The production and preservation of trilobite resting and furrowing traces

The production and preservation of trilobite resting and furrowing traces is discussed with special reference to specimens from Upper Cambrian strata in North Walcs, Spain, and Poland. Resting excavations are attributed largely or completely to animal activity, and do not normally rcsult from currcnt scouring. Resting excavations, furrows, and many other trace fossils are formed at the sediment-water interface and not bencath a thin sand layer or by burrowing between beds. The factors which allow the initial production of the traces on the ocean floor muds (e.g. cohesion, shear strength, low pore water pressures) will also favour their preservation during deposition of the casting sand.     

On the Origin of Thymic Lymphocytes

In the leopard frog (Rana pipiens), thymic lymphocytes do notoriginate from blood-borne stem cells that migrate into thethymus anlage; rather they arise in situ from elements in thethymic rudiment itself. After thymic differentiation, the lymphocytes(or their descendants) leave the thymus and extensively seedthe peripheral lymphoid organs. Indeed, virtually all the lymphocytesin the spleen, kidney, and bone marrow are ontogenically derivedfrom thymic cells. In postmetamorphic life, the thymus representsan organ in which lymphopoiesis is genuinely self-sustaining.Throughout the juvenile life of the frog, there is no indicationof an inward afferent stream of cells entering the intact thymus.     

Stratigraphical classification and all that

Quantitative treatment of the geological sciences must remain limited; there continues to be need for a language of words. This is keenly felt in stratigraphy, where the practising geologist is dismayed by the effusions of the stratigraphical philosophers. The situation, in so far as it appears to remain in some ways obscure or troublesome, is briefly reviewed in terms of lithostratigraphy. biostratigraphy. and chronostratigraphy. A revised version of a controversial diagram is presented, in which the role of the boundary stratotype as anchor point is believed to be clarified. Finally a synthesis is attempted of relationships between the various stratigraphical procedures and the provision of dates in years through the parallel discipline of geochronometry.     

The Membranes of a Eukaryotic Cell

The evolution of internal membrane systems has introduced manyadditional control steps into pathways that, although basicallysimilar in prokaryotic cells, are less sophisticated and leavemany aspects up to chance. Temporal and spatial control of secretion,quality control of the proteins secreted or inserted into theplasma membrane, and exquisite control over the selective degradationof macromolecules, for example, are apparently indispensiblerequirements in multicellular organisms, but are relativelyunimportant in bacteria. Although complicated in detail, mostmembrane traffic in the eukaryotic cell can be reduced to afew basic principles. Such a reductionist's view provides aconceptual framework that allows the reader to organize an otherwiseoverwhelming amount of data on cellular membrane architectureand dynamics.     

Complete Elimination of Mitosis and DNA Synthesis in the Lens of the Hypophysectomized Frog: Effects on Cell Migration and Fiber Growth

Three weeks after hypophysectomy mitosis and DNA synthesis are absent in the lens of the leopard frog ( Rana pipiens ). By labeling germinative zone cells in their last DNA synthetic period we were able to follow them in the absence of further proliferation. By this means it has been demonstrated that migration of epithelial cells to the equator of the lens is stopped. Based on indirect measurements it seems that lens fiber formation also ceases. A correlation coefficient of 0.86 was computed between average labeling index and average distance migrated. In hypophysectomized animals the correlation coefficient between time and migration was 0.02 while in intact animals it is 0.87.     

Does Late-acting Self-incompatibility (LSI) Show Family Clustering? Two More Species of Bignoniaceae with LSI:Dolichandra cynanchoides and Tabebuia nodosa

By means of hand pollination experiments and fluorescence microscopythe breeding systems and pollen tube growth were studied inDolichandra cynanchoides and Tabebuia nodosa, two bignoniaceousspecies of the Chaco woodland of NE Argentina. Both speciesabscised all selfed pistils within 4–8 d although selfpollen tubes grew successfully to the ovary and penetrated manyovules. Both species therefore show ‘late-acting self-incompatibility’(LSI) or ‘ovarian sterility’ (OS) type control ofselfing. LSI-OS has been reported in 17 species of the Bignoniaceae,and is also clustered in families such as the Bombacaceae-Sterculiaceae,Fabaceae and Myrtaceae. The implications of such family clusteringare discussed, with regard to current hypotheses for the actionof LSI-OS phenomena. Copyright 1999 Annals of Botany Company Bignoniaceae, Dolichandra cynanchoides, Tabebuia nodosa, Chaco, late-acting self-incompatibility, ovarian sterility.     

Covert Categories and Folk Taxonomies

Much of the recent work in ethnoscience has been concerned with the nature of folk taxonomies, an often stated definition of which requires that all folk taxa be monolexemically labeled. This paper offers evidence that unlabeled categories may also be of crucial taxonomic significance, and we feel that it is inappropriate to treat such categories apart from the named taxonomic entities of the system. More importantly, evidence presented indicates that by recognizing unnamed taxa one may gain an understanding of the structure of a particular semantic domain that is actually obscured if one focuses solely on lexically labeled units.     

Ultrastructure of the Eyespot and its Possible Significance in Phototaxis of Tetracystis excentrica*†

SYNOPSIS. The eyespot of the zoospore of Tetracystis excentrica (a green alga) has been studied by light and electron microscopy. In Tetracystis the eyespot consists of about 110 osmiophilic granules which form a plate in the anterior third of the cell. The granules are about 80 Å in diameter and are found in the outermost portion of the chloroplast; they commonly show hexagonal close packing and a hexagonal shape. The granules are confined positionally by the chloroplast envelope and an inner thylakoid. The plasmalemma over the eyespot is thickened and is separated from the chloroplast envelope by a 50 mμ space. The eyespot of Tetracystis is compared with others reported in the literature and the possible functional significance of these studies is discussed. The possibility that the eyespot plate in Tetracystis serves as a shading device rather than the primary photoreceptor is considered.     

Phase Change in Hedera helix: Induction of the Mature to Juvenile Phase Change by Gibberellin A3

A sensitive and reproducible method to obtain GA₃ induced morphological reversion of mature Hedera helix to the juvenile form has been developed. Dose response experiments indicate that GA₃ stimulates reversion over a 50–100 fold range with a half maximal response at approximately 0.5 μg GA₃ per plant. The individual characteristics involved in phase change revert to the juvenile form in a sequential manner as GA₃ dose is increased. Variations in light intensity from 1.2–3.6 × 10⁴ lux and temperature from 15 to 26°C do not affect this hormonal response. Other growth regulators including indoleacetic acid, kinetin, abscisic acid and (2-chloroethyl)phosphonic acid (Ethephon) are inactive but other gibberellins (GA₁ and a mixture of A₄–A₇) are active in stimulating reversion. Therefore, the response is specific for gibberellins as a class of hormones but non-specific for a particular form of gibberellin. The significance of this response in relation to juvenility in woody plants is discussed.