The Cycadofilicales, They Formed the Dominant Fossil Plants During Palaeozoic Age
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UNIT-2 Cycadeoideales Introduction to Cycadeoideales: The Cycadofilicales, they formed the dominant fossil plants during Palaeozoic age. The Cycadofilicales have of course definite affinities with the cycads on one side and ferns on the other, but they had no cones either in the male or in the female part of the plants, so some workers think that the Cycadofilicales form a separate group quite distinct from gymnosperms. In the Mesozoic times, however, we came across fossils plants which had cones and were definitely related to gymnosperms. So in Mesozoic the Cycadofilicales were replaced by true gymnosperms which formed strobili, and the seeds had a naked dicotyledonous embryo in them. The ovule or the seed was never enclosed in closed carpel. The Mesozoic gymnosperms can be placed into two separate groups: 1. Cycadeoidale (Bennettitales) and 2. Cycadales. Pant (1957) has placed the cycadeiods in a distinct class, the Cycadeoideopsida of the division Cycadophyta. The Cycadeoideales (Bennettitales) first appeared in the Permian they reached their highest range during the Jurassic period, after which they disappeared altogether. The second group Cycadales had a world-wide distribution during the Mesozoic period Majority of them had altogether disappeared; only a few types have been left which are confined to special parts of the East. The present day cycads are only the remnants of very large dyeing out group, i.e., they are sometimes described as living fossils, because they are on their way to extinction. The Cycadeoideales (Bennettitales) were very much like the cycads in their general appearance, and as the Mesozoic had these two prominent groups of gymnosperms, so that period sometimes described as age of cycads. These Cycadeoideales are closely related to the Cycadofilicales on one side and to cycads on the other but they have their own characteristic features which distinguish them from all other gymnosperms except the Gnetales. The important feature which separates the Cycadeoideales from other gymnosperms is the presence of bisporangiate strobili. BSC I YEAR BOTANY-II Page 1 UNIT-2 The plants of this group were diversified in their habit. Some types had short columnar stems like most of the living cycads. The short columnar stem was usually un-branched and at the apex of the plant there was a terminal crown of leaves which in most cases pinnate. Some other forms had branched stems with multiple crown. In present day cycads we know that young leaves and megasporophylls are covered up by unicellular hairy outgrowths known as ramenta. In Cycadeoideales (Bennettitales) these ramenta were not unicellular; they were scale like, flattened and were several cells in breadth. Like cycads the plants had well organized strobili or cones, but in cycads they are monosporangiate whereas in Cycadeoideales they were usually bisporangiate and they were either terminal or axiarlly in position. Majority of Cycadeoideales (Bennettitales) seem to have flowered only once in their life and after flowering the plant died out as we find in some of present day angiosperms. Classification of Cycadeoideales: According to Sporne (1965), the order Cycadeoideales (Bennettitales) has been divided into three families. They are: 1. Cycadeoideaceae. 2.Williamsoniaceae, and 3. Wielandiellaceae. Here Cycadeoidea (Bennettites) of Cycadeoideaceae and Williamsonia of Williamsoinaceae have been discussed in detail. Systematic Position of Cycadeoideales: Gymnosperms Class. Cycadopsida Order. Cycadeoideales Family. Cycadeoideaceae BSC I YEAR BOTANY-II Page 2 UNIT-2 Genus. Cycadeoidea Bennettites, by American workers have been described as Cycadeoidea. Features of Cycadeoideales: (A) Morphological features: In Cycadeoidea the stem was un-branched with a single crown of pinnate leaves at the tops, but some species had branched stem with a multiple crown. In some the stem was tuberous. In all cases the stem was covered up by persistent leaf bases as we find in Cycas. BSC I YEAR BOTANY-II Page 3 UNIT-2 BSC I YEAR BOTANY-II Page 4 UNIT-2 Bennettitales: Meaning of Bennettitales: 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. 2. Origin of Bennettitales: 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 BSC I YEAR BOTANY-II Page 5 UNIT-2 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. 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. BSC I YEAR BOTANY-II Page 6 UNIT-2 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. Gnetales Taxonomic Arrangements of Gnetales: Foster and Gifford (1959) described Gnetales as “a small group of gymnosperm-like plants” while Maheshwari and Vasil (1961) Mentioned that ” the order Gnetales, formerly included three genera, Ephedra, Welwitschia and Gnetum which were considered to be highest evolved among the gymnosperms and believed to show an approach to the angiosperms”. But mainly due to the presence of naked ovules and also because of the absence of true sty le and stigma in Ephedra, Welwitschia and Gnetum, these members can only be treated under gymnosperms and not under angiosperms. General Characteristics of Gnetales: Gnetales, believed by some botanists to be the ancestors of flowering plants or angiosperms, are the highly evolved members of gymnosperms and show following characteristics: 1. These are woody plants, of which some species are trees (Gnetum gnemon), many are lianes or shrubs and a few. are stumpy turnip-like (e.g. Welwitschia mirabilis). 2. Leaves are simple elliptical or strap-shaped or sometimes reduced to minute scales. They are generally opposite or whorled. BSC I YEAR BOTANY-II Page 7 UNIT-2 3. Vessels are present in the secondary wood. 4. „Flowers‟ are unisexual, usually dioecious and only rarely monoecious as in some species of Gnetum. 5. „Flowers‟ are arranged in compound strobili or „inflorescences‟. 6. The male flowers are surrounded by a perianth. Each male flower contains an antherophore with one to eight synangia. 7. A single erect orthotropous ovule is present in each female flower. 8. Nucellus of the ovule remains surrounded by two or three envelopes. The micropyle of each ovule remains projected in the form of a long bristle- like tube. 10. At the time of fertilization the pollen tube contains two male nuclei. 11. A unicellular primary suspensor is present in the embryo. 12. Two cotyledons are present in the embryo. Ginkgoales Meaning of Ginkgoales: