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Hesse et al 2009

  1. 1. W
  2. 2. Michael Hesse Heidemarie Halbritter Reinhard Zetter Martina Weber Ralf Buchner Andrea Frosch-Radivo Silvia UlrichPollen TerminologyAn illustrated handbook SpringerWienNewYork
  3. 3. Univ.-Prof. Dr. Michael HesseDDr. Heidemarie HalbritterAo.Univ.-Prof. Dr. Reinhard ZetterAo.Univ.-Prof. Dr. Martina WeberDipl.-Biol. Dr. Ralf BuchnerAndrea Frosch-RadivoMag. Silvia UlrichUniversity of Vienna, AustriaThis work is subject to copyright.All rights are reserved, whether the whole or part of the material is concerned,reproduction by photocopying machines or similar means, and storage in databanks.Product Liability: The publisher can give no guarantee for all the informationcontained in this book. This does also refer to information about drug dosageand application thereof. In every individual case the respective user must checkits accuracy by consulting other pharmaceutical literature. The use of registerednames, trademarks, etc. in this publication does not imply, even in the absence -tive laws and regulations and therefore free for general use.© 2009 Springer-Verlag/WienPrinted in AustriaSpringerWienNewYork is part ofSpringer Science + Business Mediaspringer.atCover Illustrations: Agrostemma githago (SEM, HH), Pinus sp. fossil (LM, RZ),Ruellia graecicans (SEM, HH), Phyllanthus x elongatus (SEM, HH), Argyranthe-mum sp. (TEM, AF-R), Acacia myrtifolia (SEM, HH), Leontodon saxatilis (SEM, HH)Layout: Dr. Ralf Buchner, Wien, AustriaPrinting: Holzhausen Druck und Neue Medien GmbH, 1140 Wien, AustriaPrinted on acid-free and chlorine-free bleached paperSPIN: 12045303With numerous (partly coloured) FiguresLibrary of Congress Control Number: 2008941153ISBN 978-3-211-79893-5 SpringerWienNewYork
  4. 4. Pollen TerminologyAn illustrated Handbook Michael HESSE, Reinhard ZETTER,Heidemarie HALBRITTER, Martina WEBER,Ralf BUCHNER, Andrea FROSCH-RADIVO, Silvia ULRICH
  6. 6. In memory of Jan MULLER and Wilhelm KLAUS, who played a prominent rolein the study of fossil and extant pollen.
  7. 7. There are more things in heaven and earth, than are dreamt of in our philosophy.PrefaceT he principal aim in compiling this book Manfred A. FISCHER Alfred GLASER information about the structure and Lynn HANNONoutlook of the extremely manifold pollen in Barbara HERMANOWSKIseed plants. Anton IGERSHEIM Pollen Terminology. An illustrated Handbook Irmgard JÄGER-ZÜRNshould not be seen as a mere collection of Christel KASSELMANNstriking and/or informative light and electron Nadja KAVCIKmicrographs. Each of the micrographs is Alexander KOCYAN Wolfgang KOLLERrelated to properties and functions of the Thomas LENDLpollen grains shown. The authors hope that Claudia LOOSthe book will be useful for experiencedresearchers as well as for beginners in paly- Wolfgang OBERSCHNEIDERnology, but also for medicine, biochemistry, Marianne PEROUTKAor even for lawyers and artists as an aid and Paul RADIVOguide for the evaluation and interpretation Saskia SAM-RICHTARZof pollen features. Johannes SCHACHNER Ursula SCHACHNER Barbara SIMONAcknowledgements Susanne SONTAG Robert STANGL Many people were involved in various Rupert STINGLaspects of this book project and the authors Matthias SVOJTKAwould like to acknowledge and thank them Angelika SYROVATKAfor their time, suggestions and encour- Enikö TWERASERagement during the various developmental Walter TILLphases of this book and/or for providing Stefan VOGELplant material. Bruno WALLNÖFER Arabella WURZINGER Among the many other colleagues, who Klaudia ZETTERcontributed substantially, the authors want The staff members of the Botanicalto thank (in alphabetical order): Garden of the University of Vienna (HBV) The staff members of the Bundesgärten Wolfram ADLASSNIG Wien_Innsbruck Elisabeth ANGER Josef BOGNER A special note of thanks is due to Paula BOMBOSI Mag. Franziska BRUGGER and to Mrs. Silvia Perica BRODARIC SCHILGERIUS of the team at Springer Vienna Thomas CROAT for their great support and patience during Bernadette DIETHART the preparation of this book. David Kay FERGUSON
  9. 9. CONTENTGENERAL CHAPTER Introduction _______________________________ 5 Guidelines ________________________________ 7 Rules for Using Prefixes__________________________ 8 Palynology _______________________________ 11 The Science of Pollen and Spores ______________ 11 A Brief History of Palynology ___________________ 12 A Tentative Outlook___________________________ 13 Pollen Morphology________________________ 15 Polarity and Symmetry ________________________ 15 Apertures_____________________________________ 17 Structure and Function ________________________ 20 Pollen Wall ___________________________________ 20 Structure and Sculpture _______________________ 23 Harmomegathy ______________________________ 23 Why Do We Need Categories?_____________ 27 Pollen Development ______________________ 35 Microsporogenesis and Microgametogenesis __ 35 Inherence of Misinterpretation _____________ 39 Tripartite Features _____________________________ 39 Apertures as Pitfalls ___________________________ 42 Pollen Features can be Ambiguous ____________ 44 Controversial or Fuzzy Terms_______________ 47 Acalymmate/Calymmate_____________________ 47 Areolae/Areolate ____________________________ 47 Pseudocolpus ________________________________ 48 Retipilate _____________________________________ 48 Zona-, Zono- etc. _____________________________ 48 Methods _________________________________ 51 Scanning Electron Microscopy ________________ 51 Acetolysis and Light Microscopy _______________ 51 Single-Grain Technique _______________________ 51 Transmission Electron Microscopy ______________ 52 Acetocarmine Staining for Light Microscopy____ 52 How to Describe a Pollen Grain ____________ 55
  10. 10. INTRODUCTIONIntroductionP ollen Terminology. An illustrated range of features. This can be achieved only Handbook is a collection of useful with micrographs, which demonstrate – a terms in palynology, well illustrated picture is telling more than thousand wordswith light (LM) and electron microscope – the often stunning diversity of features. For that reason, the explanatory poweran encyclopedic compilation of terms; in of micrographs produced with scanningthat respect see KREMP (1968). The focus is electron microscopy (SEM) and transmissionon the pollen of seed plants, predominantly electron microscopy (TEM) is used in theangiosperms, while spores are considered present volume. The numerous SEM micro-only exceptionally. Therefore the termi- graphs illustrating the astonishing diversitynology rarely includes spore or gymnosperm of pollen ornamentation. Where importantcharacteristics (e.g., leptoma, trilete mark). terms have appeared ambiguous or have Since 1994, the Glossary of Pollen been hitherto underrated, the term hasand Spore Terminology, co-authored by been reviewed and brought into focus (e.g.Wim PUNT, Stephen BLACKMORE, Siwert harmomegathy, or pollen class versus pollenNILSSON and Annick LE THOMAS, was the type).standard reference publication in paly- It is self-evident that such a book cannotnological terminology. Then, in 1999 the renounce the basics of palynology. Inonline version by Peter HOEN (http://www. this context please consult standard text-bio.uu.nl/~palaeo/glossary/glos-int.htm) books in palynology, e.g., ERDTMAN (1952),appeared, with several additions. The FÆGRI and IVERSEN (1989) or BEUG (2004).online version was published by W. PUNT, The principles of pollen development andP.P. HOEN, S. BLACKMORE, S. NILSSON and morphology are incorporated as separateA. LE THOMAS in 2007 and provides inform- chapters for purposes of clarity and in orderative schematic drawings containing the to correctly interpret the detailed struc-essentials of each term and colored to tures of the pollen wall and the full range ofindicate the wall and aperture components, ornamentation. Although extremely useful for overviewpurposes, drawings cannot show the full GENERAL CHAPTER 5
  11. 11. GUIDELINESGuidelinesT he aim of this book is to provide a fully more precisely, to show the full range of a illustrated terminology and glossary of single character). Brief information on the the most important palynological terms, method of preparation is often provided.including a substantial standardization of In preparing pollen for SEM micrographs, acetolysis was avoided as far as possible. Underrated pollen conditions, e.g., thethey belong to the terminology of fern spores, physical condition of the turgescent, life-likewhich is not considered here. A compre- pollen, are considered. The SEM micrographshensive description of pollen grains with terms usually represent the turgescent condition,mentioned in "Pollen Terminology. An illus- without further notice. Consequently, pollentrated Handbook" is easily accomplishable. grains are often shown in dehydrated stage, A strict rationalization of terms on the basis marked as “dry pollen”. The deviating char-of practical criteria has been attempted. acters in turgescent and dry pollen grainsFor consistency, phrases are standardized are designated by descriptive pictorial termsas far as possible; for example, features of such as cup-shaped, boat-shaped and aperture sunken.as “pollen wall with ….”, and pollen wall fea- Comments are provided where this maytures (or pollen shape and size) as “pollen help in the application of a term or to qualifygrain with ….”. the circumstances in which it is used. Self-explanatory general terms are usuallybeen reworded, newly circumscribed, orbrought into focus. In addition, consistent noted (e.g., circular, see outline). For moreapplication of EM techniques and the now- information on these see the appropriateadays better understanding of pollen fea- page(s) in chapter "Illustrated Glossary". Three categories of terms are used: important terms are printed in bold and areterms according to applied techniques (LM, usually illustrated; terms of minor importanceSEM, TEM) and their usage in morphological, are printed in regular script, usually withoutanatomical and/or functional context. In illustrations (if necessary, terms in chapterchapter "Alphabetic Glossary" the entries are "Alphabetic Glossary" are sometimes also illustrated in a footnote); terms printed inprovided with numbers in bold referring to italics are not recommended and often pro-the respective page in chapter "Illustrated vided with an explanatory comment.Glossary"1 and numbers in square brackets The chapter "Illustrated Glossary" is sub-referring to important literature (see chapter divided into larger topics, e.g., “Shape and"Bibliography"). Size” or “Ornamentation”. The terms them- Emphasis is given to the numerous illus- selves are listed according to their resem-trations. The worldwide largest database blance in order to provide the user with aon pollen, PalDat (http://www.paldat.org/) side-by-side spectrum of similar characters.is the main source of pictures. Each term is For a quick orientation please use the lastillustrated with LM or EM pictures in order to page of "Pollen Terminology. An illustratedpoint out the character range of a term (or, Handbook". It is a fold-out page with terms alphabetically arranged. Numbers indicate the page in chapter "Illustrated Glossary". 1 Please note: literature references are not nec-essarily the earliest publication in which the term was In contrast to chapter "Illustrated Glossary"used. The comprehensive literature list (see chapter the terms in chapter "Alphabetic Glossary""Bibliography") includes beside the references more are throughout arranged alphabetically asand other (and preferably recent) publications whichhave been selected as sources of further information. the noun and the corresponding adjectival GENERAL CHAPTER 7
  12. 12. GUIDELINES tends to become foggy, REITSMAform, if appropriate. Few terms are used resolute step to overcome this problem. A concise terminology now became available, though unfortu-exclusively as nouns or exclusively as adjec- nately not taking account of the range of variationtives. Sometimes two adjectival variants of most of the palynological features, and without(-ate, -ar) are used but, if so, in two different drawings or micrographs. FÆGRI and IVERSEN (1989, 4th ed.) restricted their glossary to terms exclusivelymeanings. For example: from the noun used in their book. MOORE et al. (1991, 2nd ed.) pro-granulum (sculptural or structural element vided a glossary of selected terms used in their pollen and spore keys. Standardization came with theof differing size and shape, less than 1 μm glossary by PUNT et al. (1994), updated in 2007. Thein diameter) derive the two adjectival forms main advance of their concise and comprehensive terminology is the consistent usage of drawings andgranular and granulate (both meaning the critical comments on terms and usage.“with granules”); these are correspondingterms used in two quite different contexts:granular describes a distinct type of infra- Rules for Usintectum hence a structural feature whereasgranulate refers to an ornamentation feature If both a Greek and a corresponding Latin– a sculpturing element. Both the singular and the plural are used consistently: panto- (not peri-), ekto-given consistently for Latin terms. The English (not ecto-), or the Greek di- (dis-), and notspelling of the Latin term is added (porus, the Latin bi- (bis-). There are few exceptionspl. pori, engl. pore) if the English form is from this rule. If the Latin form is more widelypreferable. used, then the term is treated as a nomen Cross-references are given to terms conservandum; for example, bisaccate isthat are synonyms (the preferable one is found exclusively in the literature and notprinted in bold) or that indicate the opposite the Greek form disaccate.condition (antonyms), e.g., homo- and Micro-heterobrochate. is used to denote features <1 μm: micro- Numbered literature references are reticulate, -echinate, -verrucate, -baculate,given for each term in chapter "Alphabetic -clavate, -gemmate, -rugulate. However,Glossary" and are not necessarily the earliest some possible combinations are not appli-publication in which the term was used. cable; for example, micro-striate or micro- perforate. Striae are not known to be PUNT et al. (2007) provide the basis of the presentterminology. Many terms in palynology were coinedat a time when only LM observations were available. describes a feature <1 μm.Mainly for historical reasons, inconsequent nomen- Terms not listed in the glossary belongclatural applications, enumerations of synonyms, and to fern or moss spores, or are considered asand the same term. obsolete, diffuse or redundant (e.g., multi- During the 20th century questions of terminologybecame more and more problematic. The mainreasons were the greatly increasing number of pub- because plicate pollen grains are alwayslications in palynology, dealing with sometimes insuf- equipped with several to many plicae), orand simultaneously the advent of manifold applied may be a permanent source of confusion (zon-, zona-, zoni-, zono-).authors used their own terminology. The situationbecame worse in the 1970s and 1980s, leading to a "Pollen Terminology. An illustrated Handbook"variety of terminological “schools”. aims to clearly separate the types and Nonetheless, in the 1950s attempts were made classes of pollen. Pollen type is a generalterms more precisely. A deserving, widely accepted term categorizing pollen grains by distinctbut all-too restricted list of pollen morphological terms combinations of characters and is oftenIVERSEN and TROELS-SMITH. Later, KREMP (1968), in his used in connection with a distinct taxonfamous encyclopedia, provided a monumental enu- (e.g., Polygonum aviculare type).meration of all known terms . Being aware of the danger that pollen terminology
  13. 13. GUIDELINES Pollen class2 porate, porate, synaperturate, spiraper-of pollen grains that share a single, dis- turate, lophate, clypeate and plicate. Thesetinctive character. Pollen classes refer topollen units, to aperture form and location, they have a good diagnostic, althoughor to an extremely distinctive ornamen- mostly no systematic, value. In general,tation character. Classes include the terms a pollen grain may belong to more thanpolyads, tetrads, dyads, saccate, inap- one pollen class; in such cases the moreerturate, sulcate, ulcerate, colpate, col- Pistia: plicate - inaperturate, Hemigraphis: 2 "Pollen type" is sometimes (colloquially) plicate - colporate, Typha: tetrads - ulcerate,misused; for example, Croton type, which is a distinct Rhododendron: tetrads - colporate).feature of ornamentation and is correctly termedCroton pattern. GENERAL CHAPTER 9
  14. 14. PALYNOLOGYPalynologyThe Science of Pollen and Spores haploid counterpart of the much larger diploid plant body "as we see it in nature".T he term palynology was coined after a During transport pollen grains are com- written discussion with Ernst ANTEVS and pletely separated from the parent plant and A. Orville DAHL in the Pollen Analysis perfectly adapted for their role – the transferCircular no. 8 by HYDE and WILLIAMS of male genetic material – and are able to(1944) and is a combination of the Greek resist hostile environmental stress on their (male haploid) organisms usually have as variable parameters: the pollen shape and size, the number, type and position of aper- tures and the pollen wall with its extremely diverse structure and sculpture. The char-speech”). acters of these parameters in comparative Palynology is the science of paly- pollen (and spore) morphology and plantnomorphs, a general term for all entities systematics are at least as important as anyfound in palynological samples. A domi- other morphological character of the diploidnating object of the palynomorph spectrum generation.is the pollen grain, the point of origin and the The pollen grains of seed plants andcarrier for the male gametes (sperm cells). the spores of mosses and ferns share many What makes pollen grains so unique? homologies. However, although probablyPollen grains represent an extra generation equivalent, the terminology of spore wallin seed plants, the highly reduced male strata differs, mainly for historical reasons,gametophyte (the enclosing sporoderm from the terms used for pollen grains. Someand the cellular content, consisting of two elements and/or features of spores areor three cells, and the pollen tube). Pollen unknown in pollen grains, e.g., the outermostgrains are therefore not simply parts of a wall layer in many fern spores, called theplant, such as leaves or seeds, but are the perine or perispore. HYDE and WILLIAMS (1944) The right word. Pollen Analysis Circular 8: p. 6 GENERAL CHAPTER 11
  15. 15. PALYNOLOGYA Brief History of Palynology New and better microscopes enabled Hugo von MOHL (1834) and Carl JuliusThe Very Early Beginnings FRITZSCHE (1837) to separate clearly the Assyrians are said to have known the principal layers of the pollen wall and toprinciples of pollination, but it is unclear if publish surveys on pollen morphology ofthey recognized the nature and power of many angiosperm families. The terms pol-pollen itself. Greeks and Romans, and the lenin, exine and intine go back to FRITZSCHE.Middle Ages up to the 16th century did not Johann Heinrich Robert GÖPPERT (1837)contribute substantially, as far as is known. and Christian Gottfried EHRENBERG (1838)The Era of the Light Microscope pollen grains. Eduard STRASBURGER (1882) A comprehensive historical survey is achieved ground-breaking insights intofound in WODEHOUSE (1935) and especially the development and internal structure ofin DUCKER and KNOX (1985). Only the most pollen. Hugo FISCHERimportant scientists can be mentioned here; to summarize the arguments for the phylo-the list is not exhaustive. genetic value of pollen characters. Pollen It was Nehemiah GREW who as early as1662 in his famous work "The Anatomy ofPlants" described the constancy of pollen von POSTform within the same species; in other words,he founded pollen morphology and was the The 20th century up to ca 1960 was domi- nated by the skilful use of the LM, with manypollen. Carl von LINNÉterm pollen (in Latin). During the 18th and a method for analyzing patterns of exinethe early 19th centuries there was consid- organization by light microscopy: focusingerable progress on pollen and the under- at different levels distinct features appearstanding of pollination. For example, Joseph bright (L = Lux) or dark (O = Obscuritas).Gottlieb KOELREUTER (1766), together with Textbooks by Roger WODEHOUSE (1935),Christian Konrad SPRENGEL, the founder of Gunnar ERDTMAN (1943, 1952, 1969), or Knut FÆGRI and Johannes IVERSEN (1950) sum- marized the knowledge on pollen at that time and to a great extent have maintainedimportant part in determining the characters their value. thof the offspring. century paly- SPRENGEL nology as a predominantly basic sciencepores and furrows in the pollen wall; he also “went applied”, giving rise to a series ofdemonstrated the effects of cross pollination,of dichogamy, and distinguished between in use, include aeropalynology, biostratig-entomo- and anemophily. raphy, copropalynology, cryopalynology, Johannes PURKINJE (1830) and Franz forensic palynology, iatropalynology, melisso-Andreas (Francis) BAUER, among others, palynology, paleopalynology, pharmaco-also made substantial contributions. BAUER palynology, among others.and watercolors of pollen, now held in The Era of the Electron Microscopethe Botanical Library of the Natural History As pointed out by KNOX (1984, p. 204):Museum, London. Only a few facsimiles "The terminology applied to the pollenhave been published, e.g., in KESSELER and wall is daunting, especially as it has beenHARLEY (2004). Robert BROWN (1828, 1833) developed from early light microscopy work, BAUER’s earlier and then transposed to the images seen in the transmission and scanning electronorigin of the pollen tube. microscopes".
  16. 16. PALYNOLOGY Electron Microscopy with its two most Nowadays the LM (with basic andimportant types, TEM and SEM, facilitated advanced equipment) and the two mainthe major breakthrough in palynology: the types of EM form an expedient combinationultrastructure of developing and mature of imaging techniques. The LM remains thepollen and the stunning visualization of workhorse method (TRAVERSE 2007; seepollen morphological characters. the compendia by REILLE 1992, 1995 and During the 1950s and early 1960s con- 1998) but is limiting insofar as morphologicalsiderable progress in TEM preparation and structural features at species level, not observable by LM but of diagnostic value,staining) took place. The resolving power of are routinely determinable only by SEM. Thethe TEM was the basis for new information role of SEM as an essential part in illustratingon pollen grain ultrastructure and pollen exine sculpture and ornamentation cannotdevelopment. Nevertheless, EM-based infor- be overrated (HARLEY and FERGUSONmation on ornamentation details of pollen 1990).grains was rare up to the mid-1960s. OnlyTEM-based casts or replica methods wereavailable, all of them with limited resolution A Tentative Outlookand depth of focus (e.g., the single-stagecarbon replica technique; ROWLEY and Nowadays, palynology, as an organ-FLYNN 1966, FLYNN and ROWLEY 1967). The ismic-based science, can serve as an indis-time-consuming and laborious TEM replica pensable tool for various applied sciences,procedures were an obstacle to extensive but clearly also can stand alone as one ofsurveys of pollen morphology and have now the most developed basic sciences.been successfully replaced by SEM (HARLEY In general, compared to the diplontand FERGUSON 1990). the male gametophyte in seed plants Today barely conceivable, the intro- is yet poorly investigated. From at leastduction of SEM in palynology in the second 250.000 plant species onlyca 10 percenthalf of the 1970s was a key innovation in the have been studied with respect to pollen grain morphology, and regarding pollenAdvantages of SEM include the relatively grain anatomy it is much less.simple and rapid preparation methods, the In the 21st century, no matter what roleunsurpassed depth of focus revealing anoverwhelming vividness and power. SEM of science or more probably a bundle ofquantum leap in EM (HAY and SANDBERG of our knowledge of pollen grains and in this context the enhancement of pollen termi-published by THORNHILL et al. (1965) and nology. Modern palynologists, making useERDTMAN and DUNBAR (1966). of LM as well as EM, need for descriptive Since then palynologists have been pro-vided with a plethora of beautiful micro- pollen terminology, covering the richnessgraphs. "The scanning electron micro- of features and the enormous spectrum ofscope has provided a greater impetus characters.to palynology than any other technicaldevelopment during the history of thesubject." (BLACKMORE 1992). GENERAL CHAPTER 13
  17. 17. POLLEN MORPHOLOGY Pollen Morphology A diagrammatic representation of the main morphological features of equatorial plane a palynomorph (preferably pollen grains or spores) is called palynogram. It includes parameters of symmetry, shape and size, aperture number and location, Polarity and Symmetry Mature pollen is shed in dispersal units. The post-meiotic products either remain per- manently united or become partly or usually completely disintegrated. In the latter case the dispersal unit is a single pollen grain, a monad; if the post-meiotic products remain united, dyads (a rare combination), tetrads or polyads (massulae, pollinia) are the result. Pollinaria are dispersal units of two pollinia including the sterile, interconnecting appendage. Tetrad stage orientation of microspores microspore’s center, perpendicular to the polar axis. Therefore, the equatorial plane divides the pollen grain into a proximal and a distal half. Isopolar pollen grains have identical proximal and distal poles, thus the equatorial distal poles plane is a symmetry plane. In heteropolarshaded green pollen grains the proximal and distal halves are different. Pollen shape and aperture location Polarity directly relate to pollen polarity, which is determined by the spatial orientation of the microspore in the meiotic tetrad and can be examined only in the tetrad stage. The of each microspore runs from the left: , orientated towards the tetrad isopolar center, to the distal pole at the outer tetrad right: side. The equatorial plane is located at the heteropolar GENERAL CHAPTER 15
  18. 18. POLLEN MORPHOLOGY The various arrangements of the four (probably restricted to Proteaceae, no per- microspores within permanent or disinte- manent tetrads). grating tetrads depend on the simultaneous or successive type of cytokinesis and on Aperture arrangement the type of intersporal wall formation. The spatial arrangement of microspores after simultaneous cytokinesis is usually a tetra- hedral tetrad. This arrangement is of sys- tematic relevance. The spatial arrangement of microspores after successive cytokinesis leads to different tetrad types without any systematic relevance: planar (tetragonal, linear, T-shaped) or non-planar (decussate or tetrahedral). Fischer‘s lawTetrad arrangement tetrad tetrahedral Fagus sp. Fagaceae, fossil (exceptional Garside‘s law Pollen shape refers to the P/E-ratio: the ratio of the length of the polar axis (P) to the equatorial diameter (E). In spheroidal (or isodiametric) pollen grains the polar tetrad planar axis is ± equal to the equatorial diameter. Pollen grains with a polar axis longer than Typha latifolia Typhaceae the equatorial diameter are called prolate; grains where the polar axis is shorter than the equatorial diameter are described as In pollen grains with three apertures, two oblate. types of aperture arrangement occur after Pollen shape simultaneous cytokinesis (disintegrating or permanent tetrahedral tetrads). Fischer’s law refers to the most frequent arrangement where the apertures form pairs at six points left: oblate in the tetrad (e.g., Ericaceae, permanent mid: spheroidal tetrads). Garside’s law refers to the unusual right: prolate arrangement of apertures where they form groups of three at four points in the tetrad
  19. 19. POLLEN MORPHOLOGY Pollen size an aperture are called inaperturate. Thesize the largest diameter is used. It also gymnosperm pollen, but in gymnospermsdepends on the degree of hydration and the type of aperture usually differs from thatthe preparation method. Because of this in angiosperms, since often a leptoma isand natural variation, a bandwidth desig- present. Note: unless stated otherwise, thenation is recommended. A diameter indi- following sections deal with angiospermcation in the range of, e.g., less than 1 μm is aperture constructs only.not recommended. The polarity of the pollen determines the The use of the following size categories aperture terminology. A circular aperture ismay be helpful: very small (<10 μm), small called a porus if situated equatorially or glo-(10–25 μm), medium (26–50 μm), large bally; if situated distally it is called an ulcus.(51–100 μm) and very large (>100 μm). An elongated aperture is called a colpus if situated equatorially or globally; if situated distally it is called a sulcus. A combinationApertures of porus and colpus is termed a colporus;The many facets of an allegedly simple character colpori are situated only equatorially or glo- bally. Colpi and colpori (colpi and pori) mayNomenclature and Typology be present simultaneously in some taxa; An aperture is a region of the pollen this condition is called heteroaperturate. A circular or elliptic aperture with indistinctthe wall in its morphology and/or anatomy, margins is a poroid.and is presumed to function usually as The number of equatorial aperturesthe site of germination and to play a role (pori, colpi, colpori) is indicated by the pre-in harmomegathy. Pollen grains lacking Pollen grain polarity dicots Bellis perennis Asteraceae polar view equatorial view GENERAL CHAPTER 17
  20. 20. POLLEN MORPHOLOGYPollen grain polarity monocots Allium paradoxum Alliaceaeproximal polar view distal polar view equatorial view equatorial view
  21. 21. POLLEN MORPHOLOGY Tetrad mark in spores hexa- are sometimes used. (Writing numbers 4-porate or tetraporate, 6-colpate or hexa- colpate. "Pollen Terminology. An illustrated Handbook" pollen grain with more than three apertures at the equator is also called stephanoaper- turate (stephanoporate, stephanocolpate, Polypodium sp. stephanocolporate). Pollen grains with Polypodiaceae, fossil globally distributed apertures are called monolete tetrad mark pantoaperturate. polar view The polarity gives rise to the polar and the equatorial view. In dicots there is usually one polar and one equatorial view. In monocots, due to the mostly distal aperture, there are four views: a proximal polar, a distal polar, and two different equatorial views. Proximal germination is unknown in seed plants and is restricted to spores, which germinate at the tetrad mark, the so-called Sphagnum sp. laesura (extensive overview: TRYON and Sphagnaceae, fossil LUGARDON 1991). trilete tetrad mark Pre-(prae-)pollen (microspores of certain polar view extinct seed plants) is characterized by proximal and distal apertures, and by presumed proximal germination, producing motile spermatozoids.Pre-pollen indet. Pteridaceae, fossil trilete tetrad mark polar viewpolar view Apertures are normally covered by an exinous layer, the aperture membrane. Aperture membranes can be ornamented, Cryptogramma crispa e.g., covered with various exine elements, or Pteridaceae can be smooth. In contrast, an operculum trilete tetrad mark is a thick, coherent exine shield and covers the aperture like a lid. In general, aperture membranes are infolded in dry pollen state; after acetolysis the aperture membrane may be lost. GENERAL CHAPTER 19
  22. 22. POLLEN MORPHOLOGY Number, type and position of apertures Cephalotaxus sp. are genetically determined and usually Cephalotaxaceaeexine shedding prior to sometimes vary (e.g., number of apertures in pollen tube formation stephanoaperturate pollen grains). Structure and Function The aperture usually acts as the (exclusive) germination site. Pollen tubes in inaperturate angiosperm pollen are produced without a preformed exit zone. In pollen the exine ruptures during hydration at a spe- cialized region, the tenuitas, ulcus, or papilla in the center of a circular leptoma and is subsequently shed. The intine including the protoplast is released and a pollen tube can be formed anywhere (resembling functionally an inaperturate pollen grain). Furthermore some angiosperm taxa shed the exine fresh pollen in water before pollen tube formation, e.g., in some Annonaceae. Instant pollen tubes During germination, usually a single pollen tube is formed. However, sometimes tube-like structures ("instant pollen tubes") are simultaneously formed in the anther or very quickly in shed pollen immediately after water contact. Their production is interpreted as a pre-germinative process (BLACKMORE and CANNON 1983). Scabiosa caucasica Pollen Wall Dipsacaceae In general, the pollen wall (sporoderm) of seed plants consists of two main layers: the outer and the inner intine. The exine consists mainly of sporopollenins, which are acetolysis- and decay-resistant biopolymers. The intine is mainly composed of cellulose and pectin. Commonly, the pollen wall in apertural regions is characterized by the reduction of exinous structures or by a deviant exine, and a thick, often bilayered Morina longifolia intine. Morinaceae Two layers within the exine are distin- guished: an inner endexine and an outer ektexine. The ektexine consists of a basal
  23. 23. POLLEN MORPHOLOGY supratectal pk elements pk: pollenkitt tectum ektexine sexine exine columellae pk pk foot layer nexine endexine intine tectate atectate tectate atectatefoot layer, an infratectum and a tectum, Costathe endexine is a mainly unstructured,single layer. There are many deviations fromthis principal construction: layers may bethickened, variably structured, or lacking. Inapertural regions the pollen wall is charac-terized by a different exine construction. The terms for the outer, structured,and for the inner, unstructured exine Nyssa sp.layer are widely used in light microscopy, Nyssaceae, fossilbut do not fully correspond to ekt- and equatorial viewendexine, respectively.The angiosperm pollen wall The consists in general oftectum, infratectum and foot layer. The outerlayer, the more-or-less continuous tectum,can be covered by supratectal elements.The infratectum beneath is columellateor granular (a second layer of columellae Austrobuxus nitidus Picrodendraceae, fossilmay form an internal tectum). The foot layermay be either continuous, discontinuous or broken grain, thickening around theabsent. The may be characterized endoapertureas continuous or discontinuous, spongy orcompact, is present overall, only in aper-tures, or even completely absent. Some tenuitas (see "Illustrated Glossary") and costatypical deviations of the wall thickness are (a thickening of the nexine/endexine bor-named with special terms: arcus, annulus, dering an endoaperture). GENERAL CHAPTER 21
  24. 24. POLLEN MORPHOLOGY Pollen terminology in saccate gymnosperm pollen Abies sp. Pinaceae, fossil equatorial view left: corpus right: sacci left: cappa right: leptoma Pollen types in saccate Pinus pollen Pinus sp. Pinaceae, fossil left: polar view right: equatorial viewHaploxylon-pollen-typeDiploxylon-pollen-type
  25. 25. POLLEN MORPHOLOGY Extreme examples of variable ektexine Structure and Sculpturedesign include massive forms lacking almost The internal construction of the pollenreduced forms, or even their complete wall is its structure; ornamenting elements onabsence. the pollen surface (ornamentation) are sum- The typical angiosperm aperture shows a marized under the term sculpture or sculp-thick, bilayered intine. turing. However, it is not always possible to distinguish between structure and sculptureThe Gymnosperm Pollen Wall (e.g., free-standing columellae). The “Gymnosperms” comprise cycads,Ginkgo, conifers and Gnetales. The gym- Ornamentationnosperm pollen wall differs from that in This general term in palynology isangiosperms in two characters: 1. the applied to surface features. All the orna-endexine is always lamellate in mature menting features (areola, clava, echinus,pollen stages. 2. the infratectum is never foveola, fossula, granulum, gemma, plicae,columellate. The four gymnosperm classes reticulum, rugulae, striae, verruca) are arti-exhibit diverse, special constructions of theapertures. a broad morphological series and are therefore regarded as extremely variable;endexine and intine) of the gymnosperm nevertheless, they are important in pollenpollen wall is identical to that of angiosperms. description.A tectum is present in all cycads, in Ginkgo, For practical purposes a distinct featurein all Gnetales, but not in all conifers: in can be subdivided into ornamenting ele-some taxa the tectum is completely lacking ments extending 1 μm in diameter, or if(sculpture elements are situated on the foot micro-.layer). The infratectum is either alveolate or Combinations of sculptural elements aregranular but never columellate. - A special terminology is appliedto saccate pollen, i.e., Pinaceae and because of the high plasticity of its orna-Podocarpaceae. The saccus is a large menting elements. A typical micrographhollow projection from the corpus, the characterizes sculptural elements to a muchcentral body of saccate pollen grains. It is a higher degree.typical deviation of the pollen wall confor- The arrangement of ornamenting ele-mation, composed only by the exine with an ments on the pollen surface is very oftenalveolate infrastructure. Most frequently, two disparate, particularly in apertural regions.sacci are present, in some taxa even three, Pollen coatings like pollenkitt or tryphineor only a single one. Saccate pollen grains may obscure the ornamentation.show on the proximal side of the corpus aregion termed cappa, and on the distal sidea thinned region, the leptoma. Harmomegathy In Pinus two pollen types are recognized Harmomegathic Effect (Wodehouse Effect)as of systematic value. The -pollen-type is characterized by pollen grains All living pollen grains are able to absorbwith broadly attached half-spherical air and release water; thus, each living grainsacs – in LM the leptoma shows remarkable exists in two morphologically differentthickenings (black spots). The - states: the dry and the hydrated condition.pollen-type is characterized by pollen Harmomegathic mechanisms, e.g., infoldinggrains with narrowly attached, spherical of the pollen wall, accommodate theair sacs - the leptoma does not show any change of the osmotic pressure in the cyto-thickenings. plasm during hydration or dehydration. GENERAL CHAPTER 23
  26. 26. POLLEN MORPHOLOGYHarmomegathic effect Cistus creticus Cistaceae left: spheroidal right: dry pollen prolate, lobate Galium rotundifolium Rubiaceae left: oblate right: dry pollen prolate, lobate Vriesea pabstii Bromeliaceae left: oblate right: dry pollen boat-shapedLamiastrum montanum Lamiaceae left: spheroidal right: dry pollenprolate, outline elliptic
  27. 27. POLLEN MORPHOLOGY The main purpose of the harmomegathic Infolding of the pollen wall after ace-effect is to protect the male gametophyte tolysis is mostly not comparable with that inagainst desiccation during pollen presen- dry state.tation and dispersal, and is often related to The harmomegathic effect dependspollination biology. predominantly on the various characters of In mature anthers, pollen is turgescent the pollen wall. Several pollen features (har-before shedding. After anther dehiscenceand during pollen presentation, water loss the mode of infolding and cannot betakes place and the pollen grain becomes considered separately:typically infolded, depending on aperture — apertures (the most important char- acter): their position, number and form.thinnings or thickenings. The pollen grain — pollen wall structure: thinned or thick-in proper dry state represents the genuine ened regions; in particular, internalharmomegathic effect and its shape is very girdles or endoapertures. If the ektexineoften typical for a family and/or genus and is is considerably reduced, its role is takentherefore of systematic relevance. over by other wall strata, namely, by a The harmomegathic effect is to some thick endexine or intine. On the otherdegree reversible. Rehydrated pollen with hand, if the exine is extremely rigid,water uptake at the stigma, or under labo- then the harmomegathic effect is onlyratory conditions, is again turgescent and marginal.largely recalls the shape before shedding. — ornamentation type.A second dehydration does not necessarily — pollen size: small pollen grains withresult in the typical dry shape but, if pollen thin walls exhibit a lesser degree of - infolding.athic effect can be induced several times in — pollen coatings: if abundant, pollenthe same way. In the case of thin walls, the coatings act as an insulating layer orsusceptible internal structure may become sheath against desiccation.irreversibly damaged, and the harmomeg- Terms used for common phenotypes ofathic effect may result in differing shapes, dry pollen include: apertures sunken, boat-often randomly. shaped, cup-shaped, interapertural area The harmomegathic effect is also infolded, irregularly infolded, not infolded.observed in pollen taken from herbarium In addition, technical terms such as, e.g.,material, and to some degree in fossil barrel-like, disk-like, or kidney-like might bematerial (HALBRITTER and HESSE 2004). helpful for an adequate description. GENERAL CHAPTER 25
  28. 28. WHY DO WE NEED CATEGORIES?Why Do We Need Categories?N ature itself neither needs catego- giving rise to a seamless transition between rization nor has any knowledge neighboring characters or to a combination of categories. However, for the of characters.scientist, categories are essential for classi- Seamless transitions between relatedfying natural characters in their diversity, for gemmate pollen and its “neighbor” clavatesystematic order. Nevertheless, categories pollen. Both types of ornamentation are very variable in shape and size and rather rare inindividual or collective convention, mostly their typical form.not by nature. Combination of ornamenting characters In addition to the theoretical concept, is very common. Often, the ornamentationcategorization always depends on the is composed of two or more characters,manner in which a character is perceived: such as reticulate and foveolate, or a com-i.e. on the visibility of a character, and/ bination of echinate and perforate (for examples see Illustrated Glossary). From thegreatly depends on the technical equipment observer’s viewpoint it is desirable to nameand method(s) used, as well as on the sub-jective interpretation of character(s)1. Thus, order: in the case of two or more combined characters, the most eye-catching, prom-standardize. A well known example is pollen inent character (the “leading term”) shouldsize2. However, depending on the prepa-ration method(s), the pollen sample may For example, in Aristolochia, the pollenshow pollen grains of one and the same grain surface bears very prominent verrucaecategory (pollen size categories: see "Pollen Combination of ornamenting charactersMorphology"). Moreover, sometimes the sizeof pollen grains is found just at the boundary Aristolochia arborea Aristolochiaceaebetween two adjacent pollen size cate-gories. Placing the pollen grain in one of thesize categories therefore depends entirelyon the material, the preparation method(s)and the observer’s evaluation. Characterization of pollen ornamen- - inaperturate, spheroidaltions of basic ornamentation characters or verrucate, perforatecombinations of different characters usually 1 To be successful in characterization considerthe following hints: be familiar with good microscopepractice. The microscope, LM or EM, should be in -quately high, but any enlarging of details beyond ashould be achieved. Quality of sample preparation isan all-too-often underrated item. 2 The importance for dimension measurementsis acknowledged but there is no need for decimalplaces, since dimensions vary considerably accordingto different treatments, as already shown by REITSMA surface detail(1969). verrucae and perforations GENERAL CHAPTER 27
  29. 29. WHY DO WE NEED CATEGORIES? (the “leading term”) combined with a great and perforations. In some taxa the micro- number of small perforations. Such ornamen- echini are more prominent (microechinate, tation therefore should be called verrucate, perforate), in others the perforations (per- perforate. forate, microechinate). There are also taxa, Sometimes it is debatable which feature where the two features are on a par (micro- represents the “leading term”. As a sample, echinate and perforate). Micrographs elu- in Caryophyllaceae, there are numerous, cidate the actual situation at a glance. more-or-less regularly arranged microechini Combination of ornamenting characters Stellaria media Caryophyllaceaemicroechinate, perforate Caryophyllaceae microechinate and perforate Silene succulenta Caryophyllaceaeperforate, microechinate
  30. 30. WHY DO WE NEED CATEGORIES? Distinct areas of the pollen grain surface — In Sideritis montana polar and inter-may show different ornamentation types. apertural areas are perforate to fove- The type of ornamentation may be irreg- olate, apertural regions are psilate.ularly distributed over the pollen surface, or — In Salvia austriaca the polar area isrestricted to distinct surface regions. psilate to perforate, all other areas Some examples may elucidate this being bireticulate.feature: — is an example where — The polar region of Fallopia convolvulus the polar areas are reticulate, while in is psilate to perforate, apertural regions equatorial view the ornamentation is are microechinate. striato-reticulate. Combination of ornamenting characters left: Fallopia convolvulus Polygonaceae polar view right: Sideritis montana Lamiaceae polar view Salvia austriaca Lamiaceae left: polar view right: equatorial view Solanaceae left: polar view right: equatorial view GENERAL CHAPTER 29
  31. 31. WHY DO WE NEED CATEGORIES? Interpretation ofornamenting characters Sometimes it depends on the individual researcher to interpret ornamenting fea- Sanchezia nobilis Acanthaceae tures: for example, to call Sanchezia nobilis (Acanthaceae) plicate and striate, but also reticulate? And should the rod-like ele- ments be termed clavae, or free-standing columellae? Moreover, is the aperture to be interpreted as a porus or a colporus? A special case deserves attention. In heterostylous species two different pollenoblique equatorial view types occur. Size and number of apertures, e.g., in Primula, or the ornamentation e.g., in Linum, may differ. For better illustration and Primula styled and short-styled, pin and thrum morphs) is shown here. In the short-styled-morph pollen is baculate, and the long-styled- morph clavate. In Primula veris the pollen of the short- styled morph (thrum) is larger and has more surface detail apertures than the pollen of the long-styled morph (pin). Heterostyly Linaceae short-styled morph baculate long-styled morph clavate
  32. 32. WHY DO WE NEED CATEGORIES? Heterostyly wart-like element more than 1 μm, broaderPrimula veris than high) would describe the ornamen-Primulaceae tation in a better manner. High SEM magni- granules (structure or sculpture elements of different size and shape; smaller than 1 μm). A typical rugulate left: ornamentation at SEM level is present in, short-styled e.g., Sanicula, which is quite dissimilar to the morph ornamentation seen in Ulmus at SEM high right: resolution level. long-styled morph This is a good place to mention interpre- tative pitfalls. The denotation of ornamen- tation frequently depends on the optical Terms derived from LM level cannot always . point resolution. Very many (paleo-)paly- A classical example: Ulmus pollen at LM nologists have relied on LM only. Even low- level was described as rugulate (rugulae: elongated exine elements longer than pollen grains unequivocally which are in LM 1 μm; irregularly arranged). In low SEM mag- very similar (for examples and discussion see verrucate (verrucae: FERGUSON et al. 2007). Ornamentation in LM and SEM view Ulmus laevis Ulmaceae left: rugulate (LM) right: verrucate (SEM) left: Ulmus laevis Ulmaceae surface detail verrucate, granulate right: Sanicula europaea Apiaceae surface detail rugulate GENERAL CHAPTER 31
  33. 33. WHY DO WE NEED CATEGORIES? A second example is scabrate, a term “granules” depends on the much better used for light microscopy only, describing - cation, where a “granulate ornamentation” shape and of a size close to the resolution emerges as, for example, a great number of limit of the light microscope. As an example, very small spines (microechini), the pointed Juglans pollen is scabrate in LM and (with some reservation) under low power SEM, but The allegedly granulate ornamentation of microechinate at high resolution SEM. many Poaceae is in fact microechinate; see "Illustrated Glossary". Ornamentation in Another interpretative pitfall does LM and SEM view Juglans sp. Ornamentation sometimes depends entirely Juglandaceae or to a high degree on the preparation method. A striking example is the presence or complete absence of distinct echini on pollen of many Araceae/Aroideae: fresh or dry material exhibits a distinct echinate ornamentation, whereas after acetolysis the echini are completely removed. These polar viewscabrate to psilate (LM) echini are composed of polysaccharides (singular exception) and lack sporopollenin completely. The pollen is then – correctly – called psilate (WEBER et al. 1999). An example for different possible inter- pretations in relation with a differing degree of hydration is Trichosanthes anguina (Cucurbitaceae), where the ornamentation The overview micrograph on the left shows a fully turgescent pollen, and on the right a less turgescent one. The ornamentation microechinate (SEM) can be described as either areolate, or ver- rucate or even fossulate. Perforations are clearly visible in fully turgescent pollen only. Another example for different interpreta- So ornamentation should better be called tions in LM and SEM is the term psilate. Many verrucate and perforate. pollen grains are psilate in LM view, but show a distinct ornamentation at SEM level. For Hydration example, in LM view pollen of Allium is psilate (see "Illustrated Glossary" – psilate), in SEM view it is striate and perforate (see "Pollen Morphology"). The term granulate (describing minute and of a size close to the resolution limit Trichosanthes anguina of the LM) is adequate for features at low Cucurbitaceae pollen grains of different state resolution a more adequate description of hydration is often possible. The actual shape of such
  34. 34. WHY DO WE NEED CATEGORIES? Hydration Trichosanthes anguina Cucurbitaceae left: surface detail areolate right: surface detail verrucate, perforateeven depends on peculiarities during pollen Palynology is the nomenclature question.development. Ubisch bodies are usually In Paleopalynology, for morphotaxafound as isolated particles between pollen often form-generic names are used.grains, or lining the mature locular wall The nomenclature of form-genera is(HUYSMANS et al. 1998, HALBRITTER andHESSE 2005, VINCKIER et al. 2005; equivalents not known at all (e.g., Oculopollis andare found in ferns: LUGARDON 1981). Pollen Trudopollis from the Normapolles group), orgrains of Cupressaceae and Taxaceae are "half-natural", when reference to an extantoften equipped with adhering (adnate) taxon is suspected but not proven (e.g.,Ubisch bodies, which are - strictly speaking Liliacidites). However, if reference to extant taxa is certain, then a "natural" nomen-(for example Chamaecyparis or Juniperus, clature is possible (e.g., Quercus sp.).see "Illustrated Glossary"). Nomenclature in Paleopalynology Oculopollis sp. Trudopollis sp. GENERAL CHAPTER 33
  35. 35. POLLEN DEVELOPMENTPollen DevelopmentMicrosporogenesis and Microsporogenesis tetradsMicrogametogenesisT he unicellular pollen grain represents the microspore of seed plants, the multicel- lular pollen grain the male gametophyticgeneration of seed plants and is sourceand transport unit for the male gametes (ortheir progenitor cell). The development ofa pollen grain includes (micro)sporogenesis[1-4] and (micro)gametogenesis [5-9]. Scrophularia nodosaMicrosporogenesis starts with the differenti- Scrophulariaceaeation of microspore mother cells (MMC) resp. tetrad tetrahedralpollen mother cells (PMC) [1]. These diploid iodidcells become enclosed by a thick callosewall and undergo meiosis, forming a tetradof four haploid microspores, each encasedin a second callose wall insulating them fromeach other and from the surrounding diploidtapetal cells [2]. Cytokinesis following meiotic nucleardivisions is accompanied by the formationof cleavage planes determined by the con- Spiraea sp. Rosaceaespindle axes. In the case of successive tetrad tetrahedralcytokinesis PA+TCH+SPand second meiotic divisions leading to theformation of various tetrad types (see "PollenMorphology"). During simultaneous cytoki-nesis the cleavage planes are formed con-currently after the second meiotic division;in this case microspores are arranged in atetrahedral tetrad. Pollen wall formation starts when themicrospores are still arranged in tetrads Orobanche hederae Orobanchaceaeconsists of the deposition of the , tetrad planar KMnO4surface of the microspores. The primexineforms a template where sporopollenin pre- forming a single layer of cells circumscribing sporopollenin are subse- the loculus. Tapetal cells are specializedwall. Apertures are developed where the their cellular organization and are reab-endoplasmic reticulum has prevented the sorbed. Two types of tapetum are known:deposition of primexine. the secretory (or glandular or parietal) and During pollen formation and maturation the amoeboid (or periplasmodial). In thethe tapetum plays an important role, usually secretory type (e.g., in Apiaceae) the tapetal GENERAL CHAPTER 35
  36. 36. POLLEN DEVELOPMENT The is followed byphysiological functions. In the amoeboid an asymmetric cell division, leading totapetum type (e.g., in Araceae) cells lose the formation of a smaller generative celltheir individuality in an early developmental and a larger vegetative cell [6]. When thestage by degeneration of the cell walls. The generative cell is formed it is pressed againstprotoplasts then fuse and intrude into the the pollen wall; it later separates and islocule where they enclose the pollen grains. then located within the cytoplasm of the The tapetum plays an important role vegetative cell [7]. After detachment, theduring several stages of pollen development. generative cell, which is sparse in organelles,Its main function is the nourishment of themicrospores but it also synthesizes enzymes spindle-shaped (the shape of the generative(e.g., callase), exine precursors, pollen nucleus changes correspondingly). Duringcoatings, forms Ubisch bodies and viscin the second pollen mitosis, which is followedthreads (both equivalents to the ektexine). by a symmetric cell division, the generativeThe most striking material produced bythe tapetum is pollenkitt (and tryphine in stage of gametophytic development [8]. InBrassicaceae, elastoviscin in Orchidaceae),a sticky, heterogeneous material composed the pollen grains are three-celled at the time -teins and polysaccharides. Pollenkitt serves pollen grains are shed from the anther atnumerous functions: for example, keeping a two-celled stage. In the latter case thepollen grains together during transport; second pollen mitosis takes place in theprotecting pollen from water loss, ultra- pollen tube, after germination of the pollenviolet radiation, hydrolysis and exocellular grain onto a stigma or a correspondingenzymes; maintaining sporophytic proteins structure [9].inside exine cavities. Microgametogenesis in gymnosperms Microgametogenesis in angiosperms includes several mitotic divisions. Normally, pollen grains of gymnosperms are multi-leading to the formation of the male gametes celled at anthesis, and comprise prothallial(sperm cells). Gametogenesis starts with for- cell(s), a large tube cell and a small anth-mation of a central vacuole within the uni- eridial cell. The tube cell becomes a pollennucleate microspore, pushing the nucleus tube; the antheridial cell undergoes divisiontowards the wall [5]. As long as the nucleus into the stalk cell and the spermatogenousis in a central position within the cytoplasm,the cell is called a microspore [4]. With the gametes (sperm cells or spermatozoids).dislocation of the microspore nucleus thecell becomes the young pollen grain.
  37. 37. POLLEN DEVELOPMENT Pollen development in angiospermsGENERAL CHAPTER 37
  38. 38. INHERENCE OF MISINTERPRETATIONInherence of Misinterpretation Tripartite featuresI nvestigation of recent and fossil pollen material often reveals interesting features that in some cases may be misinterpreted.Selected examples are various tripartitesurface features that may actually be oronly resemble apertures. Other examplesare conspicuous, even eye-catching orna-mentation features that are potentially mis- Abies cephalonicainterpreted as apertures, while the genuine, Pinaceaevery inconspicuous apertures might be proximal polar viewoverlooked. The study of a morphological indistinct impression markseries can be of help clarifying ambiguousfeatures.Tripartite Features Mature pollen of conifers, such as Abies,Larix and Pseudotsuga, often shows proxi-mally a Y-shaped bulge, comparable to a Larix sp.tetrad mark, which is called an impression Pinaceae, fossilmark (HARLEY 1999). The mark results from proximal polar viewthe close proximity of the four pollen grains Y-shaped impression markat the post-meiotic tetrad phase and isretained afterwards. Impression marks arealso found in palm pollen. Note: the termtetrad mark is restricted to spores, where it isthe germination feature, the impression markof pollen grains is no germination feature.are not comparable to that in gymnosperms.In recent and fossil Sapindaceae a three-armed feature (more precisely a triangle) Larix sp. Pinaceae, fossilis found. Cardiospermum has a narrow tri-angle (tenuitas) proximally, whereas other Y-shaped impression mark Tripartite features Cardiospermum corindum Sapindaceae tricolporate left: equatorial view right: proximal pole with triangular area GENERAL CHAPTER 39