Pteridophytes, Gymnosperms and Palaeobotany MSCBOT-503 PDF 2021

MSCBOT-503 M.Sc. I Semester PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY DEPARTMENT OF BOTANY SCHOOL OF SCIENCES UTTARAKHAND OPEN UNIVERSITY PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503...

MSCBOT-503 M.Sc. I Semester PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY DEPARTMENT OF BOTANY SCHOOL OF SCIENCES UTTARAKHAND OPEN UNIVERSITY PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 MSCBOT-503 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY DEPARTMENT OF BOTANY SCHOOL OF SCIENCES UTTARAKHAND OPEN UNIVERSITY Phone No. 05946-261122, 261123 Toll free No. 18001804025 Fax No. 05946-264232, E. mail [email protected] htpp://uou.ac.in UTTARAKHAND OPEN UNIVERSITY Page 1 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Expert Committee Prof. J. C. Ghildiyal Prof. G. S. Rajwar Retired Principal Principal Govt. PG College Government PG College Karnprayag Augustmuni Prof. Lalit Tewari Dr. Hemant Kandpal Department of Botany School of Health Sciences DSB Campus Uttarakhand Open University, Kumaun University, Nainital Haldwani Dr. Pooja Juyal Department of Botany School of Sciences Uttarakhand Open University, Haldwani Board of Studies Prof. C.M Sharma Prof. Y.S Rawat Department of Botany Department of Botany HNB Garhwal (Central) University, DSB Campus, Kumoun University Srinagar Nainital Prof. R.C.Dubey Prof. P.D Pant Head, Deptt of Botany & Microbiology Director, School of Sciences Gurukul Kangri University, Uttarakhand Open University, Haridwar Haldwani Dr. Pooja Juyal Department of Botany School of Sciences Uttarakhand Open University, Haldwani Programme Coordinator Dr. S.N. Ojha Assistant Professor Department of Botany, School of Sciences Uttarakhand Open University, Haldwani, Nainital UTTARAKHAND OPEN UNIVERSITY Page 2 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Unit Written By: Unit No. 1. Dr. Deewakar Bebni 1&2 Assistant Professor Department of Botany Government PG College, Jaiharikhal 2. Dr. Kiran Bargali 3, 4 & 5 Associate Professor, Department of Botany DSB Campus, Kumaun University, Nainital 3. Urmila Rana 6 Asst Prof., Department of Botany Government PG College New Tehri 4. Dr. Nishesh Sharma 7, 8 & 9 Department of Biotechnology Uttaranchal College of Applied and Life Science Uttaranchal University, Dehradun 5. Dr. Pooja Juyal 10 Department of Botany School of Sciences Uttarakhand Open University Haldwani, Nainital 6. Dr. Dheeraj Gahtori 11 Asstant Professor Department of Botany Government Post Graduate College Champawat Cheif Course Editor Prof. N.S. Bisht Head, Department of Botany, HNB Garhwal University, Pauri Campus, Pauri (Garhwal) Editorial Board Dr. Kirtika Padalia Dr. S.N. Ojha Assistant Professor (AC) Assistant Professor Department of Botany Department of Botany School of Sciences, School of Sciences Uttarakhand Open University, Haldwani Uttarakhand Open University, Haldwani UTTARAKHAND OPEN UNIVERSITY Page 3 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Dr. Pooja Juyal Dr. Prabha Dhondiyal Assistant Professor (AC) Assistant Professor (AC) Department of Botany Department of Botany School of Sciences School of Sciences Uttarakhand Open University, Haldwani, Uttarakhand Open University, Haldwani Dr. Pushpesh Joshi Assistant Professor (AC) Department of Botany School of Sciences Uttarakhand Open University, Haldwani Title : Pteridophytes, Gymnosperms and Palaeobotany ISBN No. : Copyright : Uttarakhand Open University Edition : 2021 Published By: Uttarakhand Open University, Haldwani, Nainital-263139 UTTARAKHAND OPEN UNIVERSITY Page 4 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 CONTENTS BLOCK-1 PTERIDOPHYTES PAGE NO. Unit-1-General characteristics, Salient features, Reproduction and Classification 7-28 Unit-2- Stele, Sori evolution, Heterospory and Seed habit, Fossil Pteridophyta 27-50 Unit-3-Psilophytopsida, Psilopsida, Lycopsida 51-94 Unit-4-Sphenopsida and Pteropsida 95-122 Unit-5-Eusporangiate and Leptosporangiate 123-182 BLOCK-2 GYMNOSPERM PAGE NO. Unit-6-Habitat, Reproduction, Classification, Distribution, Evolutionary trend and Economic importance 184-214 Unit-7-Brief account of the families of the Pteridospermales and Pentoxylales 215-242 Unit-8-General account of Cycadeoidales, Cordaitales, Cycadales, Ginkgoales and Benittitales 243-275 Unit-9-General account of Coniferales, Ephedrales, Taxales, Welwitschiales and Gnetales 278-310 BLOCK-3 PALAEOBOTANY PAGE NO. Unit-10-Fossils: Types, Formation and their Preservation 312-324 Unit-11-Methods of study of fossils, Geological time scale and Palaeobotanical Research 325-345 UTTARAKHAND OPEN UNIVERSITY Page 5 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 BLOCK-1- PTERIDOPHYTES UTTARAKHAND OPEN UNIVERSITY Page 6 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 UNIT-1-GENERAL CHARACTERISTICS, HABITAT, CLASSIFICATION, REPRODUCTION AND ECONOMIC IMPORTANCE OF PTERIDOPHYTES 1.1 Objectives 1.2 Introduction 1.3 General Characters of Pteridophytes 1.4 Habitat 1.5 Classification 1.6 Reproduction 1.7 Economic Importance 1.8 Summary 1.9 Glossary 1.10 Self Assessment Questions 1.11 References 1.12 Suggested Readings 1.13 Terminal Questions UTTARAKHAND OPEN UNIVERSITY Page 7 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 1.1 OBJECTIVES After reading this unit students will be able to- Explain and define the meaning of Pteridophyte. Describe the characteristic features of Pteridophytes. Distinguish and identify the Peridophytes in your surroundings. Classify the Pteridophytes. Know the distribution and economic importance of Pteridophytes. 1.2 INTRODUCTION The word Pteridophyta is of Greek origin. Pteron means “feather” and Phyton means plant. The plants of this group have feather like fronds (leaves) (Fig. 1.1). The group pteridophyta included in Cryptogams with Thallophyta (algae & fungi) and Bryophytes. The algae, fungi and bryophytes are called lower cryptogames (non- vascular cryptogams) while the Pteidophytesare called higher cryptogams (vascular cryptogames), because only pteridophytes have well developed conducting system among cryptogams. Due to this reason they are the first true land plants. All cryptogams reproduce by means of spores and do not produce seeds. The Peridophytes are assemblage of flowerless, seedless, spore bearing vascular plants that have successfully invaded the land. Fig.1.1: Feather like fronds of fern Pteridophytes have a long fossil history on our planet. They are known from as far back as 380 million years. Fossils of pteridophytes have been obtained from rock strata belonging to Silurian and Devonian periods of the Palaeozoic era. So the Palaeozoic era sometimes also called the “The age of pteridophyta”. The fossil Pteridophytes were herbaceous as well as arborescent. The tree ferns, giant horse tails and arborescent lycopods dominated the swampy landscapes of the ancient age. The present day lycopods are the mere relicts the Lepidodendron like fossil arborescent lycopods. Only present day ferns have nearby stature UTTARAKHAND OPEN UNIVERSITY Page 8 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 of their ancestors. Psilotum and Tmesipteris are two surviving remains of psilopsids, conserve the primitive features of the first land plants. In the plant kingdom, pteridophytes occupy a position in between bryophytes and gymnosperms, and therefore they have some similarities with the bryophytes on the one hand and with the gymnosperms on the other hand. The similarities with bryophytes are (i) presence of sterile jacket around the antheridium and archegonium, (ii) requirement of water and moisture for the fertilization, (iii) presence of alternation of generations, (iv) formation of spores etc. while with gymnosperms (i) sporophytic plant body and it’s independent nature, (ii) differentiation of sporophyte into root, shoot and leaves, (iii) presence of vascular tissues for conduction etc. The presence of vascular elements in pteridophytes makes their grouping with gymnosperms and Angiosperms as Trachaeophyta. The reproduction by spores and similar events of life cycle place them among lower plants. The lower plants algae, fungi, bryophytes and pteridophytes were earlier grouped together as cryptogams. Bryophytes, Pteridophytes and Gymnosperms are also classified as Archegoniatae due to the presence of a common reproductive body archegonium. 1.3 GENERAL CHARACTERITICS OF PTERIDOPHYTES 1. The main independent plant body is sporophyte with vascular system. 2. The pteridophytes grow mostly in cool, moist and shady places, but some are aquatic (Marsilea, Salvinia, Azolla etc.) and few are xerophytic (Selaginella rupestris, S. respanda, Marselia rajasthanensis, Marselia condenseta etc.). 3. Plants are differentiated into true roots, shoots and leaves. Some primitive members lack true roots and well developed leaves (e.g.; in members of order Psilophytales and Psilotales). 4. Except few woody tree ferns all living pteritophytes are herbaceous. 5. They may be dorsiventral or radial in symmetry with branched stems. 6. The leaves of ptridophyte may be Scale like leaf (e.g. Equisetum), small sessile leaves (e.g., Lycopodium and Selaginella) and large, petiolate compound leaves occurs in true ferns. 7. The stem bears leaves which may be microphyllous type in which the leaves are quite small with unbranched midrib (e.g. Lycopodium, Selaginella, Equisetum), or megaphyllous type, in which the leaves are large with branched midrib (e.g. ferns). 8. In fern, the young leaves show circinate vernation (curved inwards). 9. Primary embryonic roots are short lived and replaced by adventitious roots. 10. The pteridophytes reproduce by haploid spores which are produced within a specialized structure called sporangium. 11. Plants may be homosporous (all spores are same in shape and size) and heterosporous (spores are of two different shape and sizes, smaller one called microspore and larger one megaspore). UTTARAKHAND OPEN UNIVERSITY Page 9 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 12. In some pteridophytes the sporangia developed on stems in the axil between leaf and stem, or on leaves (mostly ventral surface of leaves). On the stem sporangia may be terminal e.g. Rhynia, lateral in Lycopodium, on the surface of leaves in Ferns. The sporangia borne on ventral side of specialized leaf and such leaf is called Sporophyll. In aquatic ferns micro and megasporangia together are covered by a common membrane and this bean shaped structure is called sporocarp. 13. In true ferns the sporangia are located on the lower surface of the leaf as clusters called sori (sorus). 14. The haploid spore is a unit of gametophyte. On germination it develops into gametophytic prothallus. 15. The Gametophytic plant is called prothallus since it more or less looks like the thallus of a primitive bryophytes. 16. Gametophyte bears sex organs archegonia and antheridia. As a result of fertilization the zygote or oospore is formed. 17. The homosporous plants are monoecious (antheridia and archegonia borne on same thallus). 18. Heterosporous plants are mostly dioecious (antheridia and archegonia borne on separate thalli). 19. Microspore gives rise to male prothallus which bears the male sex organs antheridia. 20. Megaspore gives rise to female prothallus which bears the female sex organs archegonia. 21. The sex organs are embedded or projected in the prothallus. 22. The male gametes are called antherozoids and produced inside the antherdium. 23. Antherozoids are unicellular, spirally coiled and flagellate. 24. The archegonia are flask shaped and differentiated into upper neck and lower broader venter. 25. The achegonial neck is projected and the venter is embedded.in the prothallus. 26. Water (moisture) is essential for completion of fertilization. 27. The egg and antherozoids fuse to form diploid zygote. The Zygote develops into new sporophytic plant body. 28. Clear alternation of generation takes place in the life cycle of Pteridophytes which is always heteromorphic type. 1.4 HABITAT Pteridophytes are first land vascular plants, so they are mostly terrestrial in nature, grow in cool, moist and shady places. Some pteridophytes grow in xerophytic, semi-aquatic or aquatic condition also (Fig. 1.2). (a) Terrestrial Pteridophyte: Members of Pteridophyta or ferns grow in terrestrial habitat. Some pteridopytes are Lithophytic on horizontal rocky patches. The fossil pteridophytes were terrestrial in nature. Most species of Lycopods growing in such habitat are Lycopodium clavatum, L.cernuum, L. reflexum, Selaginella chrysocoulus, S. kraussiana, I. coramandelina, etc. UTTARAKHAND OPEN UNIVERSITY Page 10 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Some pteridopytes are epiphytic. Psilotum nudum, L. phlegmaria, S. oragana and few ferns grow as epiphytes. The tall and well stratified trees in the forests provide a suitable habitat for the growth of epiphytic Pteridophytes. However, a few other ferns prefer open tree trunks and branches. These epiphytes share a common niche along with orchids and ferns. (b) Aquatic pteridophyte: Some pteridophytes grow in aquatic and semi-aquatic habitats. Isoetes panchananii and I. englemanni are semi-aquatic. Some members of ferns are commonly called water ferns. The examples of water ferns are Marsilea, Salvinia, Azolla, Regnellidium etc. (c) Xerophytic pteridophytes: Some species of Selaginella and Marsilea are xerophytic in nature. The examples are S. repanda, S. lepidophylla, M. rajasthanensis, M. condensata. Fig. 1.2 Different types of pteridophytes: A. Azolla, B. Selaginella, C. Marsilea, D. Lycopodium, E. Equisetum, F. Adiantum 1.5 CLASSIFICATION On the basis of presence and absence of seeds the vascular plants were classified by earlier taxonomists into two divisions, Pteridophyta and Spermatophyta. The division Pteridophyta included primitive vascular plants which bear no seeds. Later some fern like seed bearing fossil plants (Cycadofilicales) were discovered in 1903. The discovery eliminated the distinction between the two divisions Pteridophyta and Spematophyta. UTTARAKHAND OPEN UNIVERSITY Page 11 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Sinnott (1935) therefore introduced a new term “Tracheophyta” for a division which includes all the vascular plants. Eames (1936) on the basis of some characters of plants and position of sporangia the division Tracheophyta divided into four groups, Psilopsida, Lycopsida, Sphenopsida and Pteropsida. Zimmermann (1930) and Arnold (1947) considered these groups as divisions and Tippo (1942) considered as subphyla. 1.5.1- Classification proposed by Reimers (1954) and Followed by Sporne (1996) The classification of pteridophytes proposed by Reimers in the 1954 edition of Engler’s Syllabus der pflanzen families. Pteridophytes: UTTARAKHAND OPEN UNIVERSITY Page 12 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 1.5.2-Classification proposed by Wardlow (1955) According to the International Code of Botanical Nomenclature as amended in 1950, the name of the division should end with the suffix-phyta, and a sub-division should end with- phytina and a class with -opsida. According to recommendations of I.C.B.N, Wardlaw (1955) divided the Pteridophytes into four divisions: On the basis of ICBN recommendations Smith (1955) divided vascular cryptogams into following four divisions: 1- Division – Psilophyta which was divided into two classes, Psilophytopsida and Psilotopsida. 2- Division – Lycophyta which was divided into two classes, Eligulopsida and Ligulopsida on the basis of absence and presence of ligule at the axil of leaf. 3- Division – Sphenophyta or Calamophyta which was divided into two classes, Sphenophyllopsida and Calamopsida. 4- Division – Pterophyta or Filicophyta which was divided into four classes, Eusporangiopsida, Protoleptosporangiopsida, Leptosporangiopsida and Primopteropsida. 1.5.3-Classification proposed by Cronquist et al. (1966) and followed by Parihar (1977) Cronquist, Takhtajan and Zimmerman (1966) classified the pteridophytes into five divisions. The classification has also been followed by Parihar (1977). The outline of classification is following: 1. Division :- Rhyniophyta Class:- Rhyniatae Order:-Rhyniales 2. Division :- Psilotophyta Class:- Psilotatae Order:-Psilotales 3. Division :- Lycopodiophyta UTTARAKHAND OPEN UNIVERSITY Page 13 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Class:- Lycopodiatae Orders:-Asteroxylales, Drepenophycales, Protolepidodendrales and Lycopodiales. 4. Division :- Equisetophyta Order:-Sphenophyllales and Pseudoborniales Class: - Equisetatae Order: - Calamitales and Equisetales 5. Division :-Polypodiophyta Class:-Polypodiatae (the class divides sub classes.) Sub-classes:- Prototeridiidae, Archaeteridiidae, Ophioglossiidae, Noeggerothiidae, Marrattidae, Polypodiidae, Marsilleidae, Salviniidae) These sub-classes are further divided into orders. Accordingly ICBN amendment the four major groups of pteridophyta are 1. Class Psilopsida 2. Class Lycopsida 3. Class Sphenopsida 4. Class Pteropsida However there are no uniformity in nomenclature and accordingly. Psilopsida is equivalent to: Psilophyta (Zimmermann, 1930; Smith, 1955; Bold, 1957) Lycopsida is equivalent to: Lycophyta (Zimmermann, 1930) Lycopodophyta (Andrew, 1961) Microphylophyta (Bold,1957) Lepidophyta (Smith, 1955; Cronquist, 1960) Sphenopsida is equivalent to: Calamophyta (Smith, 1975) Sphenophyta (Benson, 1957) Equisetophyta or Arthrophyta (Bold, 1957; Andrew, 1961) Pteropsida is equivalent to: Pteridophyta (Benson, 1957) Pterophyta (Smith, 1955; Bold, 1957) Filicophyata (Cronquist, 1960) 1.5.4-Latest classification proposed by A. R. Smith (2006) and co-workers Scientists of three different countries from USA, A.R. Smith, K.M. Preyer and P.G. Wolf (Sweden), E. Schuettpelz and H Schneider (Germany) presented a revised classification of extant ferns. They divided all vascular plants into two groups on the basis of phylogenetic studies. Recent phylogenetic studies have revealed a basal dichotomy within vascular plants, separating the lycophytes (less than 1% of extant vascular plants) from the euphyllophytes. UTTARAKHAND OPEN UNIVERSITY Page 14 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Living euphyllophytes, in turn, comprise two major clades: the spermatophytes (seed plants), which are in excess of 260,000 species (Thorne, 2002; Scotland & Wortley, 2003), and the monilophytes (ferns, sensu Pryer& al., 2004b), with about 9,000 species, including horsetails, whisk ferns, and all eusporangiate and leptosporangiate ferns. Plants that are included in the lycophyte and fern clades (Monilophytes) are all spore-bearing or “seed-free”, and because of this common feature their members have been lumped together historically under various terms, such as “pteridophytes” and “ferns and fern allies”—paraphyletic assemblages of plants. The focus of this reclassification is exclusively on ferns. Within ferns, they recognized four classes (Psilotopsida; Equisetopsida; Marattiopsida; Polypodiopsida), 11 orders, and 37 families. Class 1. Psilotopsida A. ORDER Ophioglossales. 1. Family Ophioglossaceae. B. ORDER Psilotales. 2. Family Psilotaceae Class 2. Equisetopsida C. ORDER Equisetales. 3. Family Equisetaceae. Class 3. Marattiopsida D. ORDER Marattales. 4. Family Marattiaceae. Class 4. Polypodiopsida E. ORDER Osmundales. 5. Family Osmundaceae. F. ORDER Hymenophyllales. 6. Family Hymenophyllaceae G. ORDER Gleicheniale. 7. Family Gleicheniaceae. 8. Family Dipteridaceae 9. Family Matoniaceae. H. ORDER Schizaeales. 10. Family Lygodiaceae. 11. Family Anemiaceae 12. Family Schizaeaceae. I. ORDER Salviniales 13. Family Marsileaceae. 14. Family Salviniaceae J. ORDER Cyatheales. 15. Family Thyrsopteridaceae. 16. Family Loxomataceae. 17. Family Culcitaceae 18. Family Plagiogyriaceae. 19. Family Cibotiaceae 20. Family Cyatheaceae 21. Family Dicksoniaceae 22. Family Metaxyaceae K. ORDER Polypodiales 23. Family Lindsaeaceae UTTARAKHAND OPEN UNIVERSITY Page 15 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 24. Family Saccolomataceae 25. Family Dennstaedtiaceae 26. Family Pteridaceae 27. Family Aspleniaceae 28. Family Thelypteridaceae 29. Family Woodsiaceae 30. Family Blechnaceae 31. Family Onocleaceae 32. Family Dryopteridaceae 33. Family Lomariopsidaceae 34. Family Tectariaceae 35. Family Oleandraceae 36. Family Davalliaceae 37. Family Polypodiaceae 1.6 REPRODUCTION Reproduction through spores is main mode of reproduction in Pteridophytes. Although vegetative reproduction is also common in pteridophytes. 1.6.1- Vegetative reproduction The sporophyte of many pteridophytes reproduce vegetatively by following means: (i) By the formation of gemmae or bulbils: Vegetative reproduction is carried out by bulbils (bulblets) or gemmae. These are leafy side branches with wide base. The gemmae fall on the ground and grow into a new young plant. Ex. Psilotum, Lycopodium phlegmaria, L. selago etc. Certain species of Selaginella also propagate by bulbils. (ii) Fragmentation: Death and decay of older parts of stem leads the formation of fragments of stem/rhizomes. The individual fragment possessing roots develops into a new plant. It is common method of vegetative reproduction in species of Lycopodium, Selaginella, Dryopteris, Pteris, Adiantum etc. (iii) Formation of Tubers: The tubers originate from the paranchymetous regions of shoot and root at the onset of unfavourable conditions. The tubers are formed at surface of the ground, called surface tubers and those developing underground are the underground tubers. They consist of a group of cells with stored food materials and having the capacity to germinate into new plants during favourable season. In some species of Marsilea irregular tuberous bodies are formed in the stem. Few species of Lycopodium, Selaginella and Equisetum develop such tubers. (iv) Formation of Adventitious Buds: Such buds have been induced on isolated bulbil leaves. Decapitation of stem near its apex also induces the formation of such buds. Certain species of Lycopodium, Selaginella, also develope such buds. Few species of Asplenium, UTTARAKHAND OPEN UNIVERSITY Page 16 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Diplazium and Ophioglossum develop adventitious buds. In Dryopteris adventitious buds arise in the axil of leaves and form new plants after getting detached from the plant.In some ferns the root apex develops directly into a leafy bud (Platycerium and Asplenium esculentum). The leafy bud can grow into a new plant. 1.6.2-Asexual Reproduction Reproduction through spores is main mode of reproduction in Pteridophytes. Pteridophytes reproduce asexually by haploid spores, which are formed in sporangia. The sporangia develope either on the ventral surface or in the axils of the leaves. The Sporangia bearing leaves are called sporophylls. However, in Psilophytales the sporangia are cauline. The sporangia are terminal on the fine aerial branches in Rhynia. In Equisetum and Selaginella these sporopylls present in the form of compact structures called strobili or cones (Fig.1.4 &1.5). In genera, such as, Azolla, Marsilea and Salvinia the sporangia are present in specialized bodies called Sporocarps. The sporangia in higher ferns are present in the form of well organized groups called sori (singular sorus) (Fig.1.6). In case of Psilotum sporangia are trilobed structures with each lobe containing sporogenous region and this trilobed structure is called synangium (Fig 1.3). On the basis of development of sporangia Goebel (1881) classified sporangial development into two types, i.e., Eusporangiate and Leptosporangiate. The sporangium developing from a group UTTARAKHAND OPEN UNIVERSITY Page 17 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 of initial cells is called eusporangiate while the development from a single initial cell is called leptospoangiate development. 1.6.2.1-Sporophyte The spore producing body of Pteridophyta is called Sporophyte. The Sporophytic generation is dominant and conspicuous in the life cycle of Pteridophytes. The life cycle of typical Pteridophyte consists of a regular alternation of sporophytic (asexual) and gametophytic (sexual) generations. In bryophytes the gametophytic phage is dominant in life cycle, and the sporophyte is dependent on gametophyte. By contrast, in Gymnosperms and Angiosperms the gametophytic generation is reduced and is dependent on the sporophyte. Pteridophytes with an intermediate position are characterized by free living gametophytic and sporophytic generations. Nevertheless the sporophyte is a dominant generation; it soon becomes independent of the gametophyte and attains a much greater size. Pteridophytes are characterized by two basic kinds of life-cycles, homosporous and heterosporous (Fig. 1.7). The heterosporous pteridophytes form two kinds of spores, the larger megaspores and smaller microspores, from which develop two kinds of gametophytes, female and male gametophytes, respectively. Fig.1.7 Life cycles of Homosporous and Heterosporous Pteridophytes The homosporous pteridophytes form only one kind of spore from which a hermaphroditic (monoecious) gametophyte usually develops. Thus the heterosporous pteridophytes are obligatorily heterothallic, while the homosporous are usually homothallic. Some examples of UTTARAKHAND OPEN UNIVERSITY Page 18 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 heterosporus pteridophytes are Selaginella, Isoetes, Masilea, Salvinia, Azolla, Regnellidium etc. The homosporous life-cycle is found in the Psilotum, Tmesipteris, Lycopodium, Equisetum, and the homosporous Filicopsids. 1.6.3- Sexual reproductive phage: Gametophyte The gametophyte is the sexual phase in the life cycle of a plant.The haploid spore is the first cell of gametophyte. The spores are haploid and form after reduction division in the sporogenous cells of the sporangium. The spore germinates into a porothallus (Fig.1.8, 1.9 & 1.10). Generally the prothalli are green, simple, somewhat branched and aerial structures. But in some genera such as Lycopodium, they are subterranean, branched, colourless and saprophytic structure. The two sex organs antheridia and archegonia develop on the prothallus (Fig. 1.11). Generally the prothalli of homosporous pteridophytes are monoecious. But the prothalli of heterosporous pteridophytes usually are dioecious. UTTARAKHAND OPEN UNIVERSITY Page 19 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Mostly the antheridia and archegonia remain embedded in the prothallus. The antheridium is always surrounded by a jacket layer. The antheridia produce antherozoids. The antherozoids are unicellular, uninucleate and biciliate structure in Lycopodium, Selaginella etc. but they are multiciliate in Psilotum, Tmesipteris, Isoetes, Equisetum and ferns. The archegonium consists of a projecting neck and the lower embedded portion venter. The neck has neck canal cells and the venter has ventral canal cell and egg cell. Fig.1.11: Antheridum and archegonium of fern 1.6.4 -Fertilization and zygote formation The fertilization takes place with the help of water. When a film of water flow between prothallus and substratum, the cap cells of antheridia pushed open to release antherozoids. The neck canal cell and ventral canal cell in the archegonium disorganize, their protoplasm become mucilaginous which absorbs water from the surrounding jacket cells and swell out, due to which pressure is put on the four cover cells at the top of archegonium. Cover cells get apart due to inner pressure and mucilaginous substance having malic acid get accumulated at the opening of archegonium. Antherozoids are attracted chemotactically towards the archegonium, the antherozoids swim towards archegonia in response to malic acid released from mucilaginous mass (chemotactic). Many antherozoids swim down the neck of archegonium, only one of them fuses with egg to form a diploid zygote. Usually, cross- fertilization occurs due to protandrous nature of prothalli (i.e. antheridia mature before archegonia). The fertilization takes place and results into the formation of diploid zygote. The zygote is the first cell of sporophytic generation and develops into a well-developed sporophyte. 1.6.5-Embryo development The zygote develops into an embryo. The first division of Zygote is generally (if not always) transverse. After a usual transverse division of zygote, a two celled structure is formed. Transverse division followed by a vertical division and thus developed a quadrant. The successive divisions finally develop a young sporophyte (Fig. 1.12). UTTARAKHAND OPEN UNIVERSITY Page 20 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Fig.1.12. (A-G) Successive stages of development of embryo in fern 1.6.6 – Alternation of Generation Pteridophytes show a true alternation of generations. Here, the dominant sporophyte produces spores through meiosis. The gametophytic generation forms gametes by mitosis. They have distinct sexual haploid and asexual diploid stages. In these groups, a multicellular gametophyte, which is haploid with n chromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes (Fig. 1.13). Fig.1.13 Alternation of generation in pteridophytes A mature sporophyte produces spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n. The haploid spores germinate and grow into a haploid gametophyte. At maturity, the gametophyte produces gametes by mitosis. Two gametes (originating from different organisms of the same species or from the same organism) fuse to UTTARAKHAND OPEN UNIVERSITY Page 21 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 produce a zygote, which develops into a diploid sporophyte. The sporophyte and gametophyte alternate one by one in life cycle. This is called alternation of generation. This alternation of generations is a survival strategy in which a plant alternates between different reproductive techniques. 1.6.7- Abnormalities in the Life cycle The normal life cycle of the vascular plant has two alternating generations. Both haploid and diploid generations alternate regularly in the life cycle of the Pteridophytes. The regular alternation of chromosome numbers is sometimes impaired by the occurrence of two common phenomena called apospory and apogamy. (a) Apogamy: Development of sporophyte directly from the gametophyte without producing gametes or any sexual fusion (syngamy). The sporophyte has same haploid chromosome numbers as in gametophyte. It was first discovered by Farlow (1874) in Pteris cretica. It is a common and widespread phenomenon in ferns. The natural apogamy reported in a number of ferns including Pteris, Pteridim, Dryopteris, Adiantum, Osmunda, Todea, Athyrium, Asplenium etc. (b) Apospory: The development of gametophytes from the vegetative parts or cells of the sporophyte without of any meiotic division and formation of spores. Such types of gametophytes are diploid and this phenomenon of their formation is called apospory. The phenomenon of apospory was first discovered by Druery (1884) in Athyrium filix-foemina. Since then apospory has been reported in many pteridophytes including Pteridium aquilinum, Asplenium dimorphum, Osmunda regalis, Todeaetc. 1.7 ECONOMIC IMPORTANCE OF PTERIDOPHYTES Besides being a lower plant, pteridophytes are economically very important. About 170 species of Pteridophytes have been found to be used as food, flavor, dyes, medicines, biofertilisers, oil, fibers and biogas production (Manickam and Irudayraj 1992). (1) Food: Like other plants, pteridophytes constitute a good source of food and fodder. Sporocarps of Marsilea, a water fern, yield starch that is cooked and eaten by certain tribal communities. Young circinately coiled leaf tips of Diplazium esculentum, Diplazium maximum and some other ferns are eaten as vegetable. Marselia is used as a substitute for clover to feed animals. In Canada the crozires of Matteuccia struphiopteris are served as common spring vegetables. They are also stored and frozen for later use. (2) Biological fertilizer: Azolla (a water fern) has a symbiotic association with nitrogen fixing cyanobacterium Anabaena azollae. Azolla is a very small pteridophyte and has small microscopic leaves. Each leaf has a small cavity at its base. Inside this cavity the filaments of nitrogen fixing blue green alga Anabaena azollae. Due to this, Azolla has the ability f to fix UTTARAKHAND OPEN UNIVERSITY Page 22 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 atmospheric nitrogen and thus increases the fertility of the soil. It is inoculated to paddy fields to function as biofertilizer. (3) Medicines: Many plants are used for treating several human diseases. An anthelmintic drug is obtained from rhizomes of Dryopteris (Male Shield Fern). Fronds and rhizome of Adiantum caudatum used for wound healing. The leaf and root decoction of commonly occurring Adiantum lunulatum syn. Adiantum philippense has been found to be very effective in the treatment of chest complaints. Leaves of Marsilea minuta and leaves of Pteris quadrifolia used for cough and bronchitis and fresh leaves of Ophioglossum reticulatum are used in menstrual disorders. Tender leaves of Tectaria cicularia used for wound healings, eczemza and scabies. (4) Ornamentals: Some species of Lycopdium and Selaginella are used as ornamentals in big gardens and green houses because of their variously coloured, feathery moss like leaves. Ferns are grown as ornamental plants for their delicate and graceful leaves. Some such examples are Adiantum, Marattia, Pteris, Salvinia, Osmunda regalis, Lycopodium obscurumect. (5) Soil Binding: By their growth pteridophytes bind the soil even along hill slopes. The soil is protected from erosion. (6) Scouring: Equisetum stems have been used in scouring (cleaning of utensils) and polishing of metals. Equisetum species are therefore, also called scouring rushes. (7) Ecological Indicators: Pteridophytes are also used as a indicator plants. Equisetum accumulates minerals, especially gold, in their stem, so it is the ecological indicator of gold in the soil. Similarly, Asplenium adulterinum is an indicator of nickel and Actinopteris australis is a cobalt indicator plant. 1.8 SUMMARY In this unit we have discussed meaning of pteridophytes, their general characters and classification. The division Pteridophyta includes primitive vascular plants. So also called vascular cryptogams (cryptogams with vascular system). We also learnt the economic importance of Pteridophytes. It can be summarized as follows: 1. The main plant body of pteridophytes is sporophyte (diploid). 2. The gametophytic and sporophytic generations are two independent plants in the life cycle. 3. The pteridophyte shows much variation in shape, size and habitat. 4. The pteridophytes grow mostly in cool, moist and shady places. 5. Some pteridopytes are aquatic (Marsilea, Salvinia, Azolla) and few are xerophytic (Selaginella rupestris, M. rajasthanensis,) as well. UTTARAKHAND OPEN UNIVERSITY Page 23 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 6. The sporophyte plant body differentiated into true roots, shoots and leaves. Some primitive members (members of Psilotopsida) lack true roots and well developed leaves. 7. The stem bears leaves which may be microphyllous or megaphyllous. 8. The sporophytes reproduce by haploid spores which are produced within sporangia. Plants may be homosporous or heterosporous. 9. The sporangia bearing leaf is called sporophyll. In Equisetum and Selaginella these sporophylls present in the form of compact structures called strobili or cones. In water ferns sporangia are present in specialized bodies called Sporocarps. The sporangia in some higher ferns are present in the form of sori. 10. The haploid spore germinates in gametophytic prothallus. Prothallus bears sex organs archegonia and antheridia. As a result of fertilization the zygote or oospore is formed. 11. The zygote is the first cell of sporophytic generation. 12. Water medium is essential for fertilization. 14. The Zygote develops into new sporophytic plant body. 15. The clear alternation of generation takes place in the life cycle of Pteridophytes. 16. The division Pteridophyta divided into four classes- Psilopsida, Lycopsida, Sphenopsida and Pteropsida. 17. Pteridophytes are economically very important. They are used as food, flavor, dyes, medicines, biofertilisers, oil, fibers and biogas production etc. 1.9 GLOSSARY Asexual: Having no sexual organs. Bio-fertilizer: the fertilizer obtained from living microbes or plants e.g., Azolla Cauline: Arising from upper part of the stem. Dioecious: Having male and female sexes on different prothalli. Embryo: A young organism in early stages of development. Fertilization: The union of male and female nuclei. Fossil: Petrified plant parts found in rocks. Gametes: Sexual cells. Gametophyte: The haploid phase meant for the production of gametes. Habitat: The locality or external environment in which a plant lives. Haploid: Having x number of chromosome (n). Heterosporus: Producing two kinds of spores, i.e. Microspores and Megaspores. Lithophyte: Plants that grow on rocks. Megasporophyll: A fertile leaf attached with megasporangium. Microsporophyll: A fertile leaf attached with microsporangium. Monoecious: Having male and female sexes on same prothallus. Pinnate: A compound leaf having leaf lets on each side of an axis or mid rib. Prothallus: Gametophytic plant body. Rachis: The axis bearing leaf-lets. Sporophyte: The diploid phase producing spores. UTTARAKHAND OPEN UNIVERSITY Page 24 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 1.10 SELF ASSESSMENT QUESTIONS 1.10.1 Short answer type questions: Q.1. What are vascular cryptogams? Q.2. Which is the most important character of pteridophytes? Q.3. Which generation is dominant in pteridophytes? Q.4. What is the example of aquatic pteridophyte? Q.5. What is the name of sporangium bearing leaf? Q.6. Which pteridophyte have sori? Q.7. Name some pteridophytes having sporocarps. Q.8. Which era is called the age of pteridophytes? Q.9. Which pteridphyte is used as biofertilizer? Q.10. Which pteridophyte has cone/strobillus? 1.10.2-Multiple choice questions: 1. Vascular cryptogams are: (a) Bryophytes (b) Pteridophytes (c) Algae (d) fungi 2. Bio-fertilizer obtained from: (a) Rhynia (b) Equisetum (c) Psilophyton (d) Azolla 3. Spores of pteridophytes are: (a) Haploid (b) Diploid (c) Triploid (d) Tetraploid 4. Sporophytic generation is dominant in- (a) Bryophytes (b) Pteridophytes (c) Algae (d) Fungi 5. Prothallus occurs in: (a) Pyrophyta (b) Pteridophyta (c) Gymnosperm (d) Angiosperm 1.10.3-Fill up the following blanks: 1. Pteridophyte is also known as ………………….. 2. The two categories in which the pteridophyte may be grouped on the basis of size of leaves are……..and……… 3. Many sporangia grouped together to form a …………in higher fern. 4. The aquatic pteridophyte which used for biofertilizer is……… 5. The main stage of Pteridophyta is………… 6. Lycopodium, Equisetum and Adiantum are ……….ferns. UTTARAKHAND OPEN UNIVERSITY Page 25 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 7. The production of two different sizes, structures and functions of spores by the same species of a plant is known as……… 8. The haploid spore is a unit of ………………. 9. The sporangia of Equisetum organized in the form of…… 10. The sporocarp is found in………. 1.10.1 Answer key: 1. Peridophytes; 2. Seedless and Vascular system; 3. Sporophytic; 4. Marsilea; 5. Sporophyll; 6. Rynia; 7. Prothallus; 8.Palaeozoic; 9. Azolla; 10.Equisetum. 1.10.2 Answer key: 1.(b); 2.(d); 3.(a); 4.(b); 5.(b). 1.10.3 Answer key: 1. Vascular cryptogams; 2. Megasporophylls and Microsporophylls; 3. Sori; 4. Biofertilizer; 5. Sporophytic; 6. Homosporous; 7. Heterospory; 8. Gametophyte; 9. Cone; 10. Water fern. 1.11 REFERENCES Abraham, A; Ninan, C.A. and Mathus, P A; 1962; J. Indian bot. soc. 41: 339(Cytology of pteridophyte). Beddome, R N; 1976; Handbook of the ferns of British India, Celon and Malaya Peninsula; Today and tomorrow Publishers, New Delhi. Chandra, S; 2000; the fern of India, International book Distributers, Dehradun. Gopal Krishnan; V and Ravi; S; 1990; Indian fern J. 7:94 (Lycopsida). Pandey, A K and Rout, S D; 2006; Ethnobot. 18:102 (Ethnobotany). Sharma, O P; 2012; Pteridophyta, McGrow Hill Education New Delhi. Vashihta, P C, Sinha, A K; Kumar, A; 2010; Pteridophyta, S. cand& company New Delhi. 1.12 SUGGESTED READINGS A Text Book of Botany: V. Singh, P.C. Pande and D.K. Jain (2008). Botany for Degree Students –Pteridophyta (vascular cryptogams) : P.C. Vashishta, A.K. Sinha and A. Kumar (2006). College Botany. Vol. 2: H.C. Ganguly and A.K. Kar (1999). Pteridophyta, Sharma, O P (2012). Pteridophyta: Rashid, A (1999). 1.13 TERMINAL QUESTIONS Q.1. Describe the characteristic features of pteridophytes. Q.2. Discuss the classification of pteridophyte. Q.3. Describe about habit and habitat of pteridophytes in detail. Q.4. Write an essay on Alternation of generation of pteridophytes. Q.5. Explain the economic importance of pteridophytes giving suitable examples. UTTARAKHAND OPEN UNIVERSITY Page 26 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 UNIT-2- STELE, SORI EVOLUTION, HETEROSPORY AND SEED HABIT 2.1 Objectives 2.2 Introduction 2.3 Stele 2.3.1 Types of Steles 2.4 Sorus (pl. Sori) 2.5 Heterospory and Seed habit 2.5.1 Origin of heterospory 2.5.2 Significance of Heterospory 2.6 General account of Fossil Pteridophytes 2.6.1 Fossil Psilopsids 2.6.2 Fossil Lycopsids 2.6.3 Fossil Sphenopsids 2.6.4 Fossil Pteropsids 2.7 Summary 2.8 Glossary 2.9 Self Assessment Question 2.10 References 2.11 Suggested Readings 2.12 Terminal Questions UTTARAKHAND OPEN UNIVERSITY Page 27 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 2.1 OBJECTIVES After reading this unit learners will be able to: Definition and types of Steles. What is sorus, structure and types of sori and their evolution? What is heterospory? Heterospory in pteridophyta. Heterospory and seed habit. General description of Fossil Pteridophytes. 2.2 INTRODUCTION The land plants have complex internal organization. The pteridophytes are first land plants on the earth, who have well developed vascular system. They have central vascular cylinder. The central vascular cylinder of plants is called stele. Pteridophytes resemble higher land plants (Gymnosperms and Angiosperms) in having complex internal organization with vascular elements but differ from them lacking seed habit. Some of the pteridophytes are heterosporous, they approach closely to seed habit. The phenomenon of development of two types of spores (differing in size, structure and function) by the same species is known as heterospory. Of the two different sizes the smaller (microspores) are produced in large number and larger (megaspores) are produced in comparatively much smaller number. Heterospory has been a well debated and intereit is the pre-requisite for the origin of seed. In homosporous genera the sex determination is observed in the gametophytic stage but in heterosporous genera it is observed in sporophytic stage. So it is clear that the heterospory ultimately leads to seed development. The Selaginella is common heterosporous genus, but complete and well developed seeds are not found in Selaginella and other heterosporous genera. Absence of integuments, permanent retention of megaspores within megasporangia and histological union between the megasporangial wall and mega spore are the short comings in the formation of complete and true seeds. Heterospory was present in many fossil genera of Lycopsida, Sphenopsida and Pteropsida. They were very common in late Devonian and early Carboniferous periods. During this period the important heterosporous Lycopsids genera were Lepidocarpon, Lepidodendron, Lipidostrobus, Pleuromea, Sigilariostrobus etc. The members of Sphenopsida were Calamocarpon, Calamostachys and Palaeostachya. Among them certain members had an advance stage of heterospory. Some carboniferous Lycopsids and Sphenopsids were arborescent (tree like). There contribution in present day economy in the form of coal cannot be ignored. UTTARAKHAND OPEN UNIVERSITY Page 28 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 2.3 STELE The term stele has been derived from a Greek word meaning pillar. The stele is defined as a central vascular cylinder, with or without pith. Endodermis is the boundary between cortex and stele. The central cylinder or core of vascular tissue, consisting of xylem, phloem, pericycle and sometimes mudullary rays and pith, is technically called the stele. Van Tieghem and Douliot (1886) interpreted the plant body of vascular plant in the different way. They developed stelar theory. According to them, the fundamental parts of a shoot are the cortex and a central cylinder (known as stele). Foster and Gifford (1959) have mentioned that the most debated and controversial aspect of the stelar theory on the basis of the anatomical boundaries, which separate the cortex from stele. According to Van Tieghem and Douliot (1886), the endodermis represents the inner boundary of the cortex. But in the stem of many seed plants, the characteristic endodermal layer is not present. Some have mentioned that in such cases, the pericycle serves as the separating layer between the stele and the cortex. But the pericycle is itself the layer of stele. However, stele word is applicable to Pteridophytes while vascular bundle word is used in case of seed plants. 2.3.1 Types of Steles Jeffrey (1898), for the first time pointed out the stelar theory from the point of view of the phylogeny. On the basis of steler theory, various types of vascular cylinder can be recognised in shoot and root. Most researchers (Jeffrery 1898 and 1917, Esau, 1953; Smith, 1955; Foster and Gifford, 1959) recognise two main types of stelar organisations: Protostele and Siphonostele. 1- Protostele The stele was named protostele by Jeffery (1898). There is no pith in protostele. In protostele, the vascular tissue is a solid mass and the central core of the xylem is completely surrounded by the strand of phloem. This is the most primitive and simplest type of stele. There are several forms of protostele. Researchers such as Brebner (1902), Wordsdell (1902) and Zimmerman (1930) categorized protostele into following types: (a) Haplostele: It was named haplostele by Brebner (1902). This is the most primitive type of protostele. Here the central solid smooth core of xylem remains surrounded by phloem (Fig 2.1.A & 2.2.A). It is found in several fossil genera like Rhynia, Horneophyton and many living genera Selaginella crysocaulos, S. Kraussiana. Selaginella selaginoids, Lygodium etc. (b) Actinostele: This is the modification of the haplostele and somewhat more advanced in having the central xylem core with radiating ribs (Fig.2.2.B). In this type the xylem core is star shaped or stellate. (e.g., in Psilotum, Astroxylon, Lycopodium serratum etc). UTTARAKHAND OPEN UNIVERSITY Page 29 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Fig.2.1 Types of arrangement of vascular tissues in stele, A. Protostele, B. Siphonostele, c. Dictyostele Fig.2.2, A. Haplostele, B. Actinostele (c) Plectostele: This is the most advanced type of protostele. Here the central core of xylem is divided into number of plates arranged parallel to each other (Fig.2.3A). The phloem alternates with the xylem (e.g., in Lycopodium clavatum and L. volubile). Fig. 2.1, (A-C) Stelar System: A. Plectostele B. Mixed protostele C. Mixed protostele with pith (d) Mixed protostele: Here xylem groups are uniformly scattered in the ground mass of the phloem, called mixed protoslele (Fig.2.3 B). e.g., Lycopodium cerrnuum. (e) Mixed-pith prostotele: Here the xylem elements (i.e., tracheids) are mixed with the parenchymatous cells of the pith (Fig.2.3-C). This type is found in primitive fossils and living UTTARAKHAND OPEN UNIVERSITY Page 30 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 ferns. They are treated to be the transitional types in between true protosteles on the one hand and siphonosteles on the other (e.g., in Gleichenia. spp., Hymenophyllum dilatatum; Lepidodendron spp.; Osmunda spp.). 2- Siphonostele Medullated protostele is called siphonostele. It is characteristic of Filicophyta. This is the modification of protostele. During the development of siphonostele, the central core of xylem is replaced by paranchymatous cells hence definite pith surrounded by xylem appears in the centre. Such stele contains a tubular vascular region and a paranchymatous central region. On the basis of branch and leaf gaps Jeffrey (1910), distinguished two types of siphonosteles, cladosiphonic siphonosteles and phyllosiphonic siphonosteles. In one type, the leaf gaps are not found and are known as cladosiphonic siphonosteles while in the other type both leaf and branch gaps are present and are known as phyllosiphonic siphonosteles. Jeffrey (1898) classified Siphonostele into following two types, on the basis of position of phloem. (a) Ectophloic siphonostele: In this type of siphonostele, the pith is surrounded by concentric xylem cylinder and next to xylem the concentric phloem cylinder. It means the phloem is restricted only on the external sides of the xylem. (e.g., Osmunda and Schizea).The phloem is externally surrounded by pericycle and endodermis (Fig.2.4A). (b) Amphiphloic siphonostele: In this type of siphonostele the pith is surrounded by the vascular tissue. The concentric inner phloem cylinder surrounds the central pith. Next to the inner phloem is the concentric xylem cylinder which is immediately surrounded by outer phloem cylinder. It means phloem is present both sides of xylem. (e.g., Marsilea rhizome) (Fig.2.4B). Fig.2.2 (A-B): A. Ectophloic siphonostele, B. Amphiphloic siphonostele UTTARAKHAND OPEN UNIVERSITY Page 31 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Other modifications of Siphonostele 3-Solenostele If the siphonostele is perforated at any place due to the origin of the leaf trace, such a condition is known as Solenostele. It is of following two types- (a) Ectophloic solenostele: This type of solenostele derived from the ectophloic siphonostele. So, here the phloem is present on the outer side of xylem. (b) Amphiphloic solenostele: This type of solenostele derived from the amphiphloic siphonostele. In this case the phloem is present on either the sides of the xylem. Fig.2.3- Different types of steles arranged in evolutionary sequence UTTARAKHAND OPEN UNIVERSITY Page 32 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 4. Dictyostele In case of Pteropsida, the successive leaf gaps may overlap each other. Brebner (1902) called the siphonosteles with overlapping leaf gaps as dictyostele. So, in this case the solenostele is broken into a network of separate vascular strands due to crowded leaf gaps. In such cases each separate vascular strand is known as meristele. Each meristele is of protostelic type. Hence the dictyostele is a ring of many meristeles. 5. Eustele According to Brebner (1902), there is one more modification of the siphonostele known as eustele (Fig.2.6). Here the stele splits into distinct collateral vascular bundles. So the vascular system consists of a ring of collateral or bicollateral vascular bundles situated on the periphery of the pith. In such steles, the inter-fascicular areas and the leaf gaps are not distinguished from each other very clearly. The example of this type is Equisetum. Fig.2.4- Suggested stages in the evolution of the eustele (Taylor and Taylor,1993) 6. Atactostele In atactostele the vascular strands are scattered. It occurs in monocotyledons (Fig.2.7). George Brebner (1902) coined the term atactostele (Greek atact—without order) for vein arrangement seen in transverse view which has been described later as “scattered” by Berg (1997). Fig.2.5- Atactostele in Monocots UTTARAKHAND OPEN UNIVERSITY Page 33 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 7. Polycyclic Stele When the vascular tissue present in the form of two or more concentric rings, such a stele is called polycyclic stele. A typical polycyclic stele possesses two or more concentric rings of vascular tissue. This may be a solenostele or a dictyostele. Two concentric rings of vascular tissue are found in Pteridium aquilinum and three in Matonia pectinata. 8- Polystele When more than one stele is present in the axis of some pteridophytes. Such a condition is called Polystelic condition. Certain species of Selaginella have polystelic condition (Fig. 2.8). Fig.2.8: Polystelic (distelic) condition in Selaginella Stem 2.4 SORUS (pl. Sori) A sorus (pl. sori) is a group of sporangia (Fig. 2.9). Sorus is a Greek word that means ‘stack, pile, or heap. In ferns, these form a yellowish or brownish mass on the edge or underside of a fertile frond. In some species, they are protected during development by a scale or film of tissue called the indusium, which forms an umbrella-like cover. The sporangia within produce haploid spores. As the sporongia mature, the indusium shrivels so that spore release is unimpeded. The sporangia then burst and release the spores. Protection of the sporangial cluster from exposure, drying, and other hazards is accomplished in various ways, such as by the formation of the sori in grooves or pockets or by the production of various forms of covers. One is the so-called false indusium which is a rolled- over leaf margin under which sporangia form and mature. The true indusium is a separate and unique formation, in some ferns the sori are naked and do not from sori. Their sporangia scatter on veinlets of leaves. Thesori have variation in shape, size and arrangement. They are mostly linear, circular and rainform. Sometimes the sori may be large in size due to the fusion, they are called coenosori. UTTARAKHAND OPEN UNIVERSITY Page 34 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Fig.2.9: Arrangement of sori in different ferns The shape, arrangement, and location of the sori are often valuable clues in the identification of fern taxa. They may be arranged in rows, either parallel or oblique to the veins or randomly arranged. Their location may be marginal or away from the margin on the frond lamina. The presence or absence of indusium is also used to identify fern taxa. A widespread type of indusium among members of the family Cyatheaceae is one shaped like a cup, which arises around the base of the sorus, often enclosing the sorus until the sporangia are mature (e.g., Cyathea). In some genera, marginal sori are protected by a two- lipped, or valvate, indusium (e.g., Dennstaedtia, Dicksonia, and Hymenophyllum). When sori fuse laterally to form continuous lines, or coenosori, indusia also tend to fuse. The sorus is classified on the basis of origin as marginal sorus (origin on margin of pinnae), intramarginal and abaxial (superficial) sorus. While on the basis of development of sporangia, they may be simple, gradate and mixed sorus. (a) Simple sorus: A sorus in which all the sporangia appear, grow and mature at the same time is called simple sorus. (b) Gradate sorus: A sorus in which centre or apex is occupied by oldest sporangium and the successive youngest sporangia are present towards the base is called a gradate sorus. Such sorusis also called basipetal sorus. (c) Mixed Sorus: When sporangia of different ages are present in a sorus, without any definite arrangement such sorus is called mixed sorus. 2.5 HETEROSPORY AND SEED HABIT Heterospory is a phenomenon in which two kinds of spores are borne on the same plant. The spores are differing in size, structure and function. The smaller one is known as microspore and largerone as megaspore. Such Pteridophytes are known as heterosporous. Most of the UTTARAKHAND OPEN UNIVERSITY Page 35 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Pteridophytes produce one kind of similar spores, Such Peridophytes are known as homosporous and this phenomenon is known as homospory. The sporangia show greater specialization. They are differentiated into micro and megasporangia. The microsporangia contain large number of microspores whereas megasporangia contain few megaspores. The production of two types of spores with different sexuality was first evolved in pteridophytes. Even though, the condition of heterospory is now represented byonly eight living species of pteridophytes, they are Selaginella, Isoetes, Marsilea, Salvinia, Azolla, Regnellidium, Pilularia and Stylites. 2.5.1 Origin of Heterospory The fossil and developmental studies explain about the origin of heterospory. A number of fossil records proved that heterospory existed in many genera of Lycopsida, Sphenopsida and Pteropsida. They are very common in late Devonian and early Carboniferous periods. During this period the important heterosporous Lycopsidsgenera were Lepidocarpon, Lepidodendron, Lipidostrobus, Pleuromea, Sigilarios robus etc. The members of Sphenopsida were Calamocarpon, Calamostachys and Palaeostachya. Among them certain members had an advance stage of heterospory. Williamson and Scott (1894) discovered and described two species of Calamostachys. These species are C. binniana and C. casheana. Former C. binniana showed homospory while later C. casheana showed heterospory. The megasporangia contained small and aborted spores. This shows that abortion of spores leads to the differences in the size and number. Chaloner (1958), reported that in Stauropteris bruntis landica the megasporangia contained tetrad of megaspores in which two spores were large and two small. Only one megaspore was functional and rest aborted in case of Lepdocarpon. All these evidences from fossil records indicate that heterospory occurred quite early in plants and originated by the degeneration of spores in the sporangia. In living representative of heterosporous pteridophytes the sex determinant exert their influence during the differentiation of spore mother cells (Sporocytes). In Selaginella the development up to sporocyte formation is same in micro and megasporangia. After this stage they follow the different course. In the microsporangia all the microspore mother cells are functional and undergo meiosis to form tetrahedral tetrads of microspores while in megasporangia few or mostly one megaspore mother cell remains functional and other start degenerating. In Marselia there is no difference between the microsporangial and megasporangial development before the stage of meiosis. In case of microsporangia, all the 64 spores survive but in megasporangium only one out of 64 spores survives. In case of Salvinia only one out of 32 megaspores survives but no degeneration takes place in microsporangium. Here it is clear that in some cases the heterospory originate at the pre-meiotic stage but in others the heterospory originates at the post meiotic stage. So there is no generalisation of the origin of UTTARAKHAND OPEN UNIVERSITY Page 36 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 heterospory. The experimental studies also proved that the nutritional environment of sporangia may alter the pattern of spore formation. 2.5.2 Significance of heterospory Heterospory is an expression of sex determination in plant. Because the microspore gives rise to male gametophyte bearing male sex organs and megaspore develops into female gametophyte bearing female sex organs. So differentiation in the size of spore is directly related to the differentiation of sex of the gametophyte. In homosporous genera the sex determination is observed at gametophytic stage during the formation of antheridia and archegonia. In heterosporous genera the differentiation observed during sporogenesis at sporophytic stage. The differentiation of microspores and megaspores and their dependence upon the sporophyte has certain advantages. The gametophytes of the ferns are however, dependent for their nutrition upon soil and environmental conditions, whereas in case of Selaginella, as far as the nutrition of gametophytes is concerned they derive it from the sporophyte and therefore they are more independent to the external condition than those of ferns. The phenomenon of Heterospory is of great biological advantage because of the fact that a large megaspore which contains female gametophyte derives its food from the sporophyte, and is independent of the external conditions which might interfere with the growth of a free living gametophyte. Thus it forms a better starting point for the new embryo than an independent green prothallus which has to manufacture its own food. Heterospory has the considerable importance in the development of seed. Heterospory is rather a pre-requisite for seed habit. The Gametophyte depends upon sporophyte, reduction of gametophytic tissue, reduction in the number of megaspores, retention of megaspore in the megasporangium, all these lead to the seed habit. In brief, the origin of seed habit is associated with the following important prerequisites: 1. The production of two kinds of spores (i.e., heterospory). 2. Development of only one megaspore in the megasporangium. 3. The retention and germination of the megaspore within the megasporangium. 4. Retention of megaspore inside megasporangium till the formation of female gametophyte and after fertilization. 5. The female gametophyte absorbs nourishment from sporophyte. In Selaginella, there is remarkable approach to the seed habit on account of the following important features: 1. Selaginella shows heterospory. 2. The megaspore usually germinates within the megasporangium and their times of release from the megasporangia vary from species to species. 3. There is reduction to one megaspore in some species, e.g., S. rupestris and S. monospora and a confirmed tendency of reduction in others. UTTARAKHAND OPEN UNIVERSITY Page 37 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 4. In Selaginella rupestris the megaspore is never shed and germinates as a gametophyte. Fertilization, embryo development and finally young sporophyte develops on parent plant (Fig. 2.10 & 2.11). The development of young sporophyte on parent plant of Selaginella can be linked to viviparous habit of angiosperms. It becomes quite evident that Selaginella has considerably advanced towards the seed habit in a few species, but its approach to the true seed is not complete due to the following features: 1. The megasporangium lacks an integument or covering. 2. The permanent retention of the megaspore within the megasporangium has not become established. 3. After the development of the embryo, the resting period is not there. 4. No histological union between megaspore and megasporangium. Fig. 2.10, Yong sporophyte developing upon the strobilus of parent plant in Selaginella rupestris Fig. 2.11- Diagrammatic life cycle of Selaginella UTTARAKHAND OPEN UNIVERSITY Page 38 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 2.6 GENERAL ACCOUNT OF FOSSIL PTERIDOPHYTES Pteridophytes have a long fossil history. They have been recognized in the late Silurian period of the Paleozoic era. These plants have the dominance in whole of the Palaeozoic era. The middle and the late Palaeozoic era can be regarded as the age of ferns or ages of pteridophytes. The giant lycopsids, the horse tails and the arborescent tree ferns dominated the whole biota at that time. The main fossil groups of pteridophytes are discussed as follows: 2.6.1 Fossil Psilopsids They are the simplest extinct vascular plants that were discovered among the rocks of the early Devonian period of the Palaeozoic era. In this class few genera was included like Rhynia, Horneophyton, Cooksonia, Zosterophyllum, Psilophyton, Asteroxylon etc. These plants were distributed during the late Silurian, Devonian and the Upper Carboniferous periods. These plants lack true roots, leaves and the other structures. However, developed vascular system can be seen in these groups. Among these plants, the first to be recorded in 1858 was Sir William Dawson’s Psilophyton princeps that was discovered from the Gaps sandstone. It is the first plant with which our knowledge of the division Psilophyta started. 1-Cooksonia Cooksonia is an extinct & oldest known land plant. The Cooksonia dates from the middle of the Silurian to the early Devonian with a total time span of 433 to 393 million years ago. Cooksonia includes primitive known land plant to have a stem with vascular tissue. Cooksonia is a transitional form between the primitive non-vascular bryophytes and the vascular plants. Fig.2.12: Reconstruction of Cooksonia The sporophytes of Cooksonia were small, a few centimeters tall, and had a simple structure. The stem width varies from about 0.03 mm to 3 mm. They lacked leaves and roots, although rhizome was present. Plant had a simple stalk that branched dichotomously a few times. Each branch ended in a sporangium (Fig.2.12). UTTARAKHAND OPEN UNIVERSITY Page 39 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 2-Rhynia In 1917 the genus Rhyniawas discovered by Kidston and Lang from Rhynie chert bed in Aberdeenshire district of Northern Scotland. The plant was found in different stem, leaves and the isolated sporangia. Typical dichotomous division can be seen in the stems of the Rhynia plants. The aerial shoots of Rhynia, were photosynthetic in nature and had stomata all over. The surface of the axis of smaller species bears numerous conspicuous emergences or hemispherical projections. Rhynia possesses two species R. gwynne- vaughani and R. major (Fig. 2.13 A & B). Fig.2.13: Rhynia, External features, A- R. major, B- R. gwynne-vaughani and C-sporangium R. major was larger and attained a height of 50 cm. with a diameter of 1.5 to 6 mm., while R. gwynne-vaughani had a height of 20 cm. and a diameter of 1 to 3 mm. The larger size, absence of hemispherical outgrowths and the absence of adventitious branches are characters that differentiate R. major from the smaller R. gynne-vaughani. The transverse section of aerial stem reveals distinct epidermal layer, an outer and inner cortex and vascular strands. The pericycle and endodermal layers were lacking. Stele is a typical protostele. There are no roots but rhizoids present. The rhizoids are unicellular and grow in tufts from the ventral surface of rhizome. The aerial shoots have typically terminal sporangia. The sporangia were about 12 mm. in length and about 4 mm. in diameter. The wall of sporangia is three layered. The sporangium contained numerous spores in the form of spore tetrads. Rhynia was homosporous. 3. Horneophyton Horneophyton, is the fossil pteridophytes and it is the linkage between the fossil psilotales and the other living members of the Sphenopsida. These members were also reported from the Rhynie chert bed of the Scotland in 1920. UTTARAKHAND OPEN UNIVERSITY Page 40 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Fig. 2.14: Reconstruction of Horneophyton Fig.2.15: Psilophyton The plant body is dichotomous (Fig.2.14) and the sporangium having the central columella. The presence of the columella in the Horneophyton shows its affinity and link with the bryophytes whereas the presence of the tracheids in the vascular tissues shows the resemblance with the higher plants. Internal structure of stem resemble with the Rhynia. 4. Psilophyton Psilophyton is an important extinct genus of order Psilophytales. Specimens of Psilophyton collected from Lower Devonian rocks of Gaspe Canada, named by J. W. Dawson. The name Psilophyton princeps was given to the reconstruction. This included examples of the markedly spiny stems, and relatively smooth upper branch system that bears sporangia. Psilophyton princeps was having creeping rhizome, dichotomously forked aerial branches, spiny lower part of stem, young branch tips circinately curled and terminally paired sporangia on the fertile aerial shoots. More than 20 species of Psilophyton have been discovered (Fig.2.15). 2.6.2-Fossil Lycopsids The Lycopsids has a very long evolutionary history. All the modern living Lycopods are herbaceous plants but their Palaeozoic relatives were herbaceous as well as large trees. The large treelike lycopods were the major part of forest at that time and they contributed in our present day economy in the form of coal and other biofuel. Lepidodendron and Sigillaria are the best examples of arborescent lycopods. They were heterosporous. The herbaceous lycopods were Protolepidodendron and Baragwanathia and these were homosporous. The both types of fossil Lycopods described here in brief. (i) Herbaceous homosporous fossil Lycopods Protolepidodendrales The Protolepidodendrales comprising the fossil homosporous herbaceous plants (Fig.2.16). Protolepidodendrales consists of Palaeozoic fossils extending from the Silurian to the Devonian. They are usually placed within a single family Protolepidodendraceae which may be rather artificial as the details of most of the genera are not known. UTTARAKHAND OPEN UNIVERSITY Page 41 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 The sporophytes were herbaceous with the branches densely covered by microphyllous, eligulate leaves. Sporophylls were dispersed and not localised on definite strobili. The sporangia, where known, were homosporous. The Protolepidodendrales include genera: Baragwanathia, Archaeosigillaria, Leclercqia and Protolepidodendron. Many were short, herbaceous plants, but some reached heights of at least 50 cm. They had rhizomatous rooting structures, dichotomous branching and were probably homosporous. Baragwanathia longifolia from the Silurian of Australia is better known. Even this is older than the Psilophytes. The plant was probably larger than the present day Lycopodium. Microphyllous narrow leaves with single median vein up to 4 cm long and 1 mm wide are laxly borne on the stout, dichotomous stems 1 to 5 cm in diameter. The tips of the aerial shoots and the prostrate system are closely covered with small, spinous, eligulate leaves which bifurcate at the tips. On the middle part of the stem sporophylls are laxly borne intermixed with vegetative leaves. Reniform sporangia, about 2 mm in diameter, are present in certain areas of the stem in axillary position of leaves (sporophylls) intermixed with vegetative leaves. Although it is not definite whether the sporangia were adaxial on the leaves, the general pattern seems to be Lycopodiaceous. The stele, though not well preserved, seems to be actinostelic with annular tracheides in the 12 or more xylem rays. The sporophylls resemble the vegetative leaves with bifurcate tips and bear single sporangium on the adaxial face and little above the base. The stems are protostelic with triradiate exarch xylems. The metaxylem contains scalariform tracheides. Leaf traces depart from the angles of the xylem and median veins are present in the leaves but it is not clear if the two were continuous. Leaf scars were not formed on the stem when the leaves were shed. Fig.2.16, Members of Protolepidodendrales, A. Baragwanathia longifolia, B. Drepanophycusspinae formis, C. Protolepidodendron scheryanum, D. Sporophyll of Protolepidodendron (after Lang & Cookson) UTTARAKHAND OPEN UNIVERSITY Page 42 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Protolepidodendron is a type genus of this group. It was 30 cm. in height and 1 cm. in diameter. The branches were prostrate as well as erect. The plant body covered with small bifurcated leaves. The sporangia are arranged on upper surface of some leaves (Fig.2.16 C&D). The Stem had a triangular protostele with lobed xylem. (ii) Arborescent (Tree like), Heterosporous fossil Lycopods Lepidodendrales Lepidodendrales were big trees attaining a height of 30 m or more and a diameter of about 2 m at the base (Fig.2.17). The plants belonging to Lepidodendrales appeared on the Earth during the upper Devonian period i.e. about 359 to 345 million years ago. They were in extreme abundance during carboniferous period (345 to 280 millions years ago). They started declining on the earth during early Permian and became extinct by the end of Permian period. The two most common genera of lepidodendrales found in Pennsylvanian rocks are Lepidodendron and Sigillaria. Lepidodendron stems have diamond shaped leaf scars that are inspiral rows around the tree trunk. Sigillaria stems have somewhat rounded leaf scars that are arranged spirally, but vertical ridges between the scars give the appearance that the scars are in vertical rows. The long grass like leaves and reproductive cones of these lycopods are known from fossils. Lycopods were trees of moist, swampy areas and many species became reduced in abundance or extinct as the climate became drier in the Late Pennsylvanian and Permian. Fig. 2.17: Reconstruction of Lepidodendron Lepidodendron is the form-genus for impressions of the outer bark of large arborescent lycophytes. Lepidodendron is also the name that paleobotanists use to refer to the biological genus for entire plant, including all of its individual parts. Lepidodendron grew to over 100 feet (30 meters) tall and preferred the wetter, areas in swamps. Lepidodendron is recognized by the diamond-shaped pattern of leaf scars, each scar being generally longer than wide. Lepidodendron also called ‘scale tree’ (Fig.2.18). Rooting pattern of the plant indicates presence of humid environment. In all the members of Lepidodendrales, the root-bearing underground axes are called rhizomorph and the detached UTTARAKHAND OPEN UNIVERSITY Page 43 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 rhizomorph and their roots are called Stigmaria which are mostly found as siliceous casts or molds (Fig. 2.19). Fig.2.18: Bark pattern of Lepidodendron Fig.2.19: Stump cast of Stigmaria Fig. 2.20 L.S. of Flemingites cone The roots of lycopod trees are commonly preserved as casts in clay beds (under clays) beneath coal beds. These structures bear a series of circular pits that represent the attachment points of rootlets. Stigmaria ficoides, the commonest species of Stigmaria, was a large trunk base that divided dichotomously into four large massive descending axes. The Stigmaria spread over an area of about 20 ft (6 m) across. The Lepidodendron was a large tree with a prominent trunk. The ultimate dichotomies bear leaves. The branches and the foliage formed a spreading crown bearing cones at their tips. The plant had bipolar growth, thus the main axis developed branches at both ends. The aerial branches formed three-dimensional dichotomies bearing branches and foliages, similarly the basal branches formed three-dimensional dichotomies bearing stigmarian root system. The leaf cushions-are rhomboidal in shape and broader in their vertical dimension than their transverse length. A leaf scar is situated just above the middle line of the cushion. In most species, secondary growth is characteristic of the genus, which was initiated by the unifacial activity of the cambium. Thus, only secondary xylem was produced externally and the cambium did not produce secondary phloem. There was massive extrastellar secondary growth by the meristematic activity of cortical parenchyma. Bisporangiate cones called Flemingites (Lepidostrobus) were borne terminally (Fig.2.20). The sporophylls were helically attached to the central cone axis. The microsporophylls bearing microsporangia usually borne in the apical portion, while megasporophylls bearing megasporangia occupied UTTARAKHAND OPEN UNIVERSITY Page 44 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 the basal portion of the cones. Hence lepidodendron was heterosporous and the strobili were described as Lepidostrobus and Lepidocarpon. Lepidocarpon is the generic name given to megasporangiate cones in which a single megaspore was present. It developed into megagametophyte. The development was in situ because gametophyte was seen retained in the megasporangium. According to Phillips (1979) the entire structure was shed like a seed from the plant. Ovules and seed had not been reported. Due to this it cannot be considered as a true seed rather it is a false seed or seed like structure. 2.6.3- Fossil Sphenopsids Sphenopsids (horsetails) first appeared in the Devonian and reached the peak of its development and diversity during the Carboniferous forming a major component of the coal- swamp vegetation. They grew into stout trees that produce a small amount of secondary xylem. Sphenopsids are characterized by jointed stems with whorls of leaves and branches borne at the joints (or nodes). The internodal part of the stem is vertically ridged and spores are produced in rings of sporangia arranged in cones, usually at the tips of the fertile shoots. One of the best-known fossil genus, Calamites included tree-like forms that grew up to 30m in height. Another common fossil Sphenopsid, Sphenophyllum, was a slender plant with a ribbed stem only 1–7 mm in diameter but up to several metres in length, that probably succeeded in competition over other vegetation. They are type members of two major groups among sphenophytes the Calamitales and Sphenophyllales. 1-Calamitales The order Calamitales was at its peak of development during the upper carboniferous and become extinct at the end of Permian. Although Neocalamites and Equisetites were recovered from Mesozoic strata. Fig.2.21: Foliage (Annularia) of Calamites Calamites is a genus of extinct arborescent (tree-like) horsetails to which the modern horsetails (genus Equisetum) are closely related. Its upright stems were woody and connected by an underground runner; however, the central part of the stem was hollow, and fossils of Calamites are commonly preserved as casts of this hollow central portion (Fig.2.21). The name Calamites was originally given to the pith casts of the hollow stems, but now also used for the whole plant. The organ genera belonging to Calamites include following: UTTARAKHAND OPEN UNIVERSITY Page 45 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Stem : Calamites Leaves : Annulariaor Asterophyllites Root : Asteromyelon Fructification : Calamostachys Calamites grew to 20 metres tall standing mostly along the sandy banks of rivers. They had upward slender branches, arranged around a bamboo-like trunk in rows spaced several meters apart and had conifer-like needles arranged around the ends of the branches. The leaves were needle-shaped and grew in whorls around the trunk. There were up to 25 leaves per whorl. The trunk was hollow but woody and was very slim so it was not very strong. Calamites could either reproduce by spores, which were stored in small sacs and organized into cones, or they could have reproduced by massive underground rhizomes. These underground rhizomes allowed the plant to produce clones of itself. So they had the ability to sprout vigorously from underground rhizomes (Fig.2.22). Annularia is a genus of Calamitalean foliage. The leaves were found at the node in the form of rosette. Asterophyllites is another type of foliage of Calamitalean plants. They are needle shaped, in the form of rosette at the node, but all leaves were directed towards the apex of the shoot. Asteromyelon given to calamitalean roots. Fig.2.22: Reconstruction of Calamites Fig.2.23: Calamostachys Fig.2.24: Plant body of Sphenophyllum (Reconstruction) Asterophyllites and Calamostachys Calamostachys is the generic name of calamitalean frutification. The sporophyll holding sporangia are inserted in between on the internodes (Fig.2.23). Calamostachys binneyana is a homosporous plant and Calostachys casheana is heterosporous. Sphenophyllales: Sphenophyllales represent a small and compact group of sphenopsids. They have left no surviving representative. The geological history of Sphenophyllales dates back to a time earlier than carboniferous period. Some of the genera like Sphenophyllum UTTARAKHAND OPEN UNIVERSITY Page 46 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 appeared during Devonian, reaching maximum development during carboniferous period. A few of these Sphenophyllales survived upto Triassic. They are typified by Sphenophyllum, a small, branching plant, probably of trailing habit. The plant body consisted of a main stem, which was slender with jointed stems rarely exceeded 1 cm in diameter and had superposed, longitudinal, superficial ribs between nodes. The vascular system contained a solid xylem core with triangular primary xylem. The leaves were wedge-shaped, usually shorter than 2 cm, and had toothed, notched, or rounded distal margins. They were attached at the nodes by their narrow ends and were in whorls of usually 6 or 9 and rarely 18. Anatomically the stem showed the occurrence of secondary growth. The central pith was completely absent. Instead, the central region was occupied by a triradiate exarch xylem mass. Secondary xylem formed a thick sheath surrounding the primary xylem. Long, terminal cones, usually called Sphenophyllostachys (Bowmanites) when found detached, contained sporangia and spores. The sporangia terminated to slender stalks, forming concentric whorls that alternated with whorls of sterile bracts. Most species were homosporous. Some species could have been heterosporous. 2.6.4-Fossil Pteropsids Fern-like plants appear in the late Devonian, specialized to colonize disturbed sites such as volcanic landscapes. By the middle Carboniferous, all major fern growth forms were present: trees, vines, and ground cover, mostly specialized to exploit disturbances. Among these plants, however, were tree ferns up to 10 meters tall that lived in swampy wetlands, and whose remains fossilized into coal. Surange (1966) had described Gondwanidium, Pecoteris, Merianopteris, Belamnopteris, Alethopteris and Cyclopteris. Under the Mesozoic ferns he described many species of Marrattiaceae, Osmundaceae, Glecheniaceae, Cyatheaceae, Dicksoniaceae, Dipteridaceae, Matoniaceae etc. Under the Coenozoic pteropsids the fossil genera of Azollaceae, Salviniaceae and Marsileaceae have been discussed. 2.7 SUMMARY Pteridophytes also called vascular cryptogams, they have central vascular cylinder and reproduce by means of spores. Van Tieghem and Douliot (1886) proposed stelar theory. Jeffrey (1898), for the first time pointed out the stelar theory from the point of view of the phylogeny. The most primitive and simplest type of stele is protostele. There is no pith in protostele. It is found in several fossil genera like Rhynia, Horneophyton etc. If the central xylem core is star shaped (e.g., in Psilotum), the protostele is called actinostele. If xylem is divided into number of plates arranged parallel to each other and the phloem alternates the xylem (e.g., Lycopodium clavatum), the stele named plectostele. when xylem groups are uniformly scattered in the ground mass of the phloem then it is called mixed protoslele (e.g. Lycopodium cerrnuum) UTTARAKHAND OPEN UNIVERSITY Page 47 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Medullated protostele is called siphonostele. It is characteristic of Filicophyta. It is of two types Ectophloic siphonostele and Amphiphloic siphonostele. If the siphonostele perforated at any place due to the origin of the leaf trace, the stele is Solenostele. Siphonosteles with overlapping leaf gaps develops in dictyosteles. When the vascular strands are scattered, the stele is atactostele, It occurs in monocotyledons. The phenomenon of development of two types of spores (differing in size, structure and function) by the same species is known as heterospory. The smaller is called microspore and larger is called megaspore. Heterospory impacts on the origin of seed. In homosporous genera the sex determination is observed in the gametophytic stage but in heterosporous genera it is observed in sporophytic stage. So it is cleared that the heterospory is rather a pre- requisite for seed habit and ultimately leads to the seed development. Heterospory was present in many fossil genera of Lycopsida, Sphenopsida and Pteropsida. They were very common in late Devonian and early Carboniferous periods. Some carboniferous Lycopsids and Sphenopsids were arborescent (tree like). 2.8 GLOSSORY Arborescent: Approaching the size and habit of the tree. Basal: Situated or attached at the base. Clone: A plant derived from the vegetative reproduction of a parent plant, Dimorphic: Occurring in 2 different forms Heterospory: Producing two kinds of spores, i.e. Microspores and Megaspores. Indusium: Outer covering of sorus s Megaspore: The larger of two kinds of sporesproduced by a heterosporous plant. Microspore: The smaller of two kinds of spores produced by a heterosporous plant. Pericycle: A layer of parenchyma or sclerenchyma cells that lies just inside the endodermis. Pith: The central parenchymatous (generally) region of a stem. Prostrate: Lying parallel with the ground. Revolute: Rolled downwards or backwards, Stele: Central vascular cylinder. Viviparous: Referring to seeds which germinate when still attached to the parent plant. 2.9 SELF ASSESSMENT QUESTION 2.9.1-Multiple choice questions: 1. Heterospory is found in: (a) Lycopodium (b) Selaginella (c) Rhynia (d) Psilotum 2. Which of the following was tree? (a) Rhynia (b) Lepidodendron (c) Psilophyton (d) Azolla UTTARAKHAND OPEN UNIVERSITY Page 48 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 3. Lepidodendron is a fossil of (a) Root (b) Fruit (c) Stem (d) Ovule 4. Medullated protostele is called (a) Haplostele (b) Atactostele (c) Siphonostele (d) Actinostele 5. In dictyostele each component is called (a) Meristele (b) Plectostele (c) Protostel (d) Actinostele 6. Psilophyton was discovered by (a) Kidston and Lang (b) Birbal Sahni (c) J.W. Dawson (d) C.A. Arnold 7. Rhynia lacks (a) Root (b) Sporangia (c) Rhizoids (d) Spore 2.9.2-Fill up the following blanks: 1. Rhynia is a.....................pteridophyte. 2. Stele in Marsilea rhizome is.................. 3. Rhynia was discovered from...................by Kidston and Lang 4. Stelar theory was proposed by.................... 5. In actinostele the xylem core is.................shaped. 6. Lepidodendron is......................fossil preridophyte. 7. Fossils of tree Lycopods have been assigned to................period. 8...............is medullated protostele. 9. Calamitesis a fossil of.................... 10. Heterospory means presence of.............types of spores 2.9.1- Answer key:1-(b), 2-(b), 3-(c), 4-(c), 5-(a), 6-(c),7-(a) 2.9.2 Answer keys: 1. Fossil, 2. Amphiphloic siphonostele, 3.Devonian rocks of rhynie of Scotland, 4.Van Teigham and Douliot, 5. Star, 6. Arborescent, 7.Carboniferous, 8. Siphonostele, 9.Sphenopsids, 10.two. 2.10 REFERENCES Aagase, S. N. 1968; Amer Fern Jour. 58: 74 (Sphenopsida). Agarwal Teena; and Danai Priyanka; 2017,. Fossil Early Psilophyton & Lycopodian, Spenopsida Lines of evolution, Br J Res 2017, 4 (3): 15.. Chandra, S; 2000; the fern of India, International book Distributers, Dehradun. Gopal Krishnan; V and Ravi; S; 1990; Indian fern J. 7:94 (Lycopsida). Pandey, A K and Rout, S D; 2006; Ethnobot. 18:102 (Ethnobotany). UTTARAKHAND OPEN UNIVERSITY Page 49 PTERIDOPHYTES, GYMNOSPERMS AND PALAEOBOTANY MSCBOT-503 Sharma, O P; 2012; Pteridophyta, McGrow Hill Education New Delhi. Vashihta, P C, Sinha, A K; Kumar, A; 2010; Pteridophyta, S. Chand & company New Delhi. 2.11 SUGGESTED REA