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-