762 Palynology

Physical properties of palladium 0.03 to 0.11 in. (0.7 to 3 mm) in length, that live under stones, in caves, and in other moist, dark Property Value places. The elongate body terminates in a slender multisegmented flagellum set with setae. In a curi- Atomic weight 106.4 Naturally occurring isotopes 102 (0.96) ous reversal of function, the pedipalps, the second (percent abundance) pair of head appendages, serve as walking legs. The 104 (10.97) first pair of true legs, longer than the others and set 105 (22.23) 106 (27.33) with sensory setae, has been converted to tactile ap- 108 (26.71) pendages which are vibrated constantly to test the 110 (11.81) substratum. See ARACHNIDA. Willis J. Gertsch Crystal structure Face-centered cubic Thermal neutron capture cross 8.0 section, barns Density at 25 C (77 F), g/cm3 12.01 Melting point, C ( F) 1554 (2829) Palynology Boiling point, C ( F) 2900 (5300) Specific heat at 0 C (32 F), cal/g 0.0584 The study of pollen grains and spores, both extant Thermal conductivity, 0.18 and extinct, as well as other organic microfossils. (cal cm)(cm2 s C) Although the origin of the discipline dates back to Linear coefficient of thermal 11.6 expansion, (µin./in./)/ C the seventeenth century, when modern pollen was Electrical resistivity at 0 C (32 F), 9.93 first examined microscopically, the term palynology µΩ-cm was not coined until 1944. Young’s modulus, lb/in.2, static, at 16.7 106 20 C (68 F) The term palynology is used by both geologists Atomic radius in metal, nm 0.1375 and biologists. Consequently, the educational back- Ionization potential, eV 8.33 ground of professional palynologists may be either Binding energy, eV 3.91 Pauling electronegativity 2.2 geologically or biologically based. Considerable over- Oxidation potential, V 0.92 lap exists between some areas of the fields, how- ever, and many palynologists have interdisciplinary training in both the earth and life sciences. Palynol- gold. Other consumer applications are in automo- ogists use a range of sophisticated methodologies bile exhaust catalysts and jewelry. See INTEGRATED and instruments in studying both paleopalynologi- CIRCUITS. cal and neopalynological problems, but the utiliza- Palladium supported on carbon or alumina is used tion of modern microscopy is fundamental in both as a catalyst for hydrogenation and dehydrogenation subdisciplines. in both liquid- and gas-phase reactions. Palladium Palynologists study microscopic bodies generally finds widespread use in catalysis because it is fre- known as palynomorphs. These include an array of quently very active under ambient conditions, and it organic structures, each consisting of a highly resis- can yield very high selectivities. Palladium catalyzes tant wall component. Examples include acritarchs the reaction of hydrogen with oxygen to give water. and chitinozoans (microfossils with unknown affini- Palladium also catalyzes isomerization and fragmen- ties), foraminiferans (protists), scolecodonts (tooth tation reactions. See CATALYSIS. and mouth parts of marine annelid worms), fun- Halides of divalent palladium can be used as homo- gal spores, dinoflagellates, algal spores, and spores geneous catalysts for the oxidation of olefins (Wacker and pollen grains of land plants. This discussion will process). This requires water for the oxygen transfer focus on the palynomorphs produced by land plants, step, and a copper salt to reoxidize the palladium beginning with a general description of pollen grains back to its divalent state to complete the catalytic and spores and then providing an overview of the cycle. See HOMOGENEOUS CATALYSIS; TRANSITION EL- primary areas of investigation within neo- and pa- EMENTS. D. Max Roundhill leopalynological subdisciplines. See MICROPALEON- Bibliography. G. W. Gribble and J. J. Li, Palladium TOLOGY. in Heterocyclic Chemistry, Pergamon, 2000; F. R. Pollen Grains and Spores: An Overview Hartley, Chemistry of Platinum and Palladium, 1973; J. Tsuji (ed.), Palladium in Organic Synthe- Spores and pollen grains are reproductive structures sis (Topics in Organometallic Chemistry), Springer, and play a paramount role in the life history of 2005; J. Tsuji, Palladium Reagents and Catalysts: land plants. The sporophyte generation of nonseed- New Perspectives for the 21st Century, Wiley, 2d ed., bearing plants (ferns, for example) produces single- 2004. celled spores that ultimately germinate to grow into the haploid gametophyte generation. Homosporous species produce a single type of spore, whereas heterosporous species produce two spore types. Microspores germinate and grow into “male” sperm- Palpigradi producing microgametophytes, and megaspores de- An order of rare arachnids comprising 21 known velop into “female” egg-producing megagameto- species from tropical and warm temperate regions. phytes. The gametophytes of most nonseed plants American species occur in Texas and California. All are multicellular and proliferate outside the spore are minute, whitish, eyeless animals, varying from wall during development. All seed-bearing plants Palynology 763

(a) (b) (c) (d)

(e) (g)

(f)

(h) (i) (j)

Fig. 1. Pollen and spore morphology, (a, b) Spores. (c–j) Pollen grains. (After Y. Iwanami, T. Sasakuma, and Y. Yamada, Pollen: Illustrations and Scanning Electron Micrographs, Kodansha and Springer, 1988)

(gymnosperms and angiosperms) are heterosporous, cellulose and pectin; as such, it is similar to most and their pollen represents the microgametophyte other plant cell walls. The outer wall, or exine, is generation. Pollen grains consist of just three to a principally composed of sporopollenin, a chemically few cells, and these remain within the microspore enigmatic macromolecule that is resistant to biolog- wall, where they originally developed. See REPRO- ical decay and geological degradation. The exine is DUCTION (PLANT). further characterized by several ultrastructural lay- Spores and pollen grains are formed in multiples ers and an array of sculptural elements. It is the very of fours following meiotic divisions. During develop- presence of the exine that allows for the spectac- ment, the four are united into a tetrad that, in most ular preservation of pollen and spores in the fossil plants, subsequently dissociates into the four individ- record. ual propagules. In nonseed plants, each spore com- monly bears a mark on its proximal surface indicating Neopalynology where it made contact with the others at the center This discussion focuses on several subdisciplines of the tetrad. In most spores this external mark is of neopalynology, including taxonomy, genetics, either straight or Y-shaped (Fig. 1), and it is typically and evolution; development, functional morphol- characterized by a suture that spans the spore wall ogy, and pollination; aeropalynology; and melissopa- and is the site through which germination occurs. lynology. In contrast, most pollen grains lack sutures and ger- Taxonomy, genetics, and evolution. Taxonomy and minate through thin areas in the wall, or apertures. systematics are concerned with classifying organ- Apertures are typically located in either a distal or isms into hierarchical ranks that reflect evolutionary, an equatorial position. Common aperture types in- or phylogenetic, relationships. Pollen and spore mor- clude elongate furrows, pores, and furrows with a phology is important systematically, with particular central pore. Aperture type, number, and position features characteristic of different taxonomic ranks. are important systematic characters by which fossil For example, distinguishing characters may include and modern taxa can be compared. Other descrip- aperture type for a family, different ornamentation tively and systematically relevant characters include patterns for its subordinate genera, and variation in size, shape, presence and structure of air bladders, exine ultrastructure for its congeneric species. Pa- surface ornamentation, and wall ultrastructure. See lynological characters are especially useful system- POLLEN. atically when evaluated in conjunction with other The wall of spores and pollen grains is known col- characters (for example, plant morphological and lectively as the sporoderm (or “skin of the spore”) molecular characters). Cladistics is one technique and actually consists of two distinct walls (Fig. 2). that has employed such an integrated approach. The inner wall, or intine, is primarily composed of Cladistic analyses are based on numerical algorithms 764 Palynology

intine exine apertural region

microgametophyte

aperture

10 ␮m (b)

sculptural rods tectum

foot layer columellae

intine (a) 10 ␮m (c) endexine 1 ␮m

Fig. 2. Pollen morphology and sporoderm ultrastructure of Cabomba caroliniana (Cabombaceae), a modern water lily and primitive flowering plant. (a) Distal view of grain. (b) Cross section through the entire grain. (c) Cross section through the sporoderm.

that produce trees, or cladograms, demonstrating the sculptural surface ornamentation of the outer phylogenetic lineages among the organisms exam- pollen wall, the exine, are dependent upon the de- ined. See PLANT EVOLUTION; PLANT TAXONOMY. positional pattern of the chemical that makes up the Assessing pollen flow is another approach used exine, sporopollenin. Sporopollenin is primarily de- to study evolutionary questions. Because pollen is rived from the developing pollen grain, but can also the sperm-producing generation, the patterns and be released from a specialized layer of cells known as rates of pollen transfer are important factors in de- the tapetum, which surrounds the developing pollen termining the spread of genes throughout a popu- grains. The inner pollen wall, or intine, is synthesized lation. Competition can occur among reproductive last. organs, and pollen flow may be influenced by the When the tapetum breaks down, or undergoes correlation of pollen characters with those of re- programmed cell death (apoptosis), it also releases productive organs. For example, the ovulate, or fe- several proteins, lipids, and other substances that be- male, cones of some gymnosperms, such as pines, come isolated within the spaces and on the surface are aerodynamically adapted for entraining airborne of the developing pollen wall. In flowering plants, pollen grains that themselves have particular aero- many tapetum-derived proteins function as recogni- dynamic characters, such as extended air bladders. tion molecules in pollination systems and are impor- Furthermore, competition may exist among individ- tant in determining the extent of compatibility of ual pollen grains. Following pollination in some flow- a particular pollen grain on a floral stigma. Other ering plants, intraspecific variation can result in the pollen-derived proteins become stored within the grains of a more reproductively fit plant producing intine and may also be involved in compatibility- faster-growing pollen tubes than others of the same incompatibility reactions. Several tapetal lipids also species. See POPULATION GENETICS. play important roles in pollination. Pollenkitt, for ex- Development, functional morphology, and pollination. ample, functions in pollen adhesion and acts as a In most seed plants, a layer of callose, a carbohy- visual and olfactory attractant. Additionally, pollen drate, surrounds the entire tetrad and separates each morphology and exine architecture may be corre- immature pollen grain during development. Forma- lated with pollination systems. For example, the tion of the pollen wall and positioning of the aper- pollen of wind-pollinated plants is typically smooth, ture begin while each grain is encased within the has a thin exine, lacks pollenkitt, and may have air callose layer. Both the internal ultrastructure and bladders, whereas that of animal-pollinated plants is Palynology 765 commonly highly ornamented and bears pollenkitt. tant in the fields addressed below. In situ grains See FERTILIZATION. occur within intact, megafossil reproductive organs Aeropalynology. Aeropalynology is the study of (like flower anthers); as such, morphological data pollen grains and spores that are dispersed into from the parent plant are available and provide for the atmosphere. Wind-pollinated plants typically better systematic evaluation. Studies of in situ pollen produce copious amounts of pollen, thereby enhanc- or spores also afford the opportunity to evaluate and ing successful pollinations. The abundance of air- interpret fossils in a biological context. For example, borne pollen commonly causes allergic reactions in developmental information can be inferred by exam- a large proportion of the human population. Polli- ining pollen-containing organs preserved in various nosis, allergen rhinitis or hay fever, is elicited when ontogenetic stages, and ancient pollination events, allergen-containing pollen makes contact with the such as pollen germination and pollen tube growth, mucous membranes lining the nose, trachea, or can be assessed when grains are recovered on re- bronchi, and the cornea of the eye. Allergens leach ceptive structures. These types of data allow pale- out of the pollen and bind to immunoglobulin E obotanists to better understand and reconstruct the antibodies. The antibodies are linked to mast cells complete life history of fossil plants. that release histamine and other inflammatory chem- Past vegetation and climate reconstruction. A sig- icals, producing allergy symptoms. Ironically, the al- nificant focus of palynology involves reconstructing lergens that induce pollinosis include many of the the Earth’s vegetational history since the last major same compatibility-incompatibility, recognition pro- glaciation event, within the past 10,000 years, or dur- teins involved in pollination. ing the Holocene Epoch. This area of postglacial pa- Knowledge of the temporal, seasonal, and environ- lynology is known as pollen analysis and primarily mental aspects of pollen dispersal is also important includes the study of palynomorphs from lake sedi- in understanding and avoiding hay fever. Flowering ments and peat deposits. Sediments are obtained by time and season vary widely for different plants, and several methods (mostly core sampling), and paly- the release of airborne pollen is typically inhibited by nomorph diversity, distribution, and abundance are high humidity or rain. To monitor risks of pollinosis, plotted on pollen profiles. Pollen analysis can pro- the diversity and quantity of various pollen types are vide historical information regarding both individual assessed by filtering the air throughout the year. See taxa and larger plant communities, including data ALLERGY; ANTIBODY. about vegetational succession. Such analyses must Melissopalynology. Honeybees are the primary consider all possible sources of palynomorphs and pollinators of many flowering plants. Honeybees, take precaution during sample preparation to avoid and other bees, visit flowers to collect nectar and contamination with extant pollen because many large quantities of pollen (pollen loads), both of modern taxa may have also existed in the Holocene. which are used as food sources for developing lar- However, because of the excellent preservation po- vae. Melissopalynologists analyze bee pollen loads tential of key palynological characters, such as those and the pollen component within honeys. Although described above, fossil pollen grains yield a high de- bees primarily produce honey from nectar, 1 mL gree of taxonomic resolution. of honey may contain more than 20,000 pollen Because many plants inhabit areas exhibiting grains. The foraging behavior of bees can be de- particular environmental regimes and have limited termined by microscopically examining their pollen geographic distributions, palynological analyses con- loads and taxonomically identifying the pollen con- tribute to an understanding of paleoclimatic condi- stituents. Honey purity can also be assessed by exam- tions. For example, a palynoflora may be indicative ining the diversity of pollen grains found within the of source vegetation occupying a polar, temperate, honey. subtropical, or tropical habitat. Therefore, palyno- logical information can also be used in conjunction Paleopalynology with other megafossil indicators of climate, such as The main fields of study within paleopalynology are tree ring data. See POSTGLACIAL VEGETATION AND CLI- discussed below. The areas addressed involve pale- MATE. obotany; past vegetation and climate reconstruction; Geochronology and biostratigraphy. Palynological geochronology and biostratigraphy; and petroleum analyses play a significant role in age determina- and natural gas exploration. tions of rocks, or geochronology. Dating the geologi- Paleobotany. Fossil pollen and spores typically cal ages of palynomorph-bearing rocks is dependent consist of only fossilization-resistant exine layers. upon knowledge of the stratigraphic ranges of ex- However, these propagules did at one time contain tinct plant groups. Because different plant groups both gametophytic cells and pectocellulosic intines, are known to have restricted geological time ranges, and functioned in a similar way to that of their ex- the pollen and spores produced by their plants are tant counterparts. Fossil pollen and spores can be characteristic of particular ages and may serve as distinguished into two categories based on the gen- index fossils. Comparisons may be made against eral way in which the palynomorphs are preserved. well-established reference palynomorphs, or index Sporae dispersae grains are those occurring within fossils, and palynofloras. Palynological dating tech- sediments in a dispersed condition; in most cases, niques are especially useful when correlated with information about the parent plants is unavailable. ages of rocks that have been radiometrically dated. Investigation of dispersed grains is especially impor- See FOSSIL; INDEX FOSSIL. 766 Pancreas

Comparisons of palynomorphs within a given rock physiology, and biochemistry of the vertebrate pan- section from one site with those of units from other creas. See CARBOHYDRATE METABOLISM. localities are important in documenting stratigraphic similarities among the rock sections, even if the Anatomy sections exhibit different thicknesses and litholo- The pancreas is a more or less developed gland con- gies. When the occurrence, diversity, and abundance nected with the duodenum. It can be considered as of fossils (palynomorphs, megafossils, or both) are an organ which is characteristic of vertebrates. used to correlate geographically separated rock se- Chordates and lower vertebrates. In Branchio- quences, this is known as biostratigraphy. Histori- stoma (Amphioxus) a pancreatic anlage is found in cally, biostratigraphic correlation has provided sup- young stages as a thickening of the gut caudal to the porting evidence for continental drift theory. For liver. The pancreas of cyclostomes, arising from the example, some present-day continents, such as gut epithelium or from the liver duct, seems to be Antarctica, Africa, and India, have distinguishing in- purely endocrine; it degenerates in later stages of de- dex fossils, of various ages, that are present on these velopment. continents and absent from others. These intercon- A true pancreas is found in selachians, with an ex- tinental correlations are supportive of the previous ocrine portion opening into the intestine and an en- existence of the single Southern Hemisphere land- docrine portion represented by cellular thickenings mass known as Gondwana. See PALEOGEOGRAPHY. of the walls of the ducts. Petroleum and natural gas exploration. Economi- Higher vertebrates. The ganoids (palaeopterygian cally, palynological biostratigraphy is an important fishes) show a diffuse pancreas—its principal mass technique used in the exploration for natural gas lying between the gut and the liver—in which typical and petroleum. Biostratigraphic correlations in this islets of Langerhans are observed. The pancreas of context are conducted on a smaller scale, typically teleosts is either of the massive or dispersed type. within an existing oil field. Besides identifying the Many species, such as the pike, show enormous islets locality, it is critical to determine the appropriate of Langerhans, 10 5 mm, from which J. McLeod × level at which to drill. For this endeavor, the paly- (1922) extracted insulin. The existence of a pancreas nologist is not necessarily interested in references of in dipneusts, such as Protopterus, is doubtful. depth, but in important palynological indicators of The compact pancreas of the amphibians is lo- known oil and gas production levels. In addition to cated in the gastrohepatic omentum and extends to- the identification of key index fossils, a color eval- ward the hilus of the liver and along the branches uation is relevant. Following standard preparations, of the portal vein. It develops from three anlagen, palynomorphs exhibit a range of colors that indicate one dorsal and two ventral, the evolution of which their degree of geothermal alteration. Certain paly- varies from one species to another. The dorsal anlage nomorph colors are characteristic of rocks with ei- would be the only source of endocrine islands. The ther oil or gas reservoirs. See PALEOBOTANY; STRATIG- pancreas of reptiles is very similar to that of amphib- RAPHY. Jeffrey M. Osborn ians; the number of excretory ducts varies from one Bibliography. K. Faegri, P. Kaland, and K. to three. Krzywinski, Textbook of Pollen Analysis, 4th ed., In birds, the massive pancreas always lies in the Wiley, Chichester, 1989; M. M. Harley, C. M. Morton, duodenal loop. It develops from many dorsal and and S. Blackmore (eds.), Pollen and Spores: Mor- two ventral thickenings of the duodenal epithelium; phology and Biology, Royal Botanic Gardens, Kew, one (sometimes two) excretory duct persists. The 2000; J. Jansonius and D. C. McGregor (eds.), Pa- median portion of the dorsal anlage develops into lynology: Principles and Applications, vols. 1–3, a single mass which subdivides into typical islets of American Association of Stratigraphic Palynologists, Langerhans. A complete ring of pancreatic tissue sur- Salt Lake City, 1996; R. O. Kapp, O. K. Davis, and J. E. rounds the portal vein. King, Ronald O. Kapp’s Pollen and Spores, 2d ed., The pancreas of mammals shows the same varia- American Association of Stratigraphic Palynologists, tions as in the fishes. The extremes are the unique, College Station, 2000; S. Nilsson and J. Praglowski massive pancreas of humans, and the richly branched (eds.), Erdtman’s Handbook of Palynology, 2d ed., organ of the rabbit. Usually, the main duct, the duct Munksgaard, Copenhagan, 1992; A. Traverse, Pale- of Wirsung, opens into the duodenum very close to opalynology, Unwin Hyman, Boston, 1988. the hepatic duct. Many rodents have this opening of the pancreatic duct as far as 40 cm (15.7 in.) from the hepatic duct. In humans, the pancreas weighs about 70 g (7.5 oz). It can be divided into head, body, and Pancreas tail. A portion called the uncinate process is more or A composite gland in most vertebrates, containing less completely separated from the head. Accessory both exocrine cells—which produce and secrete en- pancreases are frequently found anywhere along the zymes involved in digestion—and endocrine cells, small intestine, in the wall of the stomach, and in arranged in separate islets which elaborate at least Meckel’s diverticulum. See DIGESTIVE SYSTEM. two distinct hormones, insulin and glucagon, both of which play a role in the regulation of metabolism, Histology and particularly of carbohydrate metabolism. This ar- The pancreatic parenchyma is formed by two el- ticle discusses the anatomy, histology, embryology, ements; one is the exocrine tissue of which the