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Diversity of Seed Plants and their Systematics

Gymnosperms I

General Features of Gymnosperms and their Classification

Dr. NUPUR BHOWMIK

Department of Botany University of Allahabad Senate Hall Allahabad – 211002 [email protected]

Date of submission: 27/06/2006

Keywords

Coniferophyta, nucellus, microsporophylls, ovule, archegonium, Pteridosperms, epiphyte, parasite, prothallial cells, integument, dendrochronological, Cretaceous, Devonian.

GENERAL FEATURES OF GYMNOSPERMS AND THEIR CLASSIFICATION

Gymnosperms are seed plants that compose a major component of the temperate forests of the world. They are widely appreciated for the great range of variation in habit and shapes. They have some of the tallest trees known in the plant kingdom. Sequoia sempervirens, commonly called the Californian or "Coast Redwood" because of its red coloured bark and red heart wood, reaches a height of 112 metres, and a diameter of 5 metres(See Pl. 1 & Pl. 2). Another species, Sequoiadendron giganteum from California, U.S.A. commonly called "Big Tree" or the Wawona Tree, is a little shorter in length about (87m) but attains a diameter of more than 10 metres (See Pl. 3).

The age of tallest tree is estimated from 400 to 800 years. The oldest redwood on record, which has a diameter of 3.7 metres across is estimated to be 2,200 years. However, the distinction of being the oldest living tree on earth has now gone to a species of Pinus, P. aristata now called as P. longavea. A tree of this species is growing at 3275 m. above sea level in east Nevada-Utah State boundary(See Pl. 4). The age has been dendrochronologically shown to be 4900 years old (Foster & Gifford, 1974). Other estimates of age of the oldest tree of this species are mentioned as their being as old as 6000 years old (Sahni, 1990).

However, all gymnosperms are not tall and gigantic. There are some small herbaceous looking forms also, like Zamia pygmaea and Stangeria paradoxa(See Pl. 5) or the two parasitic forms, Parasitaxus (= Podocarpus) ustus(See Pl. 12) and Gnetum trinerve. While the former parasitises on Dacrydium taxoides, the latter is a parasite in Cinchona forests of Chimboraza (Pearson, 1929). A species of Zamia, Z. pseudoparasitica is exceptional among all gymnosperms in being epiphytic(See Pl. 6). A few gymnosperms may be lianas or climbers.

They are mostly evergreen, but some such as Larix and Taxodium are deciduous(See Pl. 7). The plants are mostly xerophytic but plants such as Taxodium are also known to grow in swamps or marshy places in Mexico and Florida. (See Pl. 8)

The gymnosperms include ancient lines of seed-bearing plants whose evolutionary history dates back to the Upper Devonian, roughly about 375 million years ago. One of the extinct gymnosperms consisted of an assemblage of plants whose foliage was fern-like in general appearance yet it possessed a primitive type of seed. This group of plants was named "seed ferns" or "pteridosperms". Undoubted gymnosperms had come into existence during the Lower Carboniferous (about 355 million years back) and had reigned over the plant kingdom by virtue of their dominance right upto the Cenomanian (about 96 million years from now) till the advent of angiosperms at the end of the Cretaceous.

The surviving gymnosperms no longer dominate the plant kingdom but can still claim to own about 70 genera and 800 species distributed in various parts of the world. They include the newly described genera Chigua and Wollemia. While Chigua belonging to Zamiaceae of cycads was described by D.W. Stevenson in 1990(See Pl. 9), the genus Wollemia assigned to Araucariaceae of conifers was discovered a few years later by Hills et al. (1996) (See Pl. 10). The largest group is the Coniferophyta, which includes pines, firs, cedars, spruces and junipers which form extensive forests of northern and southern hemispheres. It contrast, living representatives of the Ginkgophyta and Cycadophyta are veritable "living fossils", the former being represented by the sole living member Ginkgo biloba (See Pl. 11) existing in wild state in certain mountains in southeastern China and the latter are relicts from the past age being confined to limited areas in tropics and sub-tropics. The last group of living gymnosperms is the Gnetophyta represented by Welwitschia, Ephedra and Gnetum.

Gymnosperms are highly valued for their timber. They are also sources of starch, resins, essential oils, drugs, edible nuts etc. Many are cultivated as ornamental plants ("palms") and are found in gardens all over the world while some conifers are sold as Christmas trees. Some gymnosperms like the Ginkgo are held in high esteem and are found growing around Buddhist temples in China and Japan.

Gymnosperms (gymnos = naked, sperma = seed) include plants whose ovules are naked and freely exposed for pollination. They are borne on microsporophylls, scales, or comparable structures. In angiosperms (angios = closed, sperma = seed) ovules are enclosed in a carpel and usually completely closed at the time of pollination.

The sporophyte is usually arborescent comprising of large or small woody trees or shrubs. Few may be lianas or climbers. Most gymnosperms are evergreen but some are deciduous such as Larix and Taxodium(See Pl.7). The plants are mostly xerophytic. Gymnosperms show the following features: 1. The plants have a long lasting tap root system. The vascular cylinder is di-to polyarch, xylem exarch. Main elements of xylem are tracheids. Phloem is composed of sieve cells and lacks companion cells. Some forms exhibit additional symbiotic relationship between roots and algae in coralloid roots (Cycas) and between roots and fungi in mycorrhizic roots (Pinus). 2. The sporophyte shows unlimited growth of aerial trunk by means of apical and lateral meristem which produce secondary vascular tissues. 3. The aerial trunk is branched or unbranched and woody. Majority of gymnosperms have branched stem (except in Cycas and Zamia). In Pinus branches are of two types :- (a) long shoots or branches of unlimited growth (b) dwarf shoots or branches of limited growth bearing clusters of variable number of needle shaped leaves and collectively called as spur. Vascular bundle of stems are collateral, endarch or mesarch, open and arranged in a ring. 4. Secondary growth very pronounced. Secondary vascular tissues consisting mainly of tracheids and sieve cells. Normally xylem lacks vessel elements and the phloem lacks companion cells. But both vessels and companion cells present in members of Gnetales. Mature metaxylem and secondary xylem elements have bordered pits of various types. Wood or secondary xylem of two types namely, manoxylic and pycnoxylic. In manoxylic type (as in Cycas) wood is less, porous and soft. There is a large cortex and pith and parenchymatous rays are wide. In pycnoxylic type (as in Pinus) wood is dense or compact with small xylem rays and reduced pith and cortex. 5. Leaves are diverse in form and arrangement. They are both simple and compound ranging in size from a minute scale to leaves a few meters long. Arrangements of leaves usually spiral, they may also be whorled as in Cedrus or opposite decussate as in Cupressaceae, Welwitshchia(See Pl. 12) and Gnetum. Scale leaves are microphyllous whereas larger leaves are megaphyllous and their vascular supply always leaves a leaf gap in the stem stele. Venation may be parallel (Agathis and Welwitschia), reticulate (Gnetum) or dichotomous (Ginkgo) or as in most genera there may be a single vein. Leaves may be dorsiventral or isobilateral, amphistomatic or hypostomatic with sunken stomata. Mesophyll may be differentiated into spongy and palisade parenchyma (Cycas, Gnetum) or undifferentiated (Pinus). Transfusion tissue is a prominent feature. Leaf surface protected by a thick cuticle and sometimes by an additional waxy layer. Resin canals occur in all conifers (except Taxus) and mucilage ducts in cycads and Ginkgo and latex tubes in Gnetum. Vasculature of petiole quite variable. In Cycas, vascular bundles are arranged in a horse-shoe shaped manner or look like an inverted omega (U). They are diploxylic having both centripetal and centrifugal xylem. In Ginkgo petiole receives two vascular bundles and each half of lamina is supplied by one bundle which later dichotomizes. In Gnetum, the petiole shows an arc-shaped arrangement. 6. Vegetative methods of reproduction are rare in gymnosperms but vegetatively propagating bulbils are known in Cycas. 7. Plants are heterosporous. They may be monoecious (Pinus) or dioecious (Cycas). Reproductive structures are borne in cones or strobili that are either staminate (male) or ovulate (female) except in Cycas where ovules are borne on loose megasporophylls. Sporangia are borne on fertile leaves or leaf-like structures called microsporophylls (in male cone) and megasporophylla (in female cones) which are arranged spirally around a central axis. 8. Microsporangia are borne on abaxial or lower surface of microsporophylls. They may be numerous and grouped in sori (Cycas) or reduced to two (Pinus). Microspores are produced in tetrads. 9. Megasporangium or ovule is borne on adaxial or upper surface of megasporophyll or ovuliferous scale and are generally orthotropous. Young megasporangium consists of

a nucellus which is surrounded by a sheath or integument and inside the nucellus is single functional megaspore. A narrow passage above the nucellus in the integument is the micropyle. 10. Microspore and megaspore germination in situ, producing micro-and megagametophytes which are not autotrophic. Microspores have partly developed endosporic male gametophytes when they are transferred to micropyle of ovule. The microspore or pollen grain at the time of shedding may have only one prothallial cell (Cycas) or two prothallial cells (Pinus), the former being liberated at three-celled stage and latter at four-celled stage. 11. The microspores or pollen grains are borne by wind and enter the ovule directly through the micropylar canal. The micropyle in almost all gymnosperms secretes a sugary exudate called the "pollination drop" which not only receives the pollen grains but also transports them to nucellus of ovule. 12. The development of female gametophyte is monosporic (except in Welwitschia and Gnetum where it is tetrasporic). It is permanently retained inside the megaspore wall and dependent on parent sporophyte for its nutrition. The archegonia in female gametophyte may be two (in Ginkgo) to many (as in many conifers). Archegonia are quite large and elongated and lack neck canal cells. Often the ventral canal cell too, is eliminated. Gametophytic cells around the archegonia develop into a nutritive layer or jacket. However, the nutritive layer and archegonia are absent in Welwitschia and Gnetum. 13. Fertilization is effected by means of a pollen tube (siphonogamy). The multiflagellate spermatozoids or male cells within the pollen tube have no specialized means of locomotion and occur within the ovule. The pollen tube bursts liberating two large flagellated sperms into the cavity (archegonial chamber) directly above female gametophyte. One sperm fertilizes the large egg cell giving rise to a zygote (2n). 14. At the beginning of embryogeny, zygote shows free nuclear division in all except Welwitschia, Gnetum and Sequoia sempervirens. Later embryo becomes cellular after wall formation and gradually differentiates into a suspensor, shoot apex, cotyledons, hypocotyl and radicle. Polarity is endoscopic with the shoot end directed away from the micropyle. Embryo remains contained within the seed developed from the ovule. Mature embryo is differentiated into root, stem and leaves. 15. In most gymnosperms a common feature in embryogeny is polyembryony with young seeds having more than one embryo. It may be simple polyembryony when more than one archegonium is fertilized producing several zygotes or cleavage polyembryony when single zygote gives rise to multiple embryos due to cleavage or splitting of embryonal layer. The mature seed normally has only one embryo. 16. The young embryo draws its nutrition from the endosperm which develops before fertilization and is haploid (n). Endosperm develops from female gametophyte that has absorbed the food from nucellus. 17. During last phases of embryogeny, the nucellar tissue of ovule becomes disorganized and frequently persists only as a paper-like cap of dry tissue at micropylar end of the seed. 18. Gymnosperm ovules and seeds are unprotected and not surrounded by an ovary wall, hence true fruits like that of angiosperms are not formed. 19. Histological maturation of various layers of integument continues and the stony layer becomes an extremely hard, resistant shell which effectively encloses and mechanically protects the female gametophyte and the embryo. 20. The detached seeds of all gymnosperms (except for cycads and Ginkgo) remain dormant for sometime undergoing a resting period. 21. Gymnosperms being mostly temperate plants (except for cycads, Gnetum and Ephedra) growing in tropics and subtropics show very little activity in the development of reproductive structures during winter. The activity is renewed in the

spring. Under favourable conditions the embryo resumes growth and after rupturing the seed coat develops into a new sporophyte plant. 22. Polyploidy is rare in gymnosperms, except in Juniperus (partly) Sequoia and Ephedra (partly) where gametic number is multiple of the basic number of the families. Sequoia is the solitary gymnosperm which is hexaploid. Ephedra is the only genus where polyploidy is common.

Classification of Gymnosperms

Gymnosperms have been variously classified by different workers from time to time. The early history of classification of gymnosperms is linked with the angiosperms. Robert Brown (1827), after recognizing the gymnosperms as a distinct group of plants placed them alongwith angiosperms.

Benthem and Hooker (1862-1883) in their Genera Plantarum, placed them between the Dicotyledones and Monocotyledones. They recognised only three orders of living gymnosperms: Gnetaceae, Coniferae and Cycadaceae.

Some outstanding systems of classification of gymnosperms are being mentioned below:- Van Tieghem (1898) recognized gymnosperms as one of the two major divisions of Spermatophyta.

Spermatophyta

Gymnosperms Angiosperms (Astigmatae) (Stigmatae)

Eichler (1883), considered gymnosperms as one of the two divisions under. Phanerogamae. The second division being Angiospermae. Eichler (1889) classified the seed plants as:

Phanerogamae

Gymnospermae Angiospermae

Cycadaceae Cordaitaceae Coniferae Gnetaceae

Pinioideae Taxoideae

Abietineae Cupressineae Taxeae Podocarpineae

By the beginning of the present century, Engler (1897), Coulter and Chamberlain (1910), Engler and Prantl (1926), Rendle (1930) and others recognized gymnosperms as a primary division of Spermatophyta, Phanerogamia or Embryophyta-Siphonogama. They divided the group into classes or orders of coordinate rank, viz. (1) Pteridospermae (Cycadofilicales) (2) Cycadales (3) Bennettitales (4) Cordaitales (5) Ginkgoales (6) Coniferales and (7) Gnetales.

Sahni (1920a) recognised two main phyletic lines in the orders of gymnosperms based on the difference between the morphological nature of the ovule bearing organs of cycads and conifers. He classified gymnosperms in the following way:

Gymnosperms

Phyllospermae Stachyospermae

Pteridosperms Cycadales Bennettitales

Cordaitales Ginkgoales Coniferales Taxales (including Taxus, Torreya and Cephalotaxus)

Chamberlain (1935) classified gymnosperms into:

Gymnosperms

Cycadophytes Coniferophytes

Gymnosperms with fern-like pinnatifid Gymnosperms with profusely branched leaves, weakly branched large globose or trunks, leaves simple (needle-like, scale-like columnar trunks, having large conspicuously or laminate), stems with small pith and developed pith and cortical zones in stem. cortex. Secondary xylem cylinder massive Secondary xylem cylinder small, composed and less parenchymatous (pycnoxylic wood). mainly of tracheids and abundant The group includes extinct as well as extant parenchyma (manoxylic wood). Group well orders like:- represented in fossil record. The only surviving representatives are the modern cycads. Orders: 1. Cordaitales 2. Voltziales Extinct 3. Coniferales Extinct and 4. Ginkgoales Orders 1. Cycadofilicales living 2. Bennettitales 5. Gnetales 3. Cycadales

Arnold (1948) recognized three classes under gymnosperms.

Gymnosperms

Classes Cycadophyta Coniferophyta Chlamydospermophyta Orders 1. Pteridospermales 1. Cordaitales 1. Ephedrales 2. Cycadeoidales 2. Ginkgoales 2. Gnetales 3. Cycadales 3. Taxales 4. Coniferales

Pant (1957) suggested a classification which is a modification of Arnold's classification. The three classes of

Arnold have been raised to the rank of divisions:-

Gymnosperms

Coniferophyta Division: Cycadophyta Chlamydospermophyta (Pinophyta)

Plants with relatively small, usually unbranched or poorly Usually large sized trees, branched aerial or subterranean Peculiar angiosperm-like with profusely branched trunks; leaves usually large and forms stems and simple leaves. pinnate; stems have large pith, a Stems have small pith, thick cortex, scanty wood usually narrow cortex, abundant with wide medullary rays Class 1 Gnetopsida compact wood usually with (manoxylic) Orders 1. Gnetales narrow medullary rays 2. Welwitschiales (li)

Class 1 Pt eridospermopsida Orders 1. Lyginopteridales Class 1 Coniferopsida 2 . Medullosales 3. Glossopteridales (Pinopsida) Orders 1. Cordaitales 4 . Peltaspermales 5. Corystospermales 2. Coniferales (Pinales) 6 . Caytoniales 3. Ginkgoales

Class 2 Cycadopsida Order 1. Cycadales Class 2 Epedropsida Order 1. Ephedrales

Class 3 Pentoxylopsida Class 3 Czekanowskiopsida Order 1. Pentoxylales Order 1. Czekanowskiales

Class 4 BennettitoPanpsidat (200 2) in a Class 4 Taxopsida (Cycadeoideopsida) Order 1. Taxales Order 1. Bennettitale s

(Cycadeoideales)

Later publication suggested that separation of Taxopsida-Taxales was not valid in view of the work of Harris (1976) and others and accordingly merged this class and order with his order Coniferales (Pinales) and placed plants of this group under family Taxaceae under order Coniferales.

Andrew's (1961) classification included six divisions under the gymnosperms:

Gymnosperms

Divisions Pteridospermophyta Coniferophyta Ginkgophyta Orders 1. Coniferales Gymnosperms of uncertain Order 1. Pt eridospermales origin Order 1. Ginkgoales 2. Cordaitales Orders 1. Pentoxylales 2. Vojnovskiales

Cycadophyta Gnetophyta Orders 1. Cycadales 2. Bennettitales Order 1. Gnetales

Sporne (1974) recognized three classes under the gymnosperms:

Gymnosperms

Class 1 Cycadopsida Class 2 Coniferopsida Class 3 Gnetopsida

Orders 1. Pteridospermales Orders 1. Cordaitales Order 1. Gnetales 2. Bennettitales 2. Coniferales

3. Pentoxylales 3. Taxales 4. Cycadales 4. Ginkgoales

Steward (1983) placed under the division Trachaeophyta of plant kingdom three distinct classes :-

Plant Kingdom

Trachaeophyta Atrachaeophyta

Class 1 Progymnospermopsida Class 2 Gymnospermopsida Class 3 Gnetopsida Orders 1. Aneurophytales Orders 1. Pteridospermales Orders 1. Gnetales 2. Archaeopteridales 2. Cycadales 2. Ephedrales 3. Protopityales 3. Cycadeoidales 3. Welwitschiales 4. Caytoniales 5. Glossopteridales 6. Pentoxylales 7. Czekanowskiales 8. Ginkgoales 9. Cordaitales 10. Voltziales 11. Coniferales 12. Taxales

Meyen (1984, 1986) has suggested a classification wherein under the division Pinophyta (= Gymnospermae) he included three classes, viz. Ginkgopsida, Cycadopsida and Pinopsida. He placed the angiosperms under the division Magnoliophyta. According to Pant (2002) one of the novelties of Meyen's classification was the recognition of Ginkgopsida for the first time as a Class. The other two Classes had already been recognized earlier. Meyen (1984, 1986) had assigned nine Orders under this Class which included diverse gymnosperms, and most of the Orders included families which are based on incompletely known fossil forms. In fact, Pant (2002) believes that the Class was created to dump all such diverse imperfectly known forms which could not be fitted in the two other Classes. The Class included two living genera, Ginkgo with a single living species G. biloba besides several imperfectly known fossils and Ephedra with about fifty species. Both the genera showed resemblances with Class Pinopsida.

Class Cycadopsida had six Orders which included Gnetales and Welwitschiales and Class Pinopsida had two Orders, viz. Cordaitanthales and Pinales (Coniferales). While appreciating Meyen's system as a good survey Bhatnagar and Moitra (1996), felt the placement of Ephedrales, Gnetales and Welwitschiales appeared highly unsatisfactory.

Gifford and Foster (1989) considering the general tendency in recent years to establish several major taxa of extinct and living gymnosperms have raised the important groups to the rank of Division. Their classification of gymnosperms lists only the major Divisions and selected Orders. The occurrence of the Divisions and Orders in the geologic time is also indicated viz. Gymnosperms

Division Geologic Time

Div. 1 Progymnospermophyta Middle Devonian to Lower Carboniferous

Orders Aneurophytales Archaeopteridales

Div. 2 Pteridospermophyta Carboniferous to Permain

Orders Glossopteridales Caytoniales Permian to Triassic (Glossopteridales) Triassic to Cretaceous (Caytoniales) Div. 3 Cycadophyta

Order Cycadales Permian to Recent

Div. 4 Cycadeoidophyta Triassic to Cretaceous Order Cycadeoidales

Div. 5 Ginkgophyta Order Ginkgoales Triassic to Recent Div. 6 Coniferophyta

Upper Carboniferous to Permian

Orders Cordaitales Voltziales Coniferales Upper Carboniferous to Permian (Cordaitales) Div. 7 Gnetophyta Upper Carboniferous to Jurrasic (Voltziales) Triassic to Recent (Coniferales)

Permian (?) to Recent Orders Ephedrales Gnetales Welwitschiales

Kramer & Green (see Kubitzki, 1990) have classified the Division Gymnosperms into two Subdivision as

follows:-

Division Gymnosperms

Coniferophytina Subdivision Cycadophytina

Classes Cycadatae Gnetatae Ginkgoatae Pinatae Order Cycadales Gnetales Ginkgoales Pinales (Coniferales) Families 1. Stangeriaceae 1. Ephedraceae 1. Ginkgoaceae 1. Taxaceae 2. Boweniaceae 2. Gnetaceae 2. Cephalotaxaceae 3. Cycadaceae 3. Welwitschiaceae 3. Phyllocladaceae 4. Zamiaceae 4. Podocarpaceae 5. Araucariaceae 6. Sciadopityaceae 7. Taxodiaceae 8. Cupressaceae 9. Pinaceae

According to Bhatnagar & Moitra (1996) the clubbing of Gnetales with Cycadales is unjustified.

Bhatnagar and Moitra (1996) adopted a classification in their book entitled, "Gymnosperms", which is a synthesis of views prevalent in recent years. The classification is as following:

Division Gymnosperms

Classes Pro gymnospermopsida Cycadopsida Coniferopsida Gnetopsida

Class 1 Progymnospermopsida Class 3 Coniferopsida Orders 1. Aneurophytales Family: Aneurophytaceae Orders 1 Coniferales

2. Archaeopteridales Family: Archaeopteridaceae Families 1. Pinaceae 3. Protopityales Family: Protopityaceae 2. Taxodiaceae

3. Cupressaceae 4. Podocarpaceae

5. Araucariaceae Class 2 Cycadopsida 6. Cephalotaxaceae

Orders 1. Pteridospermales 7. Taxaceae Families 1. Calamopityaceae

2. Lyginopteridaceae Order 2 Ginkgoales Family: Ginkgoaceae 3. Medullosaceae

4. Callistophytaceae Order 3 Czekanowskiales Family: Czekanowskiaceae

Order 2 Glossopteridales Family: Glossopteridaceae Order 4 Cordaitales Family: Cordaitaceae

Order 3 Caytoniales Order 5 Voltziales Family: Voltziaceae Families 1. Caytoxiaceae

2. Corystospermaceae 3. Peltaspermaceae FOSSILIZATION AND SOME FOSSIL GYMNOSPERMS Class 4 Gnetopsida Order 4 Cycadales Family: Cycadaceae

Orders 1 Ephedrales Family: Ephedraceae Order 5 Cycadeoidales (Bennettitales) Suggested readings - Families 1. Williamsoniaceae Order 2 Gnetales Family: Gnetaceae 2. Wielandiellaceae 1. Fossil by Mark Lambert, A Kingfisher Book, Ward. Lock Limited, London. 3. Cycadeoidaceae Order 3 Welwitschiales Family: Welwitschiaceae

2. Nature Library- Fossils by Richard L. Moody. Order 6 Pentoxylales Family: Pentoxylaceae 3. The Biology and Evolution of Fossil Plants by T.N.