762 Palynology

Physical properties of palladium

Property Value

Atomic weight 106.4Naturally occurring isotopes

(percent abundance)102 (0.96)

104 (10.97)105 (22.23)106 (27.33)108 (26.71)110 (11.81)

Crystal structure Face-centered cubicThermal neutron capture cross

section, barns8.0

Density at 25 C (77 F), g/cm3 12.01Melting point, C ( F) 1554 (2829)Boiling point, C ( F) 2900 (5300)Specific heat at 0 C (32 F), cal/g 0.0584Thermal conductivity,

(cal cm)(cm2 s C)0.18

Linear coefficient of thermalexpansion, ( in./in./)/ C

11.6

Electrical resistivity at 0 C (32 F),-cm

9.93

Young’s modulus, lb/in.2, static, at20 C (68 F)

16.7 106

Atomic radius in metal, nm 0.1375Ionization potential, eV 8.33Binding energy, eV 3.91Pauling electronegativity 2.2Oxidation potential, V 0.92

µ

µΩ

gold. Other consumer applications are in automo-bile exhaust catalysts and jewelry. See INTEGRATEDCIRCUITS.

Palladium supported on carbon or alumina is usedas a catalyst for hydrogenation and dehydrogenationin both liquid- and gas-phase reactions. Palladiumfinds widespread use in catalysis because it is fre-quently very active under ambient conditions, and itcan yield very high selectivities. Palladium catalyzesthe reaction of hydrogen with oxygen to give water.Palladium also catalyzes isomerization and fragmen-tation reactions. See CATALYSIS.

Halides of divalent palladium can be used as homo-geneous catalysts for the oxidation of olefins (Wackerprocess). This requires water for the oxygen transferstep, and a copper salt to reoxidize the palladiumback to its divalent state to complete the catalyticcycle. See HOMOGENEOUS CATALYSIS; TRANSITION EL-EMENTS. D. Max Roundhill

Bibliography. G. W. Gribble and J. J. Li, Palladiumin Heterocyclic Chemistry, Pergamon, 2000; F. R.Hartley, Chemistry of Platinum and Palladium,1973; J. Tsuji (ed.), Palladium in Organic Synthe-sis (Topics in Organometallic Chemistry), Springer,2005; J. Tsuji, Palladium Reagents and Catalysts:New Perspectives for the 21st Century, Wiley, 2d ed.,2004.

PalpigradiAn order of rare arachnids comprising 21 knownspecies from tropical and warm temperate regions.American species occur in Texas and California. Allare minute, whitish, eyeless animals, varying from

0.03 to 0.11 in. (0.7 to 3 mm) in length, that liveunder stones, in caves, and in other moist, darkplaces. The elongate body terminates in a slendermultisegmented flagellum set with setae. In a curi-ous reversal of function, the pedipalps, the secondpair of head appendages, serve as walking legs. Thefirst pair of true legs, longer than the others and setwith sensory setae, has been converted to tactile ap-pendages which are vibrated constantly to test thesubstratum. See ARACHNIDA. Willis J. Gertsch

PalynologyThe study of pollen grains and spores, both extantand extinct, as well as other organic microfossils.Although the origin of the discipline dates back tothe seventeenth century, when modern pollen wasfirst examined microscopically, the term palynologywas not coined until 1944.

The term palynology is used by both geologistsand biologists. Consequently, the educational back-ground of professional palynologists may be eithergeologically or biologically based. Considerable over-lap exists between some areas of the fields, how-ever, and many palynologists have interdisciplinarytraining in both the earth and life sciences. Palynol-ogists use a range of sophisticated methodologiesand instruments in studying both paleopalynologi-cal and neopalynological problems, but the utiliza-tion of modern microscopy is fundamental in bothsubdisciplines.

Palynologists study microscopic bodies generallyknown as palynomorphs. These include an array oforganic structures, each consisting of a highly resis-tant wall component. Examples include acritarchsand chitinozoans (microfossils with unknown affini-ties), foraminiferans (protists), scolecodonts (toothand mouth parts of marine annelid worms), fun-gal spores, dinoflagellates, algal spores, and sporesand pollen grains of land plants. This discussion willfocus on the palynomorphs produced by land plants,beginning with a general description of pollen grainsand spores and then providing an overview of theprimary areas of investigation within neo- and pa-leopalynological subdisciplines. See MICROPALEON-TOLOGY.

Pollen Grains and Spores: An Overview

Spores and pollen grains are reproductive structuresand play a paramount role in the life history ofland plants. The sporophyte generation of nonseed-bearing plants (ferns, for example) produces single-celled spores that ultimately germinate to grow intothe haploid gametophyte generation. Homosporousspecies produce a single type of spore, whereasheterosporous species produce two spore types.Microspores germinate and grow into “male” sperm-producing microgametophytes, and megaspores de-velop into “female” egg-producing megagameto-phytes. The gametophytes of most nonseed plantsare multicellular and proliferate outside the sporewall during development. All seed-bearing plants

Palynology 763

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

(g)

(f)

(e)

(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,and their pollen represents the microgametophytegeneration. Pollen grains consist of just three to afew cells, and these remain within the microsporewall, where they originally developed. See REPRO-DUCTION (PLANT).

Spores and pollen grains are formed in multiplesof fours following meiotic divisions. During develop-ment, the four are united into a tetrad that, in mostplants, subsequently dissociates into the four individ-ual propagules. In nonseed plants, each spore com-monly bears a mark on its proximal surface indicatingwhere it made contact with the others at the centerof the tetrad. In most spores this external mark iseither straight or Y-shaped (Fig. 1), and it is typicallycharacterized by a suture that spans the spore walland is the site through which germination occurs.In contrast, most pollen grains lack sutures and ger-minate through thin areas in the wall, or apertures.Apertures are typically located in either a distal oran equatorial position. Common aperture types in-clude elongate furrows, pores, and furrows with acentral pore. Aperture type, number, and positionare important systematic characters by which fossiland modern taxa can be compared. Other descrip-tively and systematically relevant characters includesize, shape, presence and structure of air bladders,surface ornamentation, and wall ultrastructure. SeePOLLEN.

The wall of spores and pollen grains is known col-lectively as the sporoderm (or “skin of the spore”)and actually consists of two distinct walls (Fig. 2).The inner wall, or intine, is primarily composed of

cellulose and pectin; as such, it is similar to mostother plant cell walls. The outer wall, or exine, isprincipally composed of sporopollenin, a chemicallyenigmatic macromolecule that is resistant to biolog-ical decay and geological degradation. The exine isfurther characterized by several ultrastructural lay-ers and an array of sculptural elements. It is the verypresence of the exine that allows for the spectac-ular preservation of pollen and spores in the fossilrecord.

Neopalynology

This discussion focuses on several subdisciplinesof neopalynology, including taxonomy, genetics,and evolution; development, functional morphol-ogy, and pollination; aeropalynology; and melissopa-lynology.

Taxonomy, genetics, and evolution. Taxonomy andsystematics are concerned with classifying organ-isms into hierarchical ranks that reflect evolutionary,or phylogenetic, relationships. Pollen and spore mor-phology is important systematically, with particularfeatures characteristic of different taxonomic ranks.For example, distinguishing characters may includeaperture type for a family, different ornamentationpatterns for its subordinate genera, and variation inexine ultrastructure for its congeneric species. Pa-lynological characters are especially useful system-atically when evaluated in conjunction with othercharacters (for example, plant morphological andmolecular characters). Cladistics is one techniquethat has employed such an integrated approach.Cladistic analyses are based on numerical algorithms

764 Palynology

(a)

aperture

(c)

(b)

microgametophyte

exineintine

aperturalregion

sculptural rods tectum

intineendexine

foot layer columellae

10 !m 1 !m

10 !m

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

that produce trees, or cladograms, demonstratingphylogenetic lineages among the organisms exam-ined. See PLANT EVOLUTION; PLANT TAXONOMY.

Assessing pollen flow is another approach usedto study evolutionary questions. Because pollen isthe sperm-producing generation, the patterns andrates of pollen transfer are important factors in de-termining the spread of genes throughout a popu-lation. Competition can occur among reproductiveorgans, and pollen flow may be influenced by thecorrelation of pollen characters with those of re-productive organs. For example, the ovulate, or fe-male, cones of some gymnosperms, such as pines,are aerodynamically adapted for entraining airbornepollen grains that themselves have particular aero-dynamic characters, such as extended air bladders.Furthermore, competition may exist among individ-ual pollen grains. Following pollination in some flow-ering plants, intraspecific variation can result in thegrains of a more reproductively fit plant producingfaster-growing pollen tubes than others of the samespecies. See POPULATION GENETICS.

Development, functional morphology, and pollination.In most seed plants, a layer of callose, a carbohy-drate, surrounds the entire tetrad and separates eachimmature pollen grain during development. Forma-tion of the pollen wall and positioning of the aper-ture begin while each grain is encased within thecallose layer. Both the internal ultrastructure and

the sculptural surface ornamentation of the outerpollen wall, the exine, are dependent upon the de-positional pattern of the chemical that makes up theexine, sporopollenin. Sporopollenin is primarily de-rived from the developing pollen grain, but can alsobe released from a specialized layer of cells known asthe tapetum, which surrounds the developing pollengrains. The inner pollen wall, or intine, is synthesizedlast.

When the tapetum breaks down, or undergoesprogrammed cell death (apoptosis), it also releasesseveral proteins, lipids, and other substances that be-come isolated within the spaces and on the surfaceof the developing pollen wall. In flowering plants,many tapetum-derived proteins function as recogni-tion molecules in pollination systems and are impor-tant in determining the extent of compatibility ofa particular pollen grain on a floral stigma. Otherpollen-derived proteins become stored within theintine and may also be involved in compatibility-incompatibility reactions. Several tapetal lipids alsoplay important roles in pollination. Pollenkitt, for ex-ample, functions in pollen adhesion and acts as avisual and olfactory attractant. Additionally, pollenmorphology and exine architecture may be corre-lated with pollination systems. For example, thepollen of wind-pollinated plants is typically smooth,has a thin exine, lacks pollenkitt, and may have airbladders, whereas that of animal-pollinated plants is

Palynology 765

commonly highly ornamented and bears pollenkitt.See FERTILIZATION.

Aeropalynology. Aeropalynology is the study ofpollen grains and spores that are dispersed intothe atmosphere. Wind-pollinated plants typicallyproduce copious amounts of pollen, thereby enhanc-ing successful pollinations. The abundance of air-borne pollen commonly causes allergic reactions ina large proportion of the human population. Polli-nosis, allergen rhinitis or hay fever, is elicited whenallergen-containing pollen makes contact with themucous membranes lining the nose, trachea, orbronchi, and the cornea of the eye. Allergens leachout of the pollen and bind to immunoglobulin Eantibodies. The antibodies are linked to mast cellsthat release histamine and other inflammatory chem-icals, producing allergy symptoms. Ironically, the al-lergens that induce pollinosis include many of thesame compatibility-incompatibility, recognition pro-teins involved in pollination.

Knowledge of the temporal, seasonal, and environ-mental aspects of pollen dispersal is also importantin understanding and avoiding hay fever. Floweringtime and season vary widely for different plants, andthe release of airborne pollen is typically inhibited byhigh humidity or rain. To monitor risks of pollinosis,the diversity and quantity of various pollen types areassessed by filtering the air throughout the year. SeeALLERGY; ANTIBODY.

Melissopalynology. Honeybees are the primarypollinators of many flowering plants. Honeybees,and other bees, visit flowers to collect nectar andlarge quantities of pollen (pollen loads), both ofwhich are used as food sources for developing lar-vae. Melissopalynologists analyze bee pollen loadsand the pollen component within honeys. Althoughbees primarily produce honey from nectar, 1 mLof honey may contain more than 20,000 pollengrains. The foraging behavior of bees can be de-termined by microscopically examining their pollenloads and taxonomically identifying the pollen con-stituents. Honey purity can also be assessed by exam-ining the diversity of pollen grains found within thehoney.

Paleopalynology

The main fields of study within paleopalynology arediscussed below. The areas addressed involve pale-obotany; past vegetation and climate reconstruction;geochronology and biostratigraphy; and petroleumand natural gas exploration.

Paleobotany. Fossil pollen and spores typicallyconsist of only fossilization-resistant exine layers.However, these propagules did at one time containboth gametophytic cells and pectocellulosic intines,and functioned in a similar way to that of their ex-tant counterparts. Fossil pollen and spores can bedistinguished into two categories based on the gen-eral way in which the palynomorphs are preserved.Sporae dispersae grains are those occurring withinsediments in a dispersed condition; in most cases,information about the parent plants is unavailable.Investigation of dispersed grains is especially impor-

tant in the fields addressed below. In situ grainsoccur within intact, megafossil reproductive organs(like flower anthers); as such, morphological datafrom the parent plant are available and provide forbetter systematic evaluation. Studies of in situ pollenor spores also afford the opportunity to evaluate andinterpret fossils in a biological context. For example,developmental information can be inferred by exam-ining pollen-containing organs preserved in variousontogenetic stages, and ancient pollination events,such as pollen germination and pollen tube growth,can be assessed when grains are recovered on re-ceptive structures. These types of data allow pale-obotanists to better understand and reconstruct thecomplete life history of fossil plants.

Past vegetation and climate reconstruction. A sig-nificant focus of palynology involves reconstructingthe Earth’s vegetational history since the last majorglaciation event, within the past 10,000 years, or dur-ing the Holocene Epoch. This area of postglacial pa-lynology is known as pollen analysis and primarilyincludes the study of palynomorphs from lake sedi-ments and peat deposits. Sediments are obtained byseveral methods (mostly core sampling), and paly-nomorph diversity, distribution, and abundance areplotted on pollen profiles. Pollen analysis can pro-vide historical information regarding both individualtaxa and larger plant communities, including dataabout vegetational succession. Such analyses mustconsider all possible sources of palynomorphs andtake precaution during sample preparation to avoidcontamination with extant pollen because manymodern taxa may have also existed in the Holocene.However, because of the excellent preservation po-tential of key palynological characters, such as thosedescribed above, fossil pollen grains yield a high de-gree of taxonomic resolution.

Because many plants inhabit areas exhibitingparticular environmental regimes and have limitedgeographic distributions, palynological analyses con-tribute to an understanding of paleoclimatic condi-tions. For example, a palynoflora may be indicativeof source vegetation occupying a polar, temperate,subtropical, or tropical habitat. Therefore, palyno-logical information can also be used in conjunctionwith other megafossil indicators of climate, such astree ring data. See POSTGLACIAL VEGETATION AND CLI-MATE.

Geochronology and biostratigraphy. Palynologicalanalyses play a significant role in age determina-tions of rocks, or geochronology. Dating the geologi-cal ages of palynomorph-bearing rocks is dependentupon knowledge of the stratigraphic ranges of ex-tinct plant groups. Because different plant groupsare known to have restricted geological time ranges,the pollen and spores produced by their plants arecharacteristic of particular ages and may serve asindex fossils. Comparisons may be made againstwell-established reference palynomorphs, or indexfossils, and palynofloras. Palynological dating tech-niques are especially useful when correlated withages of rocks that have been radiometrically dated.See FOSSIL; INDEX FOSSIL.

766 Pancreas

Comparisons of palynomorphs within a given rocksection from one site with those of units from otherlocalities are important in documenting stratigraphicsimilarities among the rock sections, even if thesections exhibit different thicknesses and litholo-gies. When the occurrence, diversity, and abundanceof fossils (palynomorphs, megafossils, or both) areused to correlate geographically separated rock se-quences, this is known as biostratigraphy. Histori-cally, biostratigraphic correlation has provided sup-porting evidence for continental drift theory. Forexample, some present-day continents, such asAntarctica, Africa, and India, have distinguishing in-dex fossils, of various ages, that are present on thesecontinents and absent from others. These intercon-tinental correlations are supportive of the previousexistence of the single Southern Hemisphere land-mass known as Gondwana. See PALEOGEOGRAPHY.

Petroleum and natural gas exploration. Economi-cally, palynological biostratigraphy is an importanttechnique used in the exploration for natural gasand petroleum. Biostratigraphic correlations in thiscontext are conducted on a smaller scale, typicallywithin an existing oil field. Besides identifying thelocality, it is critical to determine the appropriatelevel at which to drill. For this endeavor, the paly-nologist is not necessarily interested in references ofdepth, but in important palynological indicators ofknown oil and gas production levels. In addition tothe identification of key index fossils, a color eval-uation is relevant. Following standard preparations,palynomorphs exhibit a range of colors that indicatetheir degree of geothermal alteration. Certain paly-nomorph colors are characteristic of rocks with ei-ther oil or gas reservoirs. See PALEOBOTANY; STRATIG-RAPHY. Jeffrey M. Osborn

Bibliography. K. Faegri, P. Kaland, and K.Krzywinski, Textbook of Pollen Analysis, 4th ed.,Wiley, Chichester, 1989; M. M. Harley, C. M. Morton,and S. Blackmore (eds.), Pollen and Spores: Mor-phology and Biology, Royal Botanic Gardens, Kew,2000; J. Jansonius and D. C. McGregor (eds.), Pa-lynology: Principles and Applications, vols. 1–3,American Association of Stratigraphic Palynologists,Salt Lake City, 1996; R. O. Kapp, O. K. Davis, and J. E.King, Ronald O. Kapp’s Pollen and Spores, 2d ed.,American Association of Stratigraphic Palynologists,College Station, 2000; S. Nilsson and J. Praglowski(eds.), Erdtman’s Handbook of Palynology, 2d ed.,Munksgaard, Copenhagan, 1992; A. Traverse, Pale-opalynology, Unwin Hyman, Boston, 1988.

PancreasA composite gland in most vertebrates, containingboth exocrine cells—which produce and secrete en-zymes involved in digestion—and endocrine cells,arranged in separate islets which elaborate at leasttwo distinct hormones, insulin and glucagon, bothof which play a role in the regulation of metabolism,and particularly of carbohydrate metabolism. This ar-ticle discusses the anatomy, histology, embryology,

physiology, and biochemistry of the vertebrate pan-creas. See CARBOHYDRATE METABOLISM.

Anatomy

The pancreas is a more or less developed gland con-nected with the duodenum. It can be considered asan organ which is characteristic of vertebrates.

Chordates and lower vertebrates. In Branchio-stoma (Amphioxus) a pancreatic anlage is found inyoung stages as a thickening of the gut caudal to theliver. The pancreas of cyclostomes, arising from thegut epithelium or from the liver duct, seems to bepurely endocrine; it degenerates in later stages of de-velopment.

A true pancreas is found in selachians, with an ex-ocrine portion opening into the intestine and an en-docrine portion represented by cellular thickeningsof the walls of the ducts.

Higher vertebrates. The ganoids (palaeopterygianfishes) show a diffuse pancreas—its principal masslying between the gut and the liver—in which typicalislets of Langerhans are observed. The pancreas ofteleosts is either of the massive or dispersed type.Many species, such as the pike, show enormous isletsof Langerhans, 10 × 5 mm, from which J. McLeod(1922) extracted insulin. The existence of a pancreasin dipneusts, such as Protopterus, is doubtful.

The compact pancreas of the amphibians is lo-cated in the gastrohepatic omentum and extends to-ward the hilus of the liver and along the branchesof the portal vein. It develops from three anlagen,one dorsal and two ventral, the evolution of whichvaries from one species to another. The dorsal anlagewould be the only source of endocrine islands. Thepancreas of reptiles is very similar to that of amphib-ians; the number of excretory ducts varies from oneto three.

In birds, the massive pancreas always lies in theduodenal loop. It develops from many dorsal andtwo ventral thickenings of the duodenal epithelium;one (sometimes two) excretory duct persists. Themedian portion of the dorsal anlage develops intoa single mass which subdivides into typical islets ofLangerhans. A complete ring of pancreatic tissue sur-rounds the portal vein.

The pancreas of mammals shows the same varia-tions as in the fishes. The extremes are the unique,massive pancreas of humans, and the richly branchedorgan of the rabbit. Usually, the main duct, the ductof Wirsung, opens into the duodenum very close tothe hepatic duct. Many rodents have this opening ofthe pancreatic duct as far as 40 cm (15.7 in.) from thehepatic duct. In humans, the pancreas weighs about70 g (7.5 oz). It can be divided into head, body, andtail. A portion called the uncinate process is more orless completely separated from the head. Accessorypancreases are frequently found anywhere along thesmall intestine, in the wall of the stomach, and inMeckel’s diverticulum. See DIGESTIVE SYSTEM.

Histology

The pancreatic parenchyma is formed by two el-ements; one is the exocrine tissue of which the