Glossary

 

Words and terminology which may be unfamiliar to you are presented here within the glossary section. Suggestions for further inclusions in later versions of this module are welcomed.

Abscission: The rejection of plant organs, such as the shedding of leaves during the autumn.

Adventitious: Growing from an unusual position.

Allochthonous: Material, particularly within sediments and sedimentary rocks, that has been transported; including organic debris, mineral and rock fragments.

Alluvial environment: A generalised term for a sedimentary environment where sediment (alluvium) is deposited by rivers and streams.

Andesite: A dark coloured extrusive igneous rock composed of sodic plagioclase minerals (e.g. andesine), and mafic minerals that may include biotite, hornblende or pyroxene. Phenocrysts are often common and may comprise one or more of these minerals. Quartz is also often present in the matrix or 'groundmass'.

Antheridium, antheridia: The reproductive structure that produces male gametes.

Apical meristem: The embryonic tissue at the 'growing tips' of stems and roots of plants.

Apodous: Not possessing legs or appendages.

Arcari: An order of the Class Arachnida (Subphylum Chelicerata) comprising the mites and ticks. The oldest fossil arcarid Protocarus crani occurs in the Rhynie chert.

Archegonium, archegonia: The reproductive structure that produces female gametes.

Arthropod: Any invertebrate possessing jointed limbs and a segmented exoskeleton. Belonging to the phylum Arthropoda, the group includes crustaceans, arachnids (e.g.: spiders, scorpions and mites), myriapods (e.g.: millipedes and centipedes) and insects.

Autochthonous: Material, particularly within sediments and sedimentary rocks, that has formed or grown in situ and has not been transported.

Autolysins: Enzymes capable of breaking down the cell walls in microbes.

Biostratigraphy: The organisation of sedimentary rocks into units on the basis of the fossils they contain (see also biozone).

Biozone: A subdivision of the rock record based on the presence of particular fossils, one of which is designated the zone fossil.

Breccia: A rock type comprised of often large angular fragments of pre-existing rock and set within a finer-grained rock matrix or cement.

Calcite: A common rock forming mineral of calcium carbonate (CaCO3), found primarily in limestones but also commonly forming a diagenetic cement in many other rock types and as a gangue mineral in many ore deposits.

Carotenoid: Orange, yellow or red-coloured accessory photosynthetic pigments, related to vitamin A, found in higher plants and photosynthetic bacteria. Carotenoids protect cells from strong UV radiation, and therefore cyanobacterial mats often appear more orange than green in colour during bright summer months.

Chloroplast: A cell with a complex internal structure of stacked, photosynthetic pigment-bearing, membranous discs set in a matrix. The chloroplasts of various algae also possess areas of starch formation (see pyrenoid).

Circinate vernation: A condition observed in plants (esp. ferns) where a leaf bud is curled up.

Clastic: A general term used in sedimentology to describe mineral and rock fragments in sediments/sedimentary rocks that have been derived from pre-existing rock. Rock and mineral fragments in this context may be referred to as clasts.

Clypeus: The area of the facial wall between the labrum and the frons (upper anterior portion of the head) in insects, usually separated from the latter by a groove.

Collophore: A peculiar tube-like structure found in collembolans or 'springtails' situated on the ventral side of the first abdominal segment.

Cortex: Plant tissue found between the epidermis and vascular tissue and comprised primarily of parenchyma cells.

Cross-stratification: Beds or strata inclined at an angle to the main bedding or stratification. Most is formed by the migration of bed-forms such as ripples and dunes.

Cutan: Complex waxy polymers that form cuticle.

Cuticle: 1. Impermeable layer made of cutan on the outer surface of the epidermis of plants. 2. Hard protecting layer covering the epidermis of arthropods.

Dichotomous branching: A simple type of branching in plants where the apical meristem splits or bifurcates at various intervals.

Enations: Non-vascularised leaf-like structures grown out from the epidermis on some early land plants. In the case of Asteroxylon mackiei, vascular traces extend from the stele to the base of the enations.

Endarch: Where the maturation and development of the xylem strand is centrifugal or 'from the inside outwards'. In this case the small xylem cells that are the first to differentiate and mature (the protoxylem) are surrounded by the later developed, longer metaxylem cells.

Epidermis: 1. The exterior tissue of leaves, roots and young stems of plants usually comprising a single layer of cells. 2. The outer layer of cells of an invertebrate (esp. arthropods).

Eukaryote: An organism whose cells contain a distinct nucleus.

Exarch: Where the maturation and development of the xylem strand is centripetal or 'from the outside inwards'. In this case the small xylem cells that are the first to differentiate and mature (the protoxylem) are peripheral to the later developed, longer metaxylem cells.

Exine: The outer, decay-resistant coat of a pollen grain or spore.

Gametangia: General term for the reproductive structures in plants.

Gamete: A haploid germ cell (e.g.: a spermatozoon or ovum) that fuses with another germ cell during fertilisation.

Gametophyte: The haploid stage in the life cycle of a plant, in species that show alternation of generations, that produces the gametes.

Gangue mineral: A mineral of no economic value, often associated with metalliferous ore deposits.

Graded bedding: Bedding in which internally the sediment shows a vertical gradual and progressive change in particle or grain-size. "Normal grading" is most common in which the particle size "fines-upwards", and frequently forms as a result of a gradually waning sediment-laden current, depositing progressively finer material as the current velocity decreases.

Gyrogonites: Dispersed fossil charophyte oogonia.

Half-graben: Term used in geology to describe a sedimentary basin where one side is bounded by an extensional or 'normal' fault.

Hydroid: An elongate, non-lignified water conducting xylem cell in some bryophytes. Analogous with tracheids in vascular plants.

Internodal cells, internodes: The long single cells on main axes, branches and branchlets in charophytes separating clusters of nodal cells.

Labrum: A sheet of cuticle on the ventral side of the head in arthropods, projecting in front of and protecting the mouthparts.

Lignin: Complex woody polymers found in vascular plants.

Lithostratigraphy: The organisation of sedimentary and volcanic rocks into distinctive named units on the basis of their lithologic character and stratigraphic position in relation to other units (compare with biostratigraphy).

Macerates: Insoluble organic residues left, for example, after dissolving sandstone samples in hydrofluoric acid.

Mesarch: Where the maturation and development of the xylem strand is both centripetal and centrifugal or 'from the outside inwards' and 'from inside outwards'. In this case the small xylem cells that are the first to differentiate and mature (the protoxylem) are embedded in the later developed, longer metaxylem cells.

Microphyll: A leaf with a single vascular strand, characteristic of lycophytes. Lycophyte leaves appear to have evolved independently from leaves seen in other vascular plants which exhibit more than one vascular strand (megaphyll).

Monopodial branching: A type of branching where lateral branches grow from a main stem.

Mycorrhizae: Symbiotic or parasitic fungi that live within intracellular and/or intercellular cavities in a plant.

Nodal cells, nodes: The short cells on the main axes and branches of charophytes from which branches, branchlets and gametangia may grow. Clusters of nodal cells are separated by long internodal cells.

Norite: A basic, coarsely crystalline plutonic igneous rock containing the plagioclase mineral labradorite as the main component and differing from gabbro by having orthopyroxene as the main ferromagnesian mineral present.

Ontogeny: The series of growth stages of a particular organism.

Oogonium, oogonia: The reproductive structure in charophytes that produces female gametes (see also gyrogonites).

Outlier: An area or group of rocks that are surrounded by rocks of an older age.

Palynology: Generally the study of all acid-insoluble organic material extracted from sedimentary rocks by acid solution techniques (see also palynomorphs), though more specifically concerned with the study of spores and pollen.

Parenchyma: Thin-walled cells in plants that may have various functions and morphologies.

Palynomorphs: Acid-insoluble organic remains, often microscopic, found in maceration residues. May include spores, pollen, acritarchs, dinoflagellates and arthropod cuticle fragments.

Permineralisation: The preservation of organisms where a mineral in-fills intercellular and intracellular cavities, but cell walls are not replaced.

Petrifaction: The preservation of organisms where a mineral in-fills intercellular and intracellular cavities and also replaces the organic material of cell walls.

Phenocrysts: Term used for relatively large conspicuous crystals set in a finer crystalline matrix in an igneous rock. These large crystals have typically grown at a much earlier stage and at a deeper level in the Earth's crust than the finer matrix in which they are set. Igneous rocks containing phenocrysts exhibit a porphyritic texture.

Phloem: The photosynthate conducting tissue in vascular plants.

Prokaryote: A primarily unicellular organism in which the cells lack a true nucleus.

Pseudomonopodial branching: A type of branching where the apical meristem appears to divide to form two branches, one of which is dominant resulting in an upright main axis with distinct side branches.

Pyrenoid: An area of starch formation found in the chloroplasts of certain algae.

Pyrite: A common metallic mineral comprising iron disulphide (FeS2). Crystallizing in the isometric system, pyrite is opaque and has a pale brass-yellow colour, metallic luster, and commonly occurs as cubic or octahedral crystals. Pyrite may occur as nodules or as finely disseminated crystals in sedimentary rocks, and may also replace fossils.

Quartz: A common rock-forming mineral of crystalline silica (SiO2) composed of silicon-oxygen tetrahedra in a 3D network. In sedimentary rocks quartz is a common detrital mineral, it also occurs in cherts as a result of the diagenesis of amorphous, hydrated forms of silica such as opal and chalcedony.

Retuse, retusoid: Showing a rounded apex with a central depression.

Sclerenchyma: Plant tissue where cells possess walls thickened and hardened with lignin thus helping to support the plant.

Sheet floods: Flooding events, particularly from rivers, where the flow of water is primarily unconfined (i.e.: not channelised) resulting in transported sediment being deposited as a broad 'sheet'.

Sinter: A porous deposit formed in hydrothermal areas by the precipitation of amorphous opaline silica from silica-saturated waters erupted from geysers and derived from hot springs.

Sporangium, sporangia: The structure in certain plants and fungi in which spores are produced.

Sporophyte: The diploid stage in the life cycle of a plant, in species that show alternation of generations, that produces asexual spores. 

Stoma, stomata: A pore in the epidermis of vascular plants used for gaseous exchange and transpiration. It is flanked by two guard cells which allow the pore to open and close.

Tapetum: The nutritive layer of cells in the sporangia of plants.

Terete: Smooth, cylindrical and tapering. Often used in describing vascular cells in plants (esp. xylem cells).

Tracheid: A water-conducting xylem cell that is thickened and hardened by lignin, contributing to the support of the plant.

Tuff: Term used for volcanic ash that has been consolidated and cemented.

Vascular tissue: The area of the plant comprising the conducting cells: the xylem and the phloem.

Vesicles: In solidified lavas this is a term used to describe 'frozen' gas bubbles. When lavas are erupted, the drop in pressure allows gases to separate and form bubbles in the molten rock. If the lava solidifies before the gas bubbles escape the 'bubbles' or vesicles are preserved in the rock. When vesicles are later in filled with a mineral they are called amygdales.

Xylem: Plant tissue through which all water and dissolved nutrients are conducted (see also tracheid and hydroid).

Zone fossil: A fossil that is characteristic of a biozone or biostratigraphic unit, and from which the biozone takes its name.

Zosterophyll: A plant belonging to an extinct group of tracheophyte plants, the Zosterophyllophytina, typified by having pseudomonopodial or dichotomous branching; lateral, stalked, reniform sporangia with a marginal dehiscence mechanism; and a terete, exarch xylem strand.