gnetales). Gymnosperms resemble pteridophytes in the absence of vessels and wood fibres.

10. Phloem consists of sieve tubes and phloem parenchyma as in pteridophytes. Companion cells are absent.

11. The wood may be scanty, loose with wide medullary rays, i.e. manoxylic (e.g. Cycadales) or may be compact with narrow medullary rays, i.e. pycnoxylic (e.g. Coniferales).

12. The leaves are of two types, i.e. foliage and scaly leaves. They are large (megaphyllous) and pinnately compound as in Cycas, or small (microphyllous) and needle-like as in Pinus.

13. Usually the leaves are arranged in a spiral manner except in Cupressaceae and Gnetales where their arrangement in cyclic. Author reported forking of rachis and leaflets in Cycas circinalis.

14. Presence of transfusion tissue is an important feature of the leaf of gymnosperms. It was first discovered by Frank (1864) in Taxus.

15. The leaf in most genera contains a single vein. Venation, however, is parallel in Agathis, reticulate in Gnetum and dichotomous in Ginkgo.

16. Resin canals occur in the leaves of all conifers. Mucilage ducts are present in cycads while latex tubes are present in Gnetum.

17. The reproductive parts are generally arranged in the form of compact and hard cones or strobilli. The cones are unisexual. Hermaphrodite cone in certain members occurs sometimes as an abnormality. Male cones are usually smaller and short-lived than female cones in certain genera (e.g. Cycas), however, reproductive structures are not present in the form of compact cone-like structures.

18. In male cones, several microsporophyll’s are arranged on the central axis, each possessing several microsporangia containing microspores or pollen grains. The microsporophyll’s may­be broad (e.g. Cycas) or peltate (e.g. Dioon, Taxus).

19. Sporangial development is similar to eusporangiate ferns.

20. Only one (Cycas) or two (Pinus) prothallial cells are present in the male gametophyte.

21. The ovules are covered by one or rarely two integuments, and are orthotropous. The integument consists of an outer fleshy, a middle stony and an inner fleshy layer. It surrounds the nucellus. An outer structure, forming an aril, is often present in Taxus.

22. Each ovule opens with a mouth opening or micropyle.

23. The ovules are not enclosed inside the ovary. Instead, they are born naked on the leafy sporophylls, and hence the name gymnosperms (Gymnos-naked; sperma-seed) is given.

24. Double fertilization, a feature unique to angiosperms, is absent in gymnosperms. Khan (1943) and Friedman (1990, 1991), however, reported double fertilization in Ephedra.

25. The pollination is direct, i.e. the pollen grains come in contact with the ovule directly. At the time of pollination a “pollination drop” is secreted by the ovule in almost all gymnosperms.

26. Gymnosperms are wind-pollinated. Insect pollination, common in angiosperms, is nearly absent in gymnosperms.

27. The female gametophyte or endosperm in post-fertilization stages remains haploid in gymnosperms. It is, however, triploid in angiosperms.

28. Embryo development is meroblastic, i.e. develops from a small part of zygote. The embryo is straight and embedded in the endosperm (except in Bennettitales).

29. Several members show polyembryony (e.g. Pinus).

30. Due to the absence of ovary, the true fruits are lacking.

31. The mature and ripened ovule forms the seed. The integuments of ovule form the seed coat.

32. Cotyledons vary from one to many (2 in Cycas, but 2-14 in Pinus) in number.

33. Polyploidy is rare in gymnosperms. It is, however, common in Ephedra. ,

34. The diploid or sporophytic phase is dominant and independent in the life cycle while the haploid or gametophytic phase is dependent and reduced.

35. Members of Cycadofilicales, Bennettitales and Cordaitales are altogether extinct and occur only as fossils, while that of Cycadales, Ginkgoales and Coniferales are living as well as extinct. Members of Gnetales are living. However, some pollen as that of Ephedra and Welwitschia from Permian period, have been reported by Delevoryas (1963).

36. Amongst the living or extant gymnosperms, Ginkgoales and Cycadales are very ancient. They have a very long fossil history, and due to this they are called “living fossils”.

37. According to Raizada and Sahni (1960) living gymnosperms in India are represented by 16 genera and 52 species mentioned in Table 1.1 :

38. Gymnospermous genera found commonly under cultivation are Cycas, Biota, Thuja, Araucaria, Ginkgo, Agathis, Cunninghamia and Taxodium.

39. Plants show alternation of generations. The scheme representing the reproductive cycle of a gymnospermous plant has been generalized by Foster and Gifford in Fig. 1.2.

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4. Essay on the Systems of Classification of Gymnosperms:

Some Old Systems of Classification:

After the recognition of gymnosperms in 1827 by Robert Brown, Brongniart (1843) included them under Dicotyledons.

Bentham and Hooker (1862-1883) placed them between Dicotyledons and Monocotyledons as under:

They included Ginkgo under Coniferae.

However, it was Van Tieghem (1898), who, on the basis of some earlier embryological and developmental studies, removed Gymnosperms from this intermediate position and considered them as follows:

Besides Cycadales, Coniferales and Gnetales, three more orders were included under Gymnosperms namely Cycadofilicales, Bennettitales and Cordaitales on the basis of the accumulation of researches on the various fossil members of this group. Engler (1897) constituted a separate order Ginkgoales due to the discovery of motile sperms in Ginkgo.

Thus, Coulter and Chamberlain (1917) recognized following seven orders in this group:

1. Cycadofilicales (Pteridospermae)

2. Bennettitales

3. Cycadales

4. Cordaitales

5. Ginkgoales

6. Coniferales

7. Gnetales

Classification Proposed by Birbal Sahni (1920):

Sahni (1920), an eminent Indian botanist, gave for the first time a phylogenetic system for the classification of Gymnosperms. He divided Gymnosperms into two groups, i.e. Phyllosperms (ovules are borne on the leaves or leaf-like organs) and Stachyosperms (ovules are borne on the stem). His classification is based on the morphological nature of the ovule-bearing organs.

A summary of his classification is given below:

Gnetales have been left in a doubtful position by Sahni (1920).

Classification Proposed by C.J. Chamberlain (1935):

Chamberlain (1935), in his book Gymnosperms-Structure and Evolution, divided Gymnosperms into two main groups, i.e. Cycadophytes and Coniferophytes as mentioned below:

Classification Proposed by C.A. Arnold (1948):

Regarding classification of gymnosperms, Arnold (1948) may be quoted as saying “….It is now believed that the term Gymnospermae as a group name has outlived its usefulness and should be dropped, or it should be retained only as a common name without taxonomic status in modern classification.”

It is also evident from the classification proposed by Arnold (1948), which is tabulated as under:

Classification Proposed by M.A. Johnson (1951):

Johnson (1951) divided Gymnosperms into following five phyla and ten orders:

Classification Proposed by D.D. Pant (1957):

Pant (1957) modified Arnold’s system of classification and proposed a new scheme to classify Gymnosperms.

Major modifications proposed by him in the Arnold’s system are under mentioned:

1. The rank of a division has been suggested for Cycadophyta, Chlamydospermophyla and Coniferophyta.

2. Suffixes for each class and order have been used strictly according to the recommendations of International Code of Botanical Nomenclature, Utrecht, 1956.

3. Orders such as Glossopteridales, Peltaspermales, Corystospermales and Caytoniales have been included under class Pteridospermopsida.

4. A new class Ephedropsida with order Ephedrales has been included under division Coniferophyta.

5. For genus Czekanowskia, a new order Czekanowskiales and class Czekanowskiopsida has been proposed.

6. The division Chlamydospermophyta has been placed in between Cycadophyta and Coniferophyta.

7. The order Pentoxylales has been included under class Pentoxylopsida under division Cycadophyta.

An outline of Pant’s (1957) scheme of classification of gymnosperms is as follows:

Classification Proposed by M.B. Raizada and K.C. Sahni (1960):

In view of Sahni’s (1948) discovery of Pentoxyleae, Raizada and Sahni (1960) proposed the following classification:

Because of their uncertain position Pentoxylales are placed in between Cycadophyta and Coniferophyta.

Classification Proposed by A.Cronquist, A. Takhtajan & W.Zimmermann (1966):

In this classification the embryo-bearing plants (Embryobionta) have been divided into eight divisions, viz. Rhyniophyta, Bryophyta, Psilotophyta, Lycopodiophvta, Equisetophyta, Polypodiophvta, Pinophyta and Magnoliophyta.

All gymnosperms are included under Pinophyta, which is further divided as under:

Classification Proposed by D.W. Bierhorst (1971):

Bierhorst (1971) divided Gymnosperms into following three classes:

1. Cycadopsida including following six orders:

1.Pteridospermales

2. Caytoniales

3. Cycadeoidales

4. Cycadales

5. Pentoxylales

6. Glossopteridales

2. Coniferopsida including following five orders:

1. Cordaitales

2. Protopityales

3. Ginkgoales

4. Coniferales

5. Taxales

3. Gnetopsida including following three orders:

1. Ephedrales

2. Gnetales

3. Welwitschiales

Classification Proposed by Sporne (1974):

Sporne (1974) adopted classification of Pilger and Melchior (1954) and recognized following three classes and nine orders:

1. Cycadopsida: Pteridospermales, Bennettitales, Pentoxylales and Cycadales

2. Coniferopsida: Cordaitales, Coniferales, Taxales and Ginkgoales

3. Gnetopsida: Gnetales

Classification Proposed by Taylor (1981):

Taylor (1981) divided gymnosperms into six major divisions (Progymnospermophyta, Ptendospermophyta, Cycadophyta, Cycadeoidophyta, Ginkgophyta and Coniferophyta) as under:

1. Division Progymnospermophyta (Progymnosperms)

2. Division Pteridospermophyta (Seed ferns)

1. Lyginoptendales

2. Medullosales

3. Callistophytales

4. Calamopityales

5. Caytoniales

6. Corystospermales

7. Peltaspermales

8. Glossoptendales

3. Division Cycadophyta (Cycads)

4. Division Cycadeoidophyta (Cycadeoids)

5. Division Ginkgophyta (Ginkgophytes)

6. Division Coniferophyta

1. Cordaitopsida (Cordaites)

2. Coniferopsida (Conifers)

1. Voltziales

2. Coniferales

3. Taxales

According to Taylor (1981) Gnetales, Vojnovskyales and Pentoxylales are the problematic gymnosperms.

Classification Proposed by Stewart (1983):

Stewart (1983) classified the gymnosperms mainly on the basis of the evidences available from the fossil records.

He divided gymnosperms into three classes (Progymnospermopsida, Gymnospermopsida and Gnetopsida) as under:

1. Progymnospermopsida:

1. Aneurophytales

2. Archaeopteridales

3. Protopityales

2. Gymnospermopsida:

1. Pteridospermales

2. Cycadales

3. Cycadeoidales

4. Caytoniales

5. Glossoptendales

6. Pentoxylales

7. Czekanowskiales

8. Ginkgoales

9. Cordaitales

10. Voltziales

11. Coniferales

12. Taxales

3. Gnetopsida (Gnetum, Ephedra, Welwitschia)

Classification Proposed by Sandra Holmes (1986):

Holmes (1986), in her book entitled “Outline of Plant Classification”, divided gymnosperms into three classes (Cycadopsida, Gnetopsida and Coniferopsida) but also described some fossil ancestors in a separate class Progymnospermopsida.

An outline of her classification is under mentioned:

Division SPERMATOPHYTA:

Sub-division Gymnospermae

Class Cycadopsida (Cycadatae):

Order Lyginoptendales (Cycadofilicales, Pteridospermales)

Order Caytoniales

Order Bennettitales

Order Pentoxylales

Order Cycadales

Order Nilssoniales

Class Gnetopsida (Gnetatae, Chlamydospermae):

Order Welwitschiales

Order Ephedrales

Order Gnetales

Class Coniferopsida:

Order Ginkgoales

Order Cordaitales

Order Coniferales (Pinales)

Order Voltziales

Order Taxales

Class Progymnospermopsida (Fossils which are now thought to be nearer to gymnosperms although not of full gymnospermous status):

Order Pityales

Order Aneurophytales

Order Protopityales

Class Cycadopsida:

Plants are palm-like or fern-like with usually compound, frond-like and pinnate leaves; wood manoxylic and vessel-less; sperms are motile in living members; flower-like structures absent; strobili are simple; ovules with only one integument; fertilization by pollen tube with one male nucleus.

Order 1. Lyginopteridales (Cycadofilicales, Pteridospermales):

Fossil Palaeozoic and Mesozoic plants which were found from Devonian to Jurassic periods; stems protostelic or polystelic; plants with large, pinnate, fern-like leaves; megasporophylls pinnately compound; ovules borne separately along margins or on surfaces of megasporophylls; microsporophyll’s pinnately compound and not in strobili; e.g. Lyginopteris, Glossopteris.

Order 2. Caytoniales (Sometimes included as a family named Caytoniaceae under order Lyginopteridales):

Extinct Mesozoic plants found from Triassic to Cretaceous periods; leaves, appearing palmate, with four terminal leaflets arranged in two pairs, ovules borne on megasporophylls in clusters; e.g. Caytonia.

Order 3. Bennettitales (Cycadeoideales):

Order of fossil plants found from Triassic to Cretaceous periods; stem covered with armour of persistent leaf bases; leaves usually pinnately compound with syndetocheilic stomata and open venation; ovules borne on dome-shaped or elongated receptacle; ovules stalked; megasporophylls absent; ovule-containing structure sometimes in the form of a flower; microsporophyll’s simple or pinnately compound; e.g. Bennettites (Cyadeoidea), Williamsonia.

Order 4. Pentoxylales:

Order of extinct Mesozoic plants from Jurassic period; probably shrubs or small-sized trees; stems polystelic; leaves simple, thick, lanceolate with diploxylic leaf trace; ovule sessile, female organ consisting of about 20 seeds; microsporophyll’s form whorl of branched sporangiophores fused basally into disc; e.g. Pentoxylon, Sahnia, Nipanioxylon.

Order 5. Cycadales:

Living as well as extinct plants found from Triassic; were at their extreme abundance in Mesozoic; palm-like or fem-like woody plants with pinnately compound leaves; leaf trace diploxylic and stomata haplocheilic; pith and cortex with mucilage canals; living members are dioecious; megasporophylls with 2-10 ovules; microsporophyll’s in strobili bearing groups of microsporangia on lower side; sperm motile with spiral band of flagella; e.g. Cycas, Zamia, Palaeocycas (extinct genus).

Order 6. Nilssoniales:

Often treated as a family (Nilssoniaceae) under order Cycadales, the members of this order are all extinct and Mesozoic; leaf trace never diploxylic; female cone negligent or inattentive and probably pendulous; e.g. Nilssonia, Beania.

Class Gnetopsida (Gnetatae, Chlamydospermae):

Most recent with hardly any known fossil history, shrubs, trees, lianas, woody plants which are neither palm-like nor fern-like; leaves elliptical or strap-shaped, simple, opposite or whorled and sometimes very minute and scaly; wood contains vessels; strobili, often called inflorescence, are compound, flower-like structures with perianth and usually dioecious; ovule single, erect, naked and often possesses a long micropyle; fertilization by pollen tube with two male nuclei; motile sperms absent.

All the three members (Welwitschia, Ephedra and Gnetum) of this group are placed in a single order Gnetales in some systems of classifications.

In other systems of classification they are treated in three separate orders (Welwitschiales, Ephedrales and Gnetales) as under mentioned:

Order 1. Welwitschiales:

Welwitschia mirabilis, the only known living species of this genus, is large, turnip-like plant with mostly underground stem and two long and leathery leaves which are opposite and bear parallel veins; both leaves persist throughout the life of plant; strobili cone like; archegonia absent.

Order 2. Ephedrales:

This unigeneric order, represented by only Ephedra, contains branched shrubs or lianas with jointed stems; leaves very small and scale-like; strobili somewhat cone-like; archegonia present.

Order 3. Gnetales:

Unigeneric order, represented by only Gnetum, contains woody plants, which are usually lianas or sometimes shrubs or trees; wood with vessels; sometimes companion cells are present: leaves opposite and venation reticulate; appears like an angiosperms; strobili not cone-like; archegonia absent.

Class Coniferopsida:

Extinct and living; usually tall trees with simple leaves of varied shape; wood pycnoxylic and vessel-less; almost in all living species, except Ginkgo biloba, motile sperms are absent; strobili simple or compound with cone-like shape; ovule unitegmic; pollen tube contains one male nucleus.

Order 1. Ginkgoales:

Represented by only one living species (Ginkgo biloba) and several extinct genera (e.g. Ginkgoites, Baiera); leaves leathery, fan-shaped or strap-shaped with dichotomous venation; mega-strobili absent; only 2 to 10 ovules develop at the tip of branches; micro-strobili axillary, un-branched, catkin-like bearing several micro-sporangiophores, each with pendulous microsporangia; sperms motile and contain spiral band of flagella.

Order 2. Cordaitales:

Large extinct trees from Devonian to Permian periods; leaves large (up to 1 m), flat, strap-like; venation parallel; fructifications in strobili; micro-strobili and mega-strobili with sterile appendages below and fertile appendages above; ovules 1 to 4; pollen chambers present; Cordaites, Poroxylon.

Order 3. Coniferales:

A large order with living (e.g. Abies, Araucaria, Cupressus, Pinus, Sequoia, etc.) as well as extinct (e.g. Lebachia) genera; mostly cone-shaped shrubs or trees from Carboniferous to present; leaves scaly, needle-like; leaves, cortex and sometimes wood contains resin canals; micro-strobili and mega-strobili are usually cone-like; sperms non-motile; 2 to many ovules are attached on ovuliferous scale.

Order 4. Voltziales:

Treated only as two families (Lebachiaceae and Voltziaceae) of order Coniferales in some classifications, this order includes only fossil members. Their major and striking difference from Coniferales includes that the mega-strobili in Voltziales were borne on short shoots.

Order 5. Taxales:

The order is represented by living (e.g. Taxus, Torreya, etc.) as well as extinct (Palaeotaxus) genera from Triassic to present; leaves linear, needle-like and spirally- arranged; leaves and wood lack resin canals; mega-strobili absent, ovules solitary and each remains surrounded by an aril; micro-strobili in small cones; non-motile sperms.

Class Progymnospermopsida (Progymnospermae or Progymnosperms)?

Members of this fossil class are not truly gymnosperms. These are thought to form a link between Psilophytopsida (pteridophytes) and gymnosperms. Formerly, these were thought to be Primofilices (pteridophytes).

But now they are considered to be nearer to gymnosperms after the discovery of the connections between the gymnosperm-like trunk of Callixylon and fern-like fronds of Archaeopteris. In some classifications they are still treated as pteridophytes.

This class is divided into three orders as under:

Order 1. Pityales:

Members with moderately complex lateral organs and compact xylem; probable ancestors of conifers; e.g. Callixylon, Pitys, Archaeopteris.

Order 2. Aneurophytales:

Members with complex pinnate lateral organs and pycnoxylic wood showing small medullary rays; probable ancestors of cycads ; e.g. Aneurophyton, Palaeopitys.

Order 3. Protopityales:

Represented by only one fossil genus (Protopitys), its leaf traces suggest affinities with cycads and its wood with conifers; probable ancestors of gymnosperms other than cycads and conifers.

Classification Proposed by Gifford and Foster (1989):

Gifford and Foster (1989) raised the major groups to the rank of Divisions and divided gymnosperms into following seven divisions:

1. Progymnospermophyta,

2. Pteridospermophyta,

3. Cycadophyta,

4. Cycadeoidophyta,

5. Ginkgophyta,

6. Coniferophyta,

7. Gnetophyta

Classification Proposed by Kramer and Green (1990):

Recently, Kramer and Green (1990; see Kubitzki, 1990) divided gymnosperms as under:

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5. Essay on the Vegetative Organs in Gymnosperms:

Majority of the gymnosperms include shrubs and trees, and there is practically no herb or climber. The roots are generally tap roots. Coralloid roots (Cycas) and mycorrhizal associations (Pinus) are also found in some genera. The stem is aerial, erect, solid and branched (Pinus) or un-branched (Cycas). The older parts of the stem are generally covered with an armour of persistent woody leaf beses (Cycas).

The leaves may be simple or compound and small or very large. The large leaves (megaphylls) of some cycads may reach up to six feet in length. The leaves are mostly evergreen, but microphylls of most of the gymnosperms are small and scaly. The venation may be parallel (Welwitschia), reticulate (Gnetum) or even dichotomous (Ginkgo).

The shape of the leaf show great variation in gymnosperms (Fig. 1.3). It may be needle-like (Pinus, Picea, Cedrus, etc.), lanceolate to ovate (Gnetum) or even wedge-shaped or fan-shaped and bilobed (Ginkgo).

The arrangement of leaves on the stem may be spiral (Taxus, Podocarpus, etc.), whorled (Cedrus) or opposite and decussate {Gnetum). In outline, the leaves may be bifacial (Ginkgo), circular (Pinus monophylla), semi­circular (Pinus sylvestris) or even triangular (Pinus roxburghii).

The stomata may be present only on the lower epidermis of the leaf (Cycas) or on both the surfaces (Ginkgo). In Pinus and several other genera the mesophyll is undifferentiated but in several members (e.g. Cycas) it is differentiated into palisade and spongy parenchyma. The resin canals may be present (Cedrus, Pinus, etc.) or absent (Ephedra).

Anatomically, the stem, when very young, contains a ring of vascular bundles which are conjoint, collateral, open and endarch. They contain narrow or broad medullary rays. The mature stem in almost all gymnosperms contains secondary wood which may be either manoxylic or pycnoxylic.

The manoxylic wood is soft and relatively sparse, with very wide parenchymatous rays (e.g. cycads), while the pycnoxylic wood is dense and compact with very small wood rays (e.g. conifers) and forms the main bulk of the main trunk and branches.

From the commercial viewpoint, the manoxylic wood is practically useless whereas the pycnoxylic wood provides world’s best timber. Except in Gnetales, wood vessels are absent in the xylem of all gymnosperms. The phloem lacks companion cells. The xylem is exarch in the roots, and endarch or mesarch in the stems.

The secondary wood tracheids usually possess bordered pits. The medullary rays may be uniseriate or multi-senate. In very young vegetative organs, the protoxylem contains spiral thickenings but in mature conditions the scalariform thickenings are present.

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6. Essay on the Reproductive Organs in Gymnosperms:

Production of two types of spores (microspores and megaspores) by all gymnosperms makes them heterosporous. Microspores develop into male gametophytes, whereas each megaspore matures into a female gametophyte. Microspores are produced in microsporangia, which are borne on microsporophyll’s, and the latter remain arranged on an axis in the form of a loose or compact male strobilus or male cone.

Similarly, a megaspore (or macrospore) is produced in a mega-sporangium (or ovule), and one or many mega-sporangia are borne on a megasporophyll. The strobilus bearing several megasporophylls is called female strobilus or female cone.

The shape, size, general structure and position of male and female cones are different in different genera as evidenced by the following examples:

In Cycas (Fig. 1.4 A,B) the male cones are compact and develop singly at the apex of male plant. Groups of microsporangia form sori on the ventral surface of the microsporophyll’s. The male cones are largest amongst gymnosperms and reach sometime up to 0.5 metre in length. The female cones are not compact. They are in the form of loosely arranged leafy structures called megasporophylls bearing ovules.

In Ginkgo (Fig. 1.4 C,D) the male strobili develop in groups in the leaf axils. They are formed by micro-sporangiophores, each with two pendant microsporangia. They are short and reach only about an inch in diameter. The female cones are in the form of two sessile ovules developing at the tip of a compact stalk. A collar is present at the base of each ovule.

In Pinus (Fig. 1.4 E,F), the male cones develop at the ends of the long shoots in compact clusters. Each male cone contains several microsporophyll’s, on the ventral surface of each of which develop two microsporangia. Each male cone is about an inch in length. The mature female cone is large and woody bearing several megasporophylls, on the adaxial surface of each of which are present two ovules.

In Taxus (Fig. 1.4 G,H), the male or staminate cone consists of over a dozen microsporophyll’s bearing pendant microsporangia. The female cone is present in the form of a single ovule.

In Ephedra (Fig. 1.4 I,J), the male strobilus develops in the axil of scaly leaves on the node. It is round or ovoid body bearing 2 to 12 pairs of bracts in opposite decussate manner. In the axil of each bract arises a male flower. Each male flower contains two bracteoles and a stamen. The female strobilus resembles male strobilus in the general morphology and bears two ovules in the axil of uppermost pair of bracts.

In Gnetum(Fig. 1.4 K,L), the male and female strobili are compound structures developing in the axil of conate bracts. The bracts fuse to form a collar or cupule. In male strobilus several male flowers are arranged on each collar in 3 to 6 rings. Each male flower consists of two bracts, a stalk and anthers. In female strobilus, a ring of 4 to 10 ovules are present just above each collar.

In Welwitschia (Fig. 1.4 M,N), the plants are dioecious and possess dichasialy branched inflorescences. Each inflorescence contains opposite decussate bracts, which are crimson or scarlet coloured. Male flowers are present in male cone while female flowers are present in the ovulate or female cone.

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7. Essay on the Ovules in Gymnosperms:

Ovules in gymnosperms are not enclosed in carpels, and the pollen grains germinate typically on their surface. They are, therefore, unprotected or naked. In Cycas the ovule (Fig. 1.5A) is orthotropous and unitegmic.

The single integument covers the ovule all over except at the micropyle. This integument is three-layered, of which outer layer (sarcotesta) is fleshy, middle layer (sclerotesta) is stony, and the inner layer is fleshy. The integument remains in close association with nucellus.

A small pollen chamber is present in the nucellar beak near the micropylar end of the ovule. The central part of the ovule is occupied by female gametophyte. An archegonial chamber develops in the female gametophyte near the pollen chamber. Two archegoma develop in the female gametophyte near the archegonial chamber.

The ventral canal cell is absent in the neck of the archegonium in Cycas while it is present in the archegonia of Ginkgo and Pinus. The archegonia occur in groups forming an archegonial complex in Cryptomeria (Fig 1.5B) and Biota.

In Gnetum, the archegonia are absent. Only a few nuclei near the micropylar end start to function as egg nuclei (Fig. 1.5C). In Ephedra, Gnetum and Welwitschia, only two integuments are present in the ovule (Fig. 1.5D). Of these two integuments, the inner one comes out in the form of a micropylar tube.

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8. Essay on the Pollen Grains in Gymnosperms:

The pollen grains or microspores (Fig. 1 6A-G) are unicellular, uninucleate and haploid structures. They are produced in a very large number in a microsporangium. In Cycas, Taxus and Ginkgo the microspores are uniaperturate, un-winged and round or somewhat spheroidal structures.

On the other hand the microspores are winged in Pinus, Abies, Picea and Cedrus. A prominent papilla is present on one side of the microspore in Cryptomeria griffthiana. In genera such as Juniperus and Cupressus the pollen grains are generally inaperturate.

In Gnetales (Ephedra, Gnetum and Welwitschia) they are usually spheroidal with thick stratified exine. In Ephedra foliata, however, two sac-like structures are present at the two ends in the elongate pollen grains. The microspore in all gymnosperms develops into a male gametophyte.

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9. Essay on the Pollination and Fertilization in Gymnosperms:

The process in which pollen is carried from the male cone to the female cone is called pollination. In gymnosperms, it mainly takes place by means of wind. Wind transfers the semi- germinated pollen grains from the male cones up to the micropyle of the ovule of female cone.

These pollen grains get entangled in the liquid of the pollination drop which oozes out through the micropyle of the ovule. As the liquid of this pollination drop dries, the entangled pollen grains are drawn into the ovule.

Once the semi-germinated pollen grains or young micro gametophytes are drawn into the ovule, the micropylar opening usually closes. In genera such as Cycas and Ginkgo, such micro gametophytes are received at the pollen chamber located at the tip of the nucellus.

In majority of the other gymnosperms, including conifers, such young micro gametophytes come in direct contact with the nucellar beak. Pollen tubes are produced from the pollen grains or young micro gametophytes.

These tubes penetrate the nucellar tissue of the ovule. In Ginkgo, Cycas and several other Cycadales, the pollen tubes function as haustorial organs. They keep on growing in the nucellar tissue for several days, absorb food from there and transfer this food to the other parts of the micro gametophyte.

After some time the pollen tube bursts at its tip and liberates the multi-flagellated male gametes into a cavity above the female gametophyte. The male gametes swim up to the archegonial neck and enter into the archegonium. One of the male gametes fuses with the female gamete or egg and results in the formation of a diploid zygote. This completes the process of fertilization.

In conifers, the non-motile male nuclei are carried to the egg cell through the pollen tube, and this process is called siphonogamy. The pollen tube in conifers thus acts only as a sperm carrier. Production of pollen tubes was absent in members of fossil genera belonging to Pteridospermales. Bennettitales and Cordaitales and fertilization in them was carried by motile spermatozoids under a process called zoidogamy.

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10. Essay on the Embryogeny in Gymnosperms:

The process leading to the formation and development of embryo is called embryogeny. In majority of the living gymnosperms, except some members (e.g. Sequoia sempervirens, Welwitschia and Gnetum) the initial changes in the development of embryo include the free-nuclear divisions.

Only four (Pinus) or as many as hundreds (Cycas) of free nuclei are formed in the diploid zygote. Soon, the wall formation starts and the embryo becomes cellular.

Organs such as suspensor, hypocotyl plumule and cotyledons are differentiated This type of embryogeny is called endoscopic because the shoot end of the embryo grows away from the micropylar end. Very commonly the polyembryony is observed in several genera.

In Pinus and several other conifers, cleavage polyembryony is observed. In these members, all the four cells of the young embryo develop into four independent embryos. However, in the later stages, only one out of these four embryos shows complete development and the remaining three abort. Cleavage polyembryony is absent in Thuja.

In Sequoia sempervirens (Fig. 1.7A-E), the zygotic nucleus does not show free-nuclear divisions. Each nuclear division is followed by wall formation and a quadrant stage is achieved (Fig. 1.7A,C). Each of these four cells of quadrant may develop into a filamentous embryo (Fig. 1.7D,E).

In Araucaria angustifolia (Fig. 1.7F-G), as many as 32-48 free-nuclei are formed. Wall formation starts at this stage. The cells show polar elongation. Enlarged cap cells are formed at the distal end, and the embryo proper develops from the cells located in the centre. Suspensors are formed from the cells present towards the micropylar end.

In Actinostrobus pyramidalis (Fig. 1 7H-K) of Cupressaceae, four nuclei are formed and then wall formation takes place forming four cells. Two of these cells are vertically placed and there is no further division in these upper cells.

Lower two cells are transversely placed, divide once to form four embryonic initials. From these initials develop the embryo. Rosette and upper tiers are absent, and the embryo contains two cotyledons. Actinostrobus shows cleavage polyembryony.

If observed broadly, there exist vast differences between the embryogeny of gymnosperms, pteridophytes and angiosperms, and this is an indication that all these three groups have evolved parallaly and not from a common ancestor.

However, some similarities (e.g. early determination of polarity, a well-developed meristematic pole, and axial development, etc.) also exist between the embryogeny of these three groups which suggest the possibility of their common ancestry to some extent.

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11. Essay on the Life Cycle in Gymnosperms:

Generalized pattern of life cycle in gymnosperms is depicted in Fig. 1.8.

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12. Essay on the Evolutionary Trends of Gymnosperms:

On the basis of the available studies of several fossil groups, although it appears that the gymnosperms constitute a heterogenous group, yet it is possible to delineate several over-all trends in the evolution in this group.

Vascular System:

The primary vascular system exhibits a narrow range among gymnosperms. Majority of the gymnosperms are monostelic, and only some exhibits polystelic condition. Some of the members also possess co-axial evolutionary cylinders of secondary wood which develops from the anomalous activity of cambium.

A change from the cauline to foliar nature has been the main evolutionary trend in the stem anatomy of different members of gymnosperms. The primary wood is suppressed in this change from cauline to foliar nature.

During this suppression the primary wood passes through vanous stages such as:

(1) Solid protostele,

(ii) Pith surrounded by mesarch strands,

(iii) Development of endarch strands adjacent to the secondary wood,

(iv) Mixed xylem, and

(v) Parenchyma.

During the entire process of these changes a gradual transference of function from primary to secondary wood takes place. The leaf trace system, which was associated with the primary wood in the earliest types, now becomes “more and more closely associated with the secondary wood”.

In members of Cordaitales, Ginkgoales and Coniferales there is a definite increase in the secondary wood. In Gnetales, however, vessels replace tracheids, though not completely but only partly.

Leaf:

The microphylls and megaphylls are the two different kinds of leaves found in gymnosperms. The microphylls are small and usually possess one or two parallel veins. The megaphylls are quite large, frond-like leaves and possess well-branched veins.

The gymnosperms of Coniferophyta-line possess microphylls while the members of Coniferophyta-line bear megaphylls. Ginkgoales, however, possess neither microphylls nor megaphylls but wedge-shaped or fan-shaped leaves of almost normal size.

Majority of the recently reported fossil members of gymnosperms indicate their affinities more with the microphylls than with the megaphylls. Several botanists consider Cycadophytes as more primitive than Coniferophytes. This indicates that megaphyllous leaves are more primitive than microphyllous leaves.

Much evolutionary significance has not been attached with the leaf anatomy of gymnosperms. A little consideration has, however, been given to the xeric conditions of these members. It is supposed that inappropriate condition of water by the tracheids has been the major reason for the xeric condition among gymnosperms.

Reproductive Organs:

Some important events of the evolution of reproductive organs among gymnosperms may be listed as under:

1. Probably, all reproductive organs in the beginning were stachyspermous (i.e. reproductive organs borne on stem). Some gymnospermous genera have managed to stay as stachyspermous even today.

2. During the course of the evolution, the structures bearing reproductive organs progressively became more leaf-like, i.e. phyllospermous. The sporophylls were thus originated.

3. In Ptendosperms, the sporophylls were leaf-like.

4. A From the compact sporophylls developed the cone. The Cordaitales were the first to produce mono-sporangiate cones. The tendency of bearing mono-sporangiate cones is also retained in Ginkgoales and Coniferales.

5. The mono-sporangiate cones or strobili evolved into bisporangiate cones or strobili during course of evolution. Gnetales (e.g. Welwitschia) possess bisporangiate strobili.

6. In Cordaitales, Ginkgoales and Coniferales, the sporophylls are distinctly attached to the axis, and, therefore, the strobili, in these groups are simple. On the other hand, compound strobili are produced in some more advanced groups such as Gnetales.

Microsporophyll’s:

In Cycadophytes, the microsporophyll’s are pinnate and peltate. They bear sori only on their lower surface. Due to these characters the microsporophyll’s of Cycads are very primitive. In different members of different groups of gymnosperms, a reduction of long sporophyll to a short discoid structure and then also to a filament is observed, and these all are the features of evolutionary significance.

Some other such features include:
(i) Formation of individual sporangia instead of synangia, and

(ii) Fast elimination of annulus from the fern-like appearance.

Ovules:

Two evolutionary trends are visible in the ovules of the gymnosperms.

These are:

(i) Degeneration of the outer fleshy layer, and

(ii) Appearance of two integuments.

In most primitive type of gymnospermous ovules, the integument is single and it is also free from the nucellus. Three distinct layers are discernible in the integument, viz. outer fleshy layer, middle stony layer and inner fleshy layer. Two vascular strands supply the ovule, of which the outer strand enters the outer fleshy layer and the inner strand enters the peripheral region of the nucellus.

In majority of the gymnosperms the integument remains fused with the nucellus in most of its part, except the micropylar region, where the nucellar beak is formed and the trilayered integument is not reduced at all.

The outer fleshy layer develops quite conspicuously in the ovules of Cycadophyta-line. In the ovules of the Pinaceae-line, the outer fleshy layer is represented by a young ovule. Instead of one, two integuments are present in the ovules of Gnetales, and, therefore, it exhibits an example of a second evolutionary trend in the gymnospermous ovules.

Male Gametophyte:

The pollen grains of most of the fossil gymnosperms, except of Coniferales and Taxales, are almost uniform in possessing a layer of parietal cells surrounding the centrally located spermatogenous region. The layer of parietal cells probably represents an antheridial jacket.

In the early stages of the evolution of gymnosperms, the vegetative prothallial cells have been in such a low number that the male gametophyte might be treated as a reduced form of an antheridium. A reduction in the antheridial jacket and the simultaneous formation of a pollen tube have been observed between the fossil gymnosperms and the extant forms (i.e. members still in existence).

A homology, probably exists between the tube cell of the modern gymnosperms and the antheridial jacket of Palaeozoic gymnosperms The male gametophyte development appears to be quite uniform within a family.

In Pinaceae, two senescent primary prothallial cells are produced from the embryonal cell of the microspore. This embryonal cell functions as an antheridial initial and results in the formation of a peripheral tube cell and a generative cell. A periclinal division in the generative cell gives rise to an outer spermatogenous cell and an inner sterile cell. Two male gametes are produced by the division of the spermatogenous cell.

In Taxaceae, Taxodiaceae, Cupressaceae and Cephalotaxaceae there is no prothallial cell, and the function of the antheridial initial is performed directly by the embryonal cell.

In Araucariaceae and Podocarpaceae, the prothallial cells show secondary proliferation. The ontogeny of male gametophyte in Araucariaceae resembles greatly with Pinaceae. In Araucariaceae and Podocarpaceae. however, the generative cell divides anticlinally and not penclinally. Very little proliferation of primary prothallial cells is observed in Podocarpaceae in comparison with that of Araucariaceae.

In Gnetales (e.g. Gnetum and Welwitschia), the prothallial cells are generally absent The function of the antheridial initial is performed by the embryonal cell. A tube cell and a generative cell are formed. Division of the generative cell results in the formation of a spermatogenous cell and sterile cell. The spermatogenous cell divides and form two male gametes.

The male gametes or sperms are ciliated and motile in Cycas. Probably the same condition existed in Ptendospermales, Cycadeoideales and Cordaitales. In Coniferales and Gnetales, however, naked nuclei are present in place of motile and ciliated ones. Pollen chamber also disappears simultaneously. The sperms are carried away by the pollen tubes.

In Ephedra, however, a deeply situated pollen chamber is present. This is, however, an indicative of a secondary origin. A well- developed archegonial neck is also present in Ephedra.

Female Gametophyte:

In gymnosperms, the female gametophyte is a massive, multicellular body. It serves the dual purpose of bearing the archegonia and providing nourishment to the young embryo.

Major evolutionary steps in the female gametophyte include:

(i) Free-nuclear divisions,

(ii) Vacuolation,

(iii) Process of wall-formation resulting into the formation parietal tissues, and

(iv) Process of wall-formation extending towards the central region and resulting into the development of endosperm.

In majority of Cycadales, Ginkgoales, Coniferales and some Gnetales (e.g. Ephedra), the early development of female gametophyte involves a fairly uniform plan. Alveoli also appear in members of almost all these groups but the pattern of alveoli vanes. The archegonia develop in the female gametophyte.

In both pteridophytes and gymnosperms, the general structure of the archegonium is almost similar. An egg cell, a ventral canal cell, a few neck cells and a venter are present in an archegonium of both the groups.

The gametophyte is, however, parasitic on sporophyte in gymnosperms while it is free-living and green in pteridophytes. Neck canal cells are absent in gymnosperms while they are present in the archegonia of ptendophytes.

These differences indicate some evolutionary trends showing a reducing capacity of gametophyte for its independent existence. On the other hand, several simulates show homologies in the structure of the female gametophyte of both ptendophytes and gymnosperms.

From Which Type of Earliest Gymnosperms Developed the Modern Gymnosperms?

The earliest gymnosperms, responsible for the evolution of this entire modern group, probably had the following characteristic features, according to Sporne (1965):

1. Stems with primary solid wood, also perhaps possessing some amount of secondary wood.

2. Plants perhaps had little distinction between leaf and stem.

3. Pollen-bearing organs were borne fully exposed at the tips of green photosynthetic branch- lets.

4. Seeds were also fully exposed.

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