RIBONUCLEIC ACID METABOLISM AND CELL EXPANSION IN OAT COLEOPTILE

RNA metabolism in oat coleoptiles was studied using physiologicalresponses to 5-FU and actinomycin D; autoradiographic detectionof RNA and protein synthesis; and estimation of ribosomal concentrationby analytical ultracentrifugation. 5-FU failed to inhibit growthof either intact coleoptiles or isolated coleoptile segmentsbut completely blocked cell division in roots. Actinomycin Dmarkedly inhibited auxin-induced expansion of coleoptile segments.When supplied to isolated segments from coleoptiles of variouslengths the RNA precursors cytidine, adenine and adenosine allshowed weak incorporation into RNA of nuclei and in some cases,to a lesser extent, RNA of cytoplasm. IAA did not affect thisRNA synthesis but it was considerably reduced by actinomycinD. A proportion of the label incorporated from RNA precursorswas not removable with either RNase, PCA or hot TCA but wasextracted by trypsin. The amount of this spurious incorporationincreased with coleoptile age, as did the ability to incorporatelabelled amino acids. The concentration of both free and boundribosomes does not increase in growing coleoptiles and may evendecline. Free ribosomes decline markedly in fully grown coleoptileswhile the proportion of bound ribosomes increases. It is concludedthat young coleoptiles contain a full complement of ribosomesnecessary for subsequent growth but normal growth is dependenton continued production of an actinomycin D-sensitive messenger-typeRNA. No evidence for auxin mediation of RNA synthesis was found. ¹Present address: Laboratory of Cell Biology, Faculty of Science,Osaka City University, Sugimoto-cho, Sumiyoshi-ku, Osaka, Japan.     

Evidence that Enzyme Polymorphisms are not Selectively Neutral

THE discovery of large amounts of electrophoretically-detectable genetic variation within natural populations has aroused considerable interest in the factors maintaining so much polymorphism. It has been proposed that these allozyme polymorphisms reflect the action of random processes^1–6, the polymorphic variation being of little or no selective significance. Alternative hypotheses have suggested that these polymorphisms may be maintained by a balance of selective forces^7–10.     

The Structure of the Anuran Amphibian Notochord and a Re-evaluation of its Presumed Role in Early Embryogenesis

For almost a century the amphibian notochord has been analyzed from either the structural or functional point of view. Relatively complete data have been collected concerning several aspects of its structure. Some of that data will be reviewed in this report. The functional role of the notochord has also occasionally been analyzed. Only recently, however, have extensive data begun to emerge which deal directly with cause-effect relationships which may or may not exist between the notochord and surrounding tissues of the embryonic axial structure system. This report will re-evaluate the role of the notochord in amphibian embryogenesis, and provide a review of the historical perspective in which recent discoveries should be interpreted.     

Linking riparian vegetation types and fluvial geomorphology along the Sabie River within the Kruger National Park, South Africa

Vegetation types were studied in relation to the fluvial geomorphology along the mixed bedrock‐alluvial Sabie River within the Kruger National Park, Mpumalanga, South Africa. Six vegetation types were identified using TWINSPAN analysis, namely: Phragmites mauritianus , Phyllanthus reticulatus , Breonadia salicina , Combretum erythrophyllum , Diospyros mespiliformis and Spirostachys africana vegetation types. Spirostachys africana and Diospyros mespiliformis vegetation types were found to occur predominantly on the stable, infrequently flooded macro‐channel banks, while the remaining four vegetation types were found almost exclusively along the more geomorphically and hydrologically dynamic macro‐channel floor. The degree of bedrock or alluvial influence was identified as being an integral factor in the distribution of the four macro‐channel floor vegetation types at both the morphological unit and the channel type scale. The geomorphological continuum from the bedrock influenced bedrock anastomosing channel types, to mixed anastomosing and pool‐rapid channel types, to the fully alluvial braided channel types, is reflected in the change in species composition from Breonadia salicina vegetation type, to Phyllanthus reticulatus and Phragmites mauritianus vegetation types, to Combretum erythrophyllum vegetation types, respectively. Given the vegetation/fluvial geomorphology links established, changes in vegetation composition are proposed in response to scenarios of geomorphological change as a result of progressive sedimentation.