sign in sign up
<<
Home , Cordaicarpus, Cordaitaceae, Cordaitales, Cordaites, Pentoxylales

CORE COURSE II BIODIVERSITY II ( III Unit- )

Unit III : A general account of the characteristic features of Gymnosperms. Origin of Gymnosperms. Classification of Gymnosperms (Sporne, 1965). General structure and interrelationships of Pteridospermales, Bennetittales, and .

1. General account on characteristic features of Gymnosperms. The word “” comes from the Greek words “gymnos”(naked) and “sperma”(), hence known as “Naked .” Gymnosperms are the seed- producing , but unlike angiosperms, they produce seeds without . These plants develop on the surface of scales or , or at the end of stalks forming a cone-like structure. Gymnosperms belong to kingdom ‘Plantae‘ and sub-kingdom ‘Embryophyta’. The fossil evidence suggested that they originated during the Paleozoic era, about 390 million years ago. Basically, gymnosperms are plants in which the are not enclosed within the ovary wall, unlike the angiosperms. It remains exposed before and after fertilisation, and before developing into a seed. The stem of gymnosperms can be branched or unbranched. The thick cuticle, needle-like leaves, and sunken stomata reduce the rate of water loss in these plants. The family of gymnosperms consist of , the , the gnetophytes, and the species of Gynkgophyta division and Ginkgo biloba. Following are the important characteristics of gymnosperms: 1. Habit: Gymnosperms are a small group of seed plants which are represented by only 900 living species.The living gymnosperms are woody, evergreen (except Larix

Dr.P.PRABAKARAN Assist .Prof. MRGAC Page 1

and a Taxodium) perennials grow as or shrubs. Tallest trees are Sequoia sempervirens (366ft) and S. gigantia (342ft). 2. Occurrence: Gymnosperms are more ancient than the angiosperms. They formed dominant vegetation on earth some 200 million years back in mesozoic era. Today they are dominant only in cold areas, where instead of rain; snow is the source of water. The living members are founding in colder regions of earth where snow (not rain) is the source of water. Only the members of cycadales and gnetales thrive in warm dry climate. 3. Plant body: The dominant plant body is sporophyte (2n) which may be dioecious or monoecious. Gametophytes are inconspicuous and endosporic i.e. develop with the spores.Sporophyte differentiated into — root, stem and leaves. 4. Root system: Tap root system is exarch and diarch to polyarch. Besides tap root, coralloid roots (in cycads) and mycorrhizal root (in coniferals) present. 5. Stem: Erect, generally branched (Cycas is un-branched) 6. Leaves: Dimorphic i.e. 2 types, foliage and scale leaves. 7. : Flowers are absent. Two types of , microsporophyll’s and megasporophylls are usually aggregated to form distinct cones or strobili, cones (male cones) and seed cones (female cones) respectively. Seeds are not formed inside a . They are naked. 8. Xerophytic Traits: Gymnosperms are xerophytes in nature due to presence of thick bark, thick hypodermis, thick cuticle, scales leaves, sunken stomata, transfusion tissue, etc.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 2

In some cases leaves modified into needle-like, scale like or small leathery. These are the adaptations to combat water stress in air and colder regions. 9. Xylem: The xylem composed of xylem parenchyma and tracheids with bordered pits. Vessels are absent (exceptin Gnetales). 10. Phloem: The phloem composed of sieve cells and phloem parenchyma but companion cells absent. 11. Heterospory: The gymnosperms are heterosporous, means 2 types of spores produced i.e. haploid microspores and megaspores. Microspores produced within micro- sporangia while megaspores produce within megasporangia (nucellus) of ovules. Both types of sporangia are formed on special -like structures called sporophylls (microsporophylls and megasporophylls). 12. Cones or Strobili: Sporophylls are spirally arranged along an axis to form compact cone or strobili i.e. male or pollen cones and female or seed cones but in Cycas female cone is loosely arranged called lax. 13. Ovules: A distinction of ovary, style and stigma is absent.The ovules are naked, sescile, geneiallyorthotropous, and unitegmic or bitegmic (in Gnetum). Ovules are orthotropous and sessile. Each is surrounded by a 3-layered integu•ment. 14. Pollination: Pollination is direct as a stigma is absent and the pollen grains directly reach the micropylar ends of ovules. Pollination is usually accomplished by wind (anemophily). 15. Fertilization: Fertilization is siphonogamous i.e. male gametes carried to female gametes by means of a pollen tube. Double fertilization, a feature unique to angiosperms, is

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 3

absent in gymnosperms, but found in Ephedra. After fertilization, Zygote develops into embryo and ovules become seed. 16. Endosperm or female gametophyte: Endosperm or female gametophyte formed before fertilization and is always haploid. (But triploid in angiosperms.) Female gametophyte contains archegonia. Seeds contain a food laden tissue or endosperm for future growth of embryo into seedling. The tissue represents the female gametophyte. 17. Male gametophyte: Male gametophyte produces only two male gametes or sperms. Generally one of them is functional. External water is not required for transport of male gametes. Instead, a pollen tube is formed by the male gametophyte for effecting fertilization (siphonogamy). 18. Embryo development: Embryo development is meroblastic i.e. develops from a small part of zygote. Most members show polyembryony i.e. development of more than one embryo, but only one survives at the end. Cotyledons 2 (in Cycas) or many (in Pinus 2- 14). 19. Connecting Links: These plants have vascular tissues which help in the transportation of nutrients and water. The lower gymnosperms like Cycadales resemble pteridophytesin absence of vessels and wood fibers whereas higher gymnosperms like Gnetales and Coniferales resemble angiosperms. Like pteridophytes, xylem does not possess vessels except in some geophytes. Phloem is without companion cells and sieve tubes. Sieve cells are not arranged end to end in rows. Vascular tissues are arranged into vascular bundles just like angiosperms. Vascular bundles of stem are open so that secondary growth is quite common

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 4

Origin of Gymnosperms Gymnosperms were the first seed plants to have evolved. The earliest seedlike bodies are found in rocks of the Upper Series (about 382.7 million to 358.9 million years ago). During the course of the evolution of the seed habit, a number of morphological modifications were necessary. First, all seed plants are heterosporous, meaning that two kinds of spores (microspores and megaspores) are produced by the sporophyte. Hence, it is assumed that the ancestors of seed plants were heterosporous. Secondly, sporangia of seedless plants typically lack an integument, which forms the seed coat in gymnosperms. Fossil ovules discovered in Scotland suggest that integuments originated during the Mississippian subdivision of the Period (about 358.9 million to 323.2 million years ago). The ovules of Genomosperma kidstonii, for example, consisted of an elongated megasporangium with one functional megaspore and featured eight elongated fingerlike processes that loosely surrounded the megasporangium. In a related species, G. latens, those eight fingerlike processes were fused at the base into a cup and covered the megasporangium rather closely. The extinct division Progymnospermophyta is thought to be ancestral to seed plants. The best-known is the Devonian Archaeopteris, originally assumed to be a , with wedge-shaped subdivided leaflets known as pinnules and sporangia borne on appendages in between the pinnules. Its wood was like that of many conifers, consisting of tracheids and vascular rays, with closely spaced circular bordered pits on the radial walls of the tracheids. Pits were clustered, separated from other clusters by an area of the wall lacking pits. At least some species are known to have been heterosporous. Archaeopteris had many of the features to be anticipated in a seed-plant ancestor and likely gave rise to more than one group of gymnosperms.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 5

Earliest gymnosperms The earliest recognized group of gymnospermous seed plants are members of the extinct division Pteridospermophyta, known as pteridosperms or seed . These plants originated in the Devonian Period and were widespread by the Carboniferous. In habit, seed ferns resembled some in that they were small trees with fernlike leaves (the equivalent of a progymnospermous flattened branch) bearing seeds. Secondary vascular tissues were common in stems of seed ferns, though the wood was composed of thin-walled tracheids and abundant vascular rays, suggesting that stems were fleshy like those of cycads.

Pteridosperm seeds were very similar to those of cycads and were often large, with a soft outer seed coat and a harder inner seed coat. Within a mature ovule was a massive female gametophyte with several archegonia. Some fossils suggest that the transport of the sperm through a pollen tube (siphonogamy) was in existence as far back as the Paleozoic. Pollen-bearing organs were variable among the pteridosperms; in many cases the microsporangia were elongated and fingerlike and were produced in clusters or were fused into compound organs. Appearance of gymnosperm divisions It is generally conceded that from the pteridosperms arose members of the division Cycadophyta. The first cycads appeared in the Period (298.9 million to 252.2 million years ago), although fragmentary fossils of older age suggest that cycads were present during the preceding Carboniferous Period. Some of these presumed cycads differ from extant members in that megasporophylls were undivided, unlike those of Cycas, considered to be primitive among cycads, in which the distal portion of the megasporophyll may be pinnately divided. Other Permian megasporophylls, from China, are more like those of Cycas. remains, especially leaves, are abundant in Mesozoic

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 6

rocks. For this reason paleobotanists often refer to the Mesozoic Era as the “age of cycads.” The earliest well-known cycads appear to have had slender stems, sometimes branched, with leaves not borne close together, unlike the situation in extant cycads in which leaves are densely crowded at the apex of the plant. There is evidence that these earliest cycads were deciduous. Megasporophylls of Mesozoic cycads are essentially like those of extant cycads. The megasporophyll of the Palaeocycas is like that of Cycas. megasporophylls are like those of most other cycads. Extant cycads are now limited in geographic distribution to the warmer parts of Earth. Coexisting with the cycads during the Mesozoic was another group of gymnosperms, the cycadeoids (division Cycadeoidophyta). Although they were superficially similar in habit to the cycads, with a squat trunk and often pinnately divided leaves, their reproductive structures were different, and their actual relationship is not close. Typically, seeds were borne on the surface of a fleshy receptacle. Among the seeds were sterile structures, called interseminal scales, that held the seeds tightly together. Pollen organs were quite similar among the forms in the sense that all had a whorl of modified microsporophylls on which were borne compound microsporangia. Conifers (division Coniferophyta) appeared first toward the end of the Carboniferous Period (about 358.9 million to 298.9 million years ago). Some of the earliest conifers (class Cordaitopsida) were trees with long strap-shaped leaves. Trunks were similar to those of extant conifers, with dense compact wood; small thick-walled tracheids; and narrow vascular rays. Reproductive axes were slender, bearing narrow in the axils of which were small budlike with helically arranged scales. On some of the topmost scales were borne elongated microsporangia. Buds on other axes bore ovules instead of microsporangia. By the late Paleozoic there came into existence another group of extinct conifers, the Voltziales (division Pinophyta). In general habit they must have

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 7

resembled some of the extant araucarias (e.g., Norfolk Island pine), with whorled flattened branches bearing helically arranged needlelike leaves. Reproductive axes were generally similar to those of the Cordaitales, but they were more compact, with the bracts on the ovule-bearing axes obscuring the axillary fertile buds. During the end of the Paleozoic and in the early Mesozoic, these axillary buds underwent further transformation. The sterile non-seed- bearing part became flattened, with the scales fused together. The ovule-bearing portion was situated toward the upper surface (away from the ). The ovuliferous scale of a seed cone, then, may be interpreted as an axis bearing bracts in the axils of which are modified woody ovuliferous scales derived from lateral buds. Modern families of conifers began to appear in the Mesozoic Era. Members of the Cupressaceae, the family to which redwoods and bald cypress are assigned, appeared first in the Jurassic Period. Dawn redwoods (Metasequoia genus) were discovered first as fossils in Miocene (23 million to 5.3 million years ago) deposits and were assumed to be extinct until M. glyptostroboides was discovered growing in Sichuan province in China in 1944.

During the late Triassic there existed a type of conifer (Compsostrobus) that had many features of the pine family (Pinaceae). Seed cones had woody ovuliferous scales with two ovules on the upper surface. More-typical pinaceous remains occurred later in the Mesozoic. The oldest known pine (Pinus mundayi) dates to about 140 million years ago; the species was identified from charred fossil remains in 2016. Conifers were the dominant vegetation just before the appearance of the angiosperms. The division Ginkgophyta, represented today by only one living species, Ginkgo biloba, was much more widespread in past ages. Gymnosperms that were presumed to be ginkgophytes existed as far back as the Permian Period. In Mesozoic rocks, Ginkgo leaves are commonly found throughout the world. The

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 8

oldest fossil ginkgophytes had leaves that were much more divided than the typical Ginkgo leaf, resembling more closely the leaves found on new growth in living ginkgoes. The fossil record of the division is obscure, and its origin is not clear. Pollen grains similar to those of Ephedra and Welwitschia are found as far back as the Permian Period. Remains of possible gnetophytan plants occur in Upper deposits (formed 100.5 million to 66 million years ago). While those fossils are unlike any extant species, the venation of the foliage is similar to that of leaves of Welwitschia, and the pollen grains are typical of the division.

Pteridospermales The Pteridospermales are popularly known as “seed-ferns”. They possessed fern-like foliage, bore seeds, and are, therefore, called pteridosperms. They first appeared on the earth in Upper Devonian times of the Palaeozoic era .They were at their climax in Carboniferous period and became extinct in Jurassic period of Mesozoic era. Members of Pteridospermales of division Spermatophyta (seed plants) resemble with Filicales (ferns) of pteridophyta in shape of their large, usually multi-

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 9

pinnate leaves and in the venation of their leaflets Seeds have been found attached to some of these fern-like fronds. Due to this, the name Pteridospermae has been suggested which indicates a group of the plants showing the characters of both ferns and seed plants. Chamberlain (1935) objected for the use of this name and preferred to name it as Cycadofilicales because of their several resemblances with both Cycadales of gymnosperms and Filicales (ferns) of pteridophytes. General Characters of Pteridospermales: 1. Extinct Palaeozoic and Mesozoic plants found from Devonian to Jurassic periods. 2. Plants possessed slender stems with large frond-like leaves as in Alethopteris, Sphenopteris, etc. 3. Primary xylem was mesarch, represented by solid or medullated protostele. Rarely, the primary xylem was exarch. Polystelic condition was also observed in some members. 4. The secondary wood was manoxylic (loose and soft) and limited in amount. 5. The radial walls of tracheids had multiseriate pits. 6. The cortex was well-developed and had longitudinally aligned fibre strand. 7. Leaves were usually fern-like, relatively large, pinnately compound, and often pinnate several times. 8. The leaves were covered by a resistant cuticle. 9. Ovules borne separately along margins of, or on surface of pinnately compound megasporophylls. 10. Ovule-bearing frond or megasporophyll was not part of a cone. 11. Megasporophylls were not arranged in strobili. 12. Megasporophylls were like foliage leaves, or specialized structures, sometimes peltate. 13. Microsporophyll’s pinnately compound and not in strobili.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 10

14. The microsporangia had no annulus and were sometimes grouped into synangia. 15. A well-developed vascular supply was present in the seed. 16. The seeds were also provided with a definite pollen chamber, e.g. Lagenostoma lomaxi. 17. The seeds of Pteridospermales resemble with those of the present day Cycads. Classification of Pteridospermales : Sporne (1974) divided Pteridospermales (Cycadofilicales) into following seven families: 1. Lyginopteridaceae 2. Medullosaceae 3. Calamopityaceae 4. Glossopteridaceae 5. Peltaspermaceae 6. Corystospermaceae 7. Caytoniaceae. Sporne (1974) mentioned that the first three families (Lyginopteridaceae, Medullosaceae and Calamopityaceae) were confined to Palaeozoic era while the last four (Glossopteridaceae, Peltaspermaceae, Corystospermaceae and Caytoniaceae) were restricted to the Mesozoic or extended from the late Palaeozoic into the Mesozoic.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 11

Affinities of Pteridospermales: Pteridospermales (Cycadofilicales) may be assigned an intermediate position between ferns and Cycadophytes due to their several resemblances with both the groups. But due to the presence of exposed seeds they belong to gymnosperms with certainty. Some of their possible affinities with ferns and Cycadophytes are mentioned below:

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 12

Similarities with Ferns: 1. Large and pinnately compound leaves. 2. Young leaves are circinately coiled. 3. Mesarch or rarely exarch condition. Leaf traces are also mesarch. 4. Absence of vessels in the xylem and companion cells in the phloem. 5. Polystelic condition of Medullosa resembles with ferns. 6. The sporangia were borne on the foliage leaves. 7. Except the presence of secondary wood, the vascular anatomy of stem is similar with that of ferns. 8. Presence of thick wall around the megaspores. Similarities with Cycadophytes: There are several common characteristics found in both Cycadales and Pteridospermales. Of these, some are under mentioned: 1. The wood is manoxylic in both Cycadales and Pteridospermales. 2. Mucilage canals are present in the cortex and pith of the stem of both cycads and “seed ferns”. 3. Extensive cortex is present in most of the Cycadales and in some Pteridospermales (e.g. Medullosa). 4. Centripetal xylem is present in some Cycadales and also in the vegetative organs of some “seed ferns” or Pteridospermales. 5. Male gametes or spermatozoids are multi-ciliate and motile in both the groups. 6. In both Pteridospermales and some Cycadales (e.g. Cycas), the megasporophylls remain spirally and loosely arranged, and they do not form a compact cone. 7. In Calymmatotheca the ovules remain attached on the proximal parts of the leaves and their distal or tip regions remain sterile. In this regard, they resemble

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 13

with the megasporophylls of Cycas, in which upper part is dissected, leafy and sterile while the lower part bears ovules. 8. The seeds of Lagenostoma lomaxi of Pteridospermales remain surrounded by an outer hard stony layer and an inner fleshy layer. They resemble the seeds of cycads to some extent. 9. The vascular supply of the ovules of cycads and some Pteridospermales also show some similarity with each other. 10. Both Cycadales and Pteridospermales possess pinnately compound leaves. 11. Pollen chamber is present in the ovules of both these groups.

Bennettitales: The name “” has been given to honour J.J. Bennett, an English botanist. The fossilized trunk of genus Bucklandia was the first specimen of Bennettitales, collected from Great Britain in 1825. A silicified trunk of Cycadeoidea etrusca was discovered in 1867 in an Etruscan tomb. This group of fossil plants flourished well during the Triassic to Lower Cretaceous periods of Mesozoic era. As the Carboniferous period is called the “Ages of Ferns “, the Mesozoic era is called the ‘Ages of Cycads’. It is due to the fact that Cycadeoideales co-existed with Cycadales during Mesozoic era from Jurassic up to Cretaceous period, and hence this period is called Age of Cycads. Bennettitales are found either in the form of compressions or petrifactions. Due to Cycad-like form of their fronds and the presence of short stems covered with an armour of presistent leaf bases Bennettitales (Cycadeoideales) have been treated under Cycadophyta by some workers. However, the two groups are quite distinct from each other and maintain their independent identity. Origin of Bennettitales:

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 14

Palaeobotanists believe that Bennettitales originated from Pteridospermales. But presence of stalked ovules in Bennettitalean members is a strong evidence against such a theory. Some workers have, however, shown resemblance between the fronds of Pteridospermales and the stalked ovules and inter-seminal scales of Bennettitales. But it is hard to imagine that stalked ovules and inter-seminal scales are homologous with the fronds. Discovery of a fossil plant, Westersheimia, from the Triassic of Austria, is again a step towards the possible Pteridospermean ancestry of Bennettitales. This genus occurs along with Bennetticarpus, the seed-bearing organs of Bennettitales. In Bennetticarpus wettsteinii and Westersheimia the ovules and inter-seminal scales were present on the ultimate segments of a pinnate structure. But it is still not clear how the entire group of Bennettitales evolved from Pteridospermales. Some peculiar characteristics present in Bennettitales and not in any other group of gymnosperms, include: (i) Bisporangiate strobili,(ii) Synangium-bearing fused microsporophyll’s,(iii) Close occurrence of ovules and inter-seminal scales, and(iv) Production of stalked ovules. Distinguishing Features of Bennettitales: 1. These extinct Mesozoic plants were present were present on the earth from Triassic to Cretaceous. 2. Bennettitales were so abundant during Mesozoic era that this period is known as ‘Age of Cycads’. 3. The members of this group are found either as compressions or petrifactions. 4. The stems were stout or slender and had a wide pith. 5. The stem grew very slowly and had manoxylic wood. 6. Resembling living Cycads, the Bennettitalean leaves were mostly pinnately compound, and only occasionally simple.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 15

7. Venation was open, and only rarely closed. 8. Syndetocheilic type of stomata were present. 9. The wall of the epidermal cells was sinuous. 10. The reproductive organs were organised in the form of hermaphrodite (e.g. Cycadeoidea) or unisexual (e.g. Wielandiella) “flowers”, which in turn were protected by many bracts. 11. The ‘flowers’ developed in the axil of leaves. 12. Male reproductive organs were borne in a whorl. They were free or fused, entire or pinnately compound. 13. Microsporangia were present abaxially in the form of synangia. 14. Microsporophyll’s sometimes surrounded megasporophylls forming hermaphrodite “flowers”. 15. Ovules were numerous and stalked and borne on a conical, cylindrical or dome-shaped receptacle. 16. Many inter-seminal bracts were present on the ovule containing receptacle. 17. The scales or bracts were united at end to form shield through which micropyle protrudes. 18. Seeds were dicotyledonous. Affinities of Bennettitales: Resemblances of Bennettitales with Ferns: Bennettitalean plants had multicellular ramenta on their entire body, a characteristic also seen in ferns. Some other features suggesting the filicean affinity of Bennettitales include the presence of: (i) Direct leaf traces, (ii) Scalariform tracheids, and (iii) Large pith. Resemblances of Bennettitales and Cycads: Bennettitales resemble cycads in the:

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 16

(i) structure of their fronds, (ii) presence of short stems covered with an armour of persistent leaf bases, (iii) presence of barrel-shaped trunk, (iv) presence of very thick cortex, relatively thin wood and large pith in the stem, (v) manoxylic wood, (vi) monocolpate pollen grains, (vii) orthotropous ovules, and (viii) dicotyledonous embryo. On the basis of several such affinities, Chamberlain (1935) opined that both Bennettitales and Cycadales arose by parallel evolution from some common ancestor. The ancestral type must have had a foliar megasporophyll having the ovules at the apex as well as on both the sides. During course of evolution, the lateral ovules must have suppressed and the megasporophyll must have reduced to a stalk-like structure, and thus the Bennettitales must have come into existence. On the other hand there exist several fundamental differences between Bennettitales and Cycadales, already listed in Table 6.1. Due to so many dissimilarities it will not be possible to visualize any phylogenetic connection between two groups. Andrews (1961) concluded on the basis of such differences that both the groups have evolved along with two different and independent lines. Resemblances and Difference between Bennettitales with Pteridospermales: Characters common in both Bennettitales and Pteridospermales include: (i) Presence of ramenta] hairs, (ii) Syndetocheilic stomata, (iii) Direct leaf traces, (iv) Similar anatomical details, (v) Leafy microsporophyll’s, and (vi) Presence of cupule.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 17

The so-called bisporangiate ‘’ of Bennettitales could be compared with the bisporangiate fronds of Ptendospermales. Scientists are of the opinion that there exist two lines of evolution from Pteridospermales. Of these, one line gave rise to Bennettitales possessing both uni- and bisporangiate forms, and the other gave rise to mono-sporangiate forms of cycads. Among the major differences include the: (i) Presence of smaller and stalked ovules in Bennettitales, which are absent in Bennettitales, and (ii) Prominent vascular supply to the nucellar tissue in Bennettitales which is not seen in Pteridospermales. Resemblances of Bennettitales with Gnetales: The two groups resemble each other in their seed structure but differ completely in several aspects. According to Rodin and Kapil (1969), “the complex and highly specialised inflorescence of the Bennettitales, the presence of inter- seminal scales and some vegetative features fail to show homologies with Gnetum”. Resemblances of Bennettitales with Angiosperms: Endarch siphonostelic vasculature of Bennettitales resemble very closely with sympetalous angiosperms. Frequent occurrence of scalariform tracheids in both the groups also brings them close together. Flowers of several primitive angiosperms (e.g. Magnoliaceae) also resemble closely with the strobili of Bennettitales, and on this basis Arber and Parkin (1907) opined that Bennettitales are the ancestors of flowering plants. Contrary to this, the Bennettitalean stamen is large, front like and compound structure, and cannot be compared with that of the stamen of Magnoliaceae. Moreover, there is no point of comparison between typical carpel of Magnolia and ovule of Cycadeoidea which is strictly gymnospermous.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 18

Ovules are naked in Bennettitales while it is not so in angiosperms. The wood rays of Bennettitales lack marginal cells which are present in angiosperms. The two groups also differ in their general habit and floral morphology. Regarding the evolutionary tendencies, Pant and Kidwai (1977) opined that Bennettitalean “flower” may represent a parallel line of evolution to angiosperms. Disappearance of Bennettitales during Cretaceous substantiates this view. During this period, however, the angiosperms were already existing by then on the earth. Differences between Bennettitales and Cycadales: Bennettitales: 1. Stomata syndetocheilic. 2. Secondary wood monoxylic. 3. Leaf traces arise always singly at the point of their origin. 4. Fructifications are flower-like structures. 5. Flowers are bisporangiate and plants are monoecious. 6. Microsporophyll’s large, uni-or bi-pinnate, arranged in whorls and fused at the base. 7. Arrangement of stalked ovules along with inter-seminal scales. 8. Seeds exalbuminous. Cycadales: 1. Stomata haplocheilic. 2. Secondary wood polyxylic. 3. Traces arise as two independent units at the point of their origin. 4. Fructifications are strobilar in nature. 5. Flowers are mono-sporangiate and plants are dioecious. 6. Microsporophyll’s simple with sori on lower side and arranged spirally. 7. Such an arrangement is not found.8. Seeds always endospermic.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 19

Cordaitales: Cordaitales, an extinct group of Palaeozoic tall trees of gymnosperms, formed “the world’s first great forests”. The name was given to honour an Australian botanist, A.J. Corda. Majority of the Cordaitales were tall, large- leaved trees attaining a height of more than 30 metres or so. The group started declining during Permian and became completely extinct by the end of this period. As per records the Cordaitales occurred side by side with Pteridospermales

Cordaitales fossils have been reported from North and South America, Europe, China, Ruccia, India, Australia and Africa indicating their world-wide occurrence during Devonian and Permian. Cordaitales in India are represented in the form of impressions or compressions of leaves, seeds and petrified woods. Representatives of the only family have been reported from India. No member of Poroxylaceae has been reported. In India, Cordaitaceae are represented in lower formations. , Dodoxylon, Noeggerathiopsis and are the Cordaitalean genera reported from India. Distinguishing Features of Cordaitales:

1. This group of fossil plants had tall trees with slender trunks and a crown of several well- developed branches. 2. Plants were present from Devonian to Permian periods of Palaeozoic era (Fig. 1.1) 3. The leaves were simple, spirally arranged and strap-shaped, grass-like or paddel-like. 4. The leaves attained a length up to 1 metre or even more, and had parallel venation.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 20

5. A scanty primary wood was present. 6. In mature stems, the secondary wood was mostly pycnoxylic. 7. Compound unisexual cones were present. 8. Each compound cone had a main axis with bracts subtending secondary fertile shoots possessing fertile and sterile appendages. 9. Mega-strobili had sterile appendages below and ovule-bearing fertile appendages above. 10. One to four ovules were present on each female fertile appendage 11. Micro-strobili had sterile appendages below and pollen-sac containing fertile appendages above. 12. Four to six terminal pollen sacs were present on each male fertile appendage. 13. Sperms have not been reported, but presence of pollen chambers suggests that motile sperms might have been formed. Pentoxylales:

This group has been discovered and named as “Pentoxyleae” by well-known Indian Palaeobotanist Professor Birbal Sahni (1948). This is a group of some fossil plants described from Rajmahal Hills in Amrapara District (Santhal Parganas) of Eastern Bihar (India) revealing their existence in Jurassic Period. Distinguishing Features of Pentoxylales: 1. Extinct Mesozoic plants found in Jurassic period. 2. Although the exact habit of these plants is not clearly established, these were probably shrubs or very small trees. 3. Long and short shoots were present on these plants. 4. Short shoots had spirally arranged leaves and terminally located reproductive organs. 5. Leaves were thick, simple, lanceolate, and had diploxylic leaf trace. 6. Stomata were formerly thought to be syndetocheilic, but now they are considered to be haplocheilic.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 21

7. Leaves possessed open venation. 8. Stems were polystelic. Basinger et al. (1974) opined that “it may be more appropriate to call each stele as vascular segment or sympodium”. 9. Wood of Pentoxylon was pycnoxylic and resembled Araucaria. 10. Ovules were sessile. 11. Female reproductive organs were like stalked mulberry, consisting of about 20 sessile seeds attached to central receptacle and surrounded by stony layer and then fleshy outer layer of integument uniting them. 12. Male reproductive organs or microsporophyll’s form whorl of branched micro-sporangiophores. 13. The micro-sporangiophores were fused basally into a disc-like structure. Stem Genera of Pentoxyleae: (i) Pentoxylon Sahnii: Pentoxylon sahnii and Nipanioxylon guptai are the stem genera of “Pentoxyleae”. The stems of Pentoxylon sahnii attained a diameter from 3mm to 2 cm. The stem has always been reported in association with the leaves called Nipaniophyllum. Presence of five steles in a cross- section of the stem has been the main reason for giving the name Pentoxylon to the genus. Many short lateral shoots or dwarf shoots were also present on the stem. Five steles (Fig. 7.1) occupied greater part of the stem in a cross-section. Each stele had its own cambium. The cambium was uniformly active in the young stems, but at maturity more secondary tissue developed towards the centre, and thus the secondary wood appeared eccentric. Primary phloem and primary xylem were present towards outer and inner sides of the cambium, respectively. Six steles have also been observed by Sahni (1948), although rarely. According to Vishnu-Mittre (1953) the number of steles varied along the length of the stem.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 22

There were present five much smaller bundles just alternating with the main bundles of the stem i.e. five steles. Each such bundle had a large amount of secondary wood. These were probably the leaf trace bundles. Medullary rays of the main steles were uniseriate, and they lacked ray tracheids, wood parenchyma and resin canals. The secondary wood resembled greatly with that of Araucaria. Uniseriate or bi-seriate bordered pits were present on the radial wall of tracheids.

(ii) Nipanioxylon: This stem genus of Pentoxyleae was discovered from the village Nipania and hence named Nipanioxylon. Village Nipania is in Rajmahal Hills, near Dumarchir in the Amrapara district (Santhal Parganas) in Bihar (India). Nipanioxylon differed from Pentoxylon in possessing larger number of bundles (steles) and less developed secondary growth in the stem. Nipanioxylon resembled Pentoxylon in other details. Leaves of Pentoxyleae: Nipaniophyllum: The leaves have been described under the name Nipaniophyllum raoi. They were found attached with the shoots or Pentoxylon sahnii. They were originally described under the name Taeniopteris. They were present on the short lateral shoots (Fig. 7.2). Each leaf was simple, petiolate, strap-shaped, and possessed a

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 23

well-developed mid rib with many parallel lateral veins. Branching has not been observed in lateral veins.

Similar to cycads, the leaf traces had centripetal and centrifugal xylems, thus exhibiting diploxylic condition. Sahni (1948) reported the presence of syndetocheilic stomata in Nipaniophyllum but Vishnu-Mittre (1953) also observed the presence of Cycadalean type of haplocheilic stomata. Later on, Sharma (1969) and Bose et al. (1985) observed that arrangement of stomata was anomocytic as in Cycads and most other gymnosperms. Vascular bundles in Nipaniophyllum were mesarch. Seed-Bearing Organ of Pentoxyleae: (i) Carnoconites: The female cones or seed-bearing organs have been described under the name Carnoconites. Two species (C. compactum and C.laxum) have been described. Both these species have, however, not been reported in organic connection with the stem genus Pentoxylon but from the same rocks. Seed-bearing organs (Fig. 7.3) were forked and found attached terminally on the lateral dwarf shoots. They were mulberry-like, and attained a length of about 1.8

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 24

cm in Carnoconites compactum and 3 cm in C. laxum. They were, however, narrower in C. laxum. About 20 sessile ovules were seen attached on the receptacle, and there were no inter-seminal scales. Any -like structure was also not reported. In-spite of these facts Sahni (1948) used the word ‘cone’ for these structures. The ovules were covered by a single integument. The nucellus was free from the integument.

(ii) Sahnia Nipaniensis: The probable microsporangiate or male organs of Pentoxyleae were named as Sahnia nipaniensis by Vishnu-Mittre (1953). They were present terminally on the , and fused basally in a shallow disc (Fig. 7.4). Vishnu-Mittre (1953) reported as many as 24 such pollen-bearing organs. Each microsporophyll possessed many pear-shaped, unilocular sporangia. The terminal position of the sporophyll was also occupied by a sporangium (Fig. 7.5). Several monocolpate and boat-shaped pollen grains were present in each microsporangium.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 25

The sexine of the pollen wall is homogeneous while its nexine is present in the form of thin dark zone. In the region of aperture, the sporoderm is highly folded. Taylor & Taylor (1987) observed a few lamellae in the region of aperture. Other details of the male flowers are not yet fully known.

Affinities of Pentoxylales: Some of their possible affinities are discussed below: Affinities with Cycadales: The two groups (Pentoxyleae and Cycadophytes) resemble each other in: (i) possession of direct leaf trace in Pentoxyleae and seedlings of some cycads,

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 26

(ii) leaf traces anatomy in showing diploxylic nature of their vascular bundles, (iii) haplocheilic stomata, (iv) vestigial polystely in the seedling stages of some modern cycads, (v) nature of wood and pittings, (vi) possessing more or less similar kind of pollen grains, and (vii) structure of their seeds and peduncles. However, vascular bundles in Pentoxylon are not arranged in the Cycadean manner. There is also no similarity between the polystelic condition of Pentoxylon and mature modern cycads. Affinities with Conifers: Some of the characteristics, in which Pentoxylales resemble with conifers, include the presence of: (i) Pycnoxylic wood, (ii) Tracheids with circular bordered pits arranged in uniseriate or bi-senate manner, (iii) Uniseriate wood rays, and (iv) dimorphism in the stems of Pentoxylon sahnii and several conifers. However, the general anatomy of the stem of Pentoxylales is not at all coniferous as mentioned also by Sahni (1948). Pentoxylales are totally stachyospermous (i.e. both the male and female organs were borne on stems, rather than on leaves) while conifer ales are partly phyllospermous and party stachyospermous. Affinities with Medullosaceae: Pentoxylales also resemble with members of family Medullosaceae of Palaeozoic Pteridospermales (e.g. Medullosa) in possessing a polystelic primary vasculature in their stems. The secondary wood of Pentoxylon was pycnoxylic, character also encountered in some species of Medullosa. The two groups also resemble each other in their mode of branching as well as nature of their steles. Coniferous type of pittings were present in the stems of Pentoxylon and some species of Medullosa.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 27

Affinities with Bennettitales: Under mentioned are some of the resemblances between Pentoxylales and Bennettitales: 1. Presence of syndetocheilic stomata, in addition to haplocheihc ones. 2. Diploxylic nature of the vascular bundles. 3. Whorled micro-sporangiophores. 4. Superficial resemblances between male flowers of both the groups. 5. The manner in which the ovules were borne in Pentoxylales was similar to that of Bennettitales. However, the inter-seminal scales, found in members of Bennettitales, were absent in Pentoxylales. 6. Stachysporous nature of their male and female organs, i.e. instead of leaves, these organs were borne on the stems. 7. Both the groups share several common characters in their dwarf shoots. 8. Presence of direct leaf trace also brings the two groups quite close to each other. However, the polystelic condition of the stems of Pentoxylon and Nipanioxylon has no similarity with that found in Bennettitales. In Pentoxyleae, the sporangiophores were erect, radial structures without any sterile part. They were spirally branched and possessed sac-like unilocular microsporangia. On the other hand, in Bennettitales these structures were completely different. They had circinate dorsiventral pinnate sporophyll with a sterile and synangium- bearing portion. Affinities with Some Other Groups: While stem dimorphism of Pentoxylon sahnii is a Ginkgoalean feature as also a coniferous one, the diploxylic vascular bundles of Pentoxylales are also seen in Cordaitales as also in Bennettitales. Meeuse (1961) observed several resemblances between Pentoxylales and Pandanus (a member of family Pandanaceae of Monocotyledons) and opined that “Pandanaceae and some related monocotyledons” have descended directly “from Pentoxylales”.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 28

Features Unique to Pentoxylales: The mulberry-like female cones or infructescences of Pentoxylales (Carnoconites compactum) with over twenty sessile ovules attached to a central receptacle is a unique feature of this group. Furthermore, these infructescences had neither any inter-seminal scales, nor anything that could be called a sporophyll, a unique feature again. The sporangiophores of Pentoxylales had spirally arranged branches and the sporangia were unilocular as well as terminal. In view of the above mentioned unique features, (e.g. wood similar to that of a conifer, leaf and pollen grains like that of cycads and cycadeoids, and ovulate cones not reported in any other gymnosperms) as well as resemblances of Pentoxylales with several groups of plant kingdom, Sahni’s (1948) remarks that Pentoxyleae “occupy a unique and rather isolated position”, or Pentoxyleae “is a group of plants that defies classification”, still hold good. This group, of course, belongs to gymnosperms, but to establish its phylogenetic relationships a lot more is still to be done.

Cordaitales 1. Cordaitaceae:

External Morphology: Cordaitaceae grew luxunently and formed large forests of tall trees during Upper Carboniferous period. Plants attained a height of more than 30 metres. They had terminal and spirally arranged well-spread branches bearing tufts of leaves (Fig. 9.1). The leaves were large, leathery, grass-like or paddle-shaped, and attained a length of about 1 metre and a breadth of about 15 cm (Fig. 9.2). They were, however, smaller than that of Cycads. Some members also had small needlelike leaves. The leaves had a dichotomous venation.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 29

The leaves of several members of Cordaitaceae were highly variable in shape and were put under a form-genus . The same name is now given to the stem as well as to the entire plant.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 30

Some other stem-genera of Cordaitaceae include Mesoxylon, Metacordaites, Parapitys, Caenoxylon, Mesopitys, Cordaicladus and Artisia. Amyelon is a root- genus while Cordaianthus is a name give to the cones or inflorescence. Seeds have been described under the form-genera Cardiocarpus, Mitrospermum and Kamaraspermum. Cridland (1964) studied and reconstructed a cordaitean plant. According to him the plants attained a height of nearly 5 metre with stilt roots similar to mangrove plants. These studies suggest the habitat of Cordaites in the swamps along the seashores. Anatomy of Cordaitaceae: 1. Stem: The stem (Fig. 9.3) resembled closely with Conifers. Both Cordaites and Mesoxylon possessed a large central pith and cortex. The wood was scanty in some species while in others it developed a large vascular cylinder, and in still other cases distinct growth rings were present. The primary wood was endarch but in Mesoxylon it was mesarch. The secondary wood consisted of pitted tracheids having multiseriate pittings. The tracheids were long and slender. Bordered pits were present, and they were confined mainly on the radial walls. In older tracheids, however, the pits were also present on the tangential walls. Medullary rays were one or two cells wide. The bordered tracheids were hexagonal in outline (Fig. 9.4) and the large pith was characteristically discoid (Fig. 9.5). Mesoxylon differed from Cordaites in the structure of the leaf trace. A network of sclerenchyma, present in the outer cortex of Mesoxylon, was absent in Cordaites. Since, technically speaking, the genus Cordaites refers to the leaves of Cordaitaceae, an alternative name Cordaioxylon was proposed by Arnold (1967).

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 31

2. Root: The roots of Cordaitales are known as Amyelon (Fig. 9.6) and resembled very much with the modern Conifers. Cridland (1964) studied the root system of Amyelon and found it to be shallow and highly branched forming stilt roots

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 32

supporting the stem. They were diarch or triarch in structure. Ectotrophic mycorrhizal fungi were present on the roots. The protoxylem had spiral tracheids while the metaxylem was scalariform in structure. Tracheids had multiseriate bordered pits. The cortex was quite large and divisible into outer and inner cortex. The secondary cortex and cambium were also quite distinct.

3. Leaf: The Cordaitalean leaf is described under the name Cordaites. Several xerophytic internal characters were present in the leaf. The epidermal and hypodermal cells were thick-walled, and the hypodermal cells on both sides were grouped into ribs. Several mesarch vascular bundles were present. Each vascular bundle was surrounded by a thick-walled strong bundle sheath. The transfusion tissue was present in the form of some elongated cells in between two vascular bundles (Fig. 9.7). The mesophyll was clearly differentiated into palisade and spongy parenchyma in species such as Cordaites lingulatus (Fig. 9.8).

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 33

Spore-producing Organs: The strobili were usually monoecious but some Cordaitales were also dioecious. They were, however, never bisporangiate. The fructifications were borne on slender branches of about 10 cm length. These branches developed on the stem among the leaves. The slender stalk had many stiff but tapering bracts. A short bud-like strobilus was present within the axil of each bract. The bracts were probably spirally arranged. Each strobilus attained a length of about 1 cm. Both male and female reproductive organs are known as Cordaianthus, but according to Fry (1955) Cordaianthus is the new name of the reproductive organs or cones of Cordaitaceae. 1. Male Strobilus: It consisted of a thick central axis possessing many spirally arranged bracts and some microsporophyll’s. At the tip of each microsporophyll were present 1-4 microsporangia (Fig. 9.9). These sporangia probably dehisced longitudinally.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 34

Three well-studied forms of male strobilus include Cordaianthus concinnus, C. penjonii (Fig. 9.9) and C. saportanus.

The microsporangium wall was probably only one-celled thick and enclosed many microspores. Taylor and Taylor (1987) studied the structure of pollen grains of Cordaitales. According to them the grains may be alete or range from monolete to trilete. They are mono-saccate with saccus attached on both distal and proximal poles. Different interpretations of male reproductive organs of Cordaites have been given by- Renault (1889), Florin (1951) and Taylor (1973). 2. Female Strobilus: Similar to male strobilus, the female strobilus also had a stout axis bearing a large number of spirally arranged bracts. The bracts were more in number than that of male strobilus. Cordaianthus pseudofluitans possessed a few elongated and dichotomously branched fertile megasporophylls (Fig. 9.10). Two or more ovules were present at the apex of each megasporophyll.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 35

In Cordaianthus williamsonii, a single ovule was present on each fertile appendage (Fig. 9.11). The ovule was bitegmic and the integuments were free in the lower part but fused above. The nucellus of the ovule was free from the integument throughout. A prominent beak with a large pollen chamber was also present. Cordaianthus zielleri differed from C. pseudofluitans in the size of megasporophylls, number of functioning megasporophylls, dichotomies and total number of ovules in each strobilus.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 36

3. Male Gametophyte: Much is not known about the male gametophyte of cordaitalean members. Only the upper part of the nucellus provides some picture of the male gametophyte (Fig. 9.12). The inner structure was multicellular and the mature pollen grains were present in the micropylar canal. It could, however, not be ascertained that whether the cells of the multicellular region were vegetative or spermatogenous in nature. Most probably, both vegetative and spermatogenous tissues were present in the pollen grains.

4. Female Gametophyte and Embryo: Not much is known about the structure of female gametophyte and embryo. Andrews and Flix (1952), however, observed some seeds with well-preserved female gametophyte in Cardio carpus. In a few specimens, they also observed archegonia. The elongated gametophyte in such specimens had only two archegonia, each of which exhibits a beaklike projection of the endosperm. Darrah (1938) reported the embryo of Cordaitales from the coal balls of Iowa, USA, and if his observations were actually cordaitalean then it is perhaps the first Palaeozoic embryo so far recognised. In many seeds, well-preserved gametophytes with megaspore wall, a tent pole and two archegonia near micropyle have been observed by many workers. Even

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 37

the starch grains have been reported in the ovule of Cardio carpus spinalus by Baxter (1964). 5. Seed: It is believed that Cordaianthus type of strobili possessed seeds known as Cardio carpus (=Cordaicarpus=Samaropsis). Mitrospermum and Kamarospermum are the other two seed-genera of Cordaitaceae. Seeds were heart-shaped, bilaterally symmetrical and crassinucellate. Central nucellus was surrounded by a two-layered envelope, of which the outer layer was probably expanded in the form of a wing. In Cardiocarpus spinatus the seeds were large and surrounded by five distinct layers, including two layers each of sarcotesta and sclerotesta and a layer of endotesta. 2. Poroxylaceae: Poroxylaceae is a unigeneric family represented by only Poroxylon, a genus of Permo-Carboniferous age. It had a combination of Pteridospermean and Cordaitalean characters. The stem structure of Poroxylon resembled with Lyginopteris of Pteridospermales. The secondary- wood in the stem of Poroxylon was ill-developed and its medullary rays were as much as three cells wide. Over a dozen primary strands were present in close contact with the secondary wood. A large pith surrounded the ring of exarch primary strands. The seeds, termed as Rhabdospermum, were similar to Cardio carpus of Cordaitaceae. Affinities of Cordaitales: Although Cordaitalean members exhibit several resemblances with and differences from the Pteridospermales, Cycadales, , Coniferales and Ephedrales, yet they show several unique features which establish them as an independent group. Some of these unique characten sties include their: (i) Arborescent habit, (ii) Scanty primary wood, (iii) Pycnoxylic secondary wood,

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 38

(iv) Multi-senate pittings on the tracheid-walls, (v) Absence of resin canals, (vi) Simple but very long (up to 1 metre) leaves with parallel venation, (vii) Compound unisexual cones, (viii) Fertile female appendages with one to four ovules, and (ix) Bilateral seeds. Their possible relation with some other groups is discussed below: Relation with Pteridospermales (=Cycadofilicales): Major similarities between Cordaitales and Ptendospermales include the presence of: (i) A large pith in their stems, (ii) Multiseriate pittings on the walls of their tracheids, (iii) Centripetally developed xylem in their primary wood, (iv) Double leaf traces, (v) Almost similar general structure and vascularization of their ovules, and (vi) General absence of embryos in their seeds. However, the differences appear more marked than resemblances when Cordaitales are compared with Pteridosperms: (i) A seed-bearing inflorescence, as reported in Cordaitales, is practically unknown in Ptendospermales, (ii) There is very little in common between the Cordaitalean leaf and pteridospermic frond, (iii) Cordaitales generally possessed pycnoxylic wood while the secondary wood in Ptendospermales was manoxylic. Relation with Cycadales: Resemblances between Cordaitales and Cycadales include the presence of: (1) The xerophytic nature of their plants, (ii) Large pith in their stems, (iii) Centripetal wood in their stems,

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 39

(iv) Large amount of sclerenchyma in their leaves, (v) Motile nature of their spermatozoids, (vi) Comparatively simple and fairly large seeds, (vii) Almost similar vasculature of their ovules, and (viii) Three distinct layers in the single integument surrounding the ovule. Cordaitales differ from Cycadales in possessing: (i) Simple leaves, (ii) Parallel venation, (iii) Straight leaf trace, (iv) Pycnoxylic wood, and (v) Compound nature of their strobili. In Cycadales, however, the leaves are pinnately compound, venation is not parallel, leaf trace is not straight, the wood is manoxylic, and the strobili are not compound. Relation with Ginkgoales: Some common characters found in both Cordaitales and Ginkgoales include the presence of: (i) Double leaf-trace, (ii) Motile sperms in Ginkgoales and the probable motility of the Cordaitalean sperms, (iii) Anatomy of leaves, and (iv) Endospermic beak in their ovules. Origin of leaf traces is, however, different in members of both the groups. In Ginkgoales, the double leaf traces originate from two separate protoxylem groups while in Cordaitales they originate from the same group of the protoxylem. Relation with Coniferales: (i) Arborescent habit of both Cordaitales and Coniferales indicates a close affinity between the two groups,

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 40

(ii) Cordaitalean leaves resemble very closely with those of Agathis and Podocarpus of Coniferales. Some of the other points of resemblances between these two groups include their (iii) Simple leaves, (iv) Parallel venation, (v) Sclerenchymatous hypodermis in the leaves, (vi) Pycnoxylic wood, (vii) Bilaterally symmetrical ovules, and (viii) Similarity between the compound strobili of Cordaitales and cones of several members of Pinaceae (e.g. Abies). Connecting links between Cordaitales and Pinaceae may be traced in some fossil coniferous genera such as Lebachia arid Ernestiodendron of Lebachiaceae and Pseudovoltzia of Voltziaceae. However, some remarkable features of cordaitalean wood include the: (i) Absence of resin canals, and (ii) Presence of multiseriate bordered pits on the walls of the tracheids. These characteristics suggest that Cordaitales have been more nearer to the than to the Pinaceae. Some workers of the phylogeny of this group have suggested that araucarians are the derivatives of Cordaitales. Relation with Ephedrales: The points of similarity between Cordaitales and Ephedrales (e.g. Ephedra) include the presence of: (i) Parallel-veined leaves, (ii) Two leaf traces, (iii) Haplocheilic stomatal apparatus, (iv) Dense pycnoxylic wood, (v) Long and slender tracheids with tapering ends, (vi) Flattened microsporophyll’s in Cordaianthus and some species of Ephedra, (vii) Terminal sporangia, (viii) Sporangia provided with main vascular bundle,

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 41

(ix) One to six microsporangia in each microsporophyll, (x) Reducing number of ovules from many to two to one, and (xi) Quite reduced and shortened megasporophylls. On the basis of the above similarities, Cordaitales appear to be more closely related to Ephedrales than to other gymnospermous groups.

Acknowledgement www.google.com All the above study materials were downloaded from Google search engine and edited by me.

Dr.P.PRABAKARAN Assist .Prof.Botany MRGAC Page 42