Histochemical Localization of Calcium Oxalate Crystals in Starch Grains of Yams (Dioscorea)

The bulbils and/or tubers of seven species of yams (Dioscorea)were examined for crystal content using light microscopy andhistochemistry. Calcium oxalate crystals in the form of raphide bundles werelocalized in the parenchymatous tissues. Within starch grains,crystals of various shapes and sizes were observed. The variationin shape and sizes of the intra-amylar crystals could be exploitedfor taxonomic purposes. Calcium oxalate crystals appear to serve a storage functionin these starch grains. Yams, Dioscorea, raphides, oxalate crystals, histochemistry     

Zooplankton associations in Zimbabwe

The specific composition and diversity of the zooplankton in 18 impoundments in Zimbabwe were analysed on the basis of samples taken in July and August 1983. All the lakes lay at altitudes over 1200 m, with the highest at 2270 m.
Thirty species of Rotifera and 20 species of Crustacea were identified, but the mean numbers of species per lake were 5·7 rotifers and 4·5 crustaceans. Some records represent considerable extensions of known ranges. Daphnia laevis was the most widespread and frequently dominant crustacean, while among the rotifers Keratella cochlearis was most frequently dominant, particularly in the lakes at the upper part of the altitudinal range.
Ordination and cluster analysis of the associations revealed a group of lakes with closely interrelated similarities which could be attributed to their lying on tributaries of the same river system and receiving trout from the same hatchery. These analyses also picked out the lowest locality as the most divergent and the one showing the most typically tropical zooplankton association. Although all the lakes were well within the tropics, their altitudes seem to have excluded some of the widespread tropical African zooplankters.
The possibility that these associations could be formed by random colonization is discussed and dismissed.
The momentary species composition of these Zimbabwean zooplankton associations was similar to that given by Pennak (1957) for the world average, although the mean number of species of Copepoda was significantly lower than the world average. Comparisons with data from Brazil and Lake Maggiore indicate the need for a further geographical analysis.
A comparative study of the species diversity in impoundments of different sizes indicates that, over several orders of magnitude of area, the number of species of Ciadocera and Copepoda shows a small increase, but the number of species of Rotifera shows a much larger and more variable increase.     

Virus Specific RNA in Cells transformed by RNA Tumour Viruses

Virus specific RNA comprises 5% of the nuclear RNA and 0.5–1.0% of the cytoplasmic RNA of cells transformed by murine sarcoma viruses. Even cryptically transformed cells which possess no detectable virus or viral proteins synthesize detectable amounts of viral RNA.     

Inhibitors of the RNA and DNA Dependent Polymerase Activities of RNA Tumour Viruses

RNA viruses of several animal leukaemias and sarcomas possess what seems so far to be a unique enzyme—an RNA dependent DNA polymerase^1–6. Specific inhibitors of the viral enzyme will not only be useful in the analysis of its possible role in neoplasia, but might provide drugs for leukaemia and cancer therapy.     

Carbon Dioxide Effects on Carbohydrate Status and Partitioning in Rice

The atmospheric carbon dioxide (CO₂) concentration has beenrising and is predicted to reach double the present concentrationsometime during the next century. The objective of this investigationwas to determine the long-term effects of different CO₂ concentrationson carbohydrate status and partitioning in rice (Oryza sativaL cv. IR-30). Rice plants were grown season-long in outdoor,naturally sunlit, environmentally controlled growth chamberswith CO₂ concentrations of 160, 250, 330, 500, 660, and 900µmolCO₂ mol¹ air. In leaf blades, the priority between the partitioningof carbon into storage carbohydrates or into export changedwith developmental stage and CO₂ concentration. During vegetativegrowth, leaf sucrose and starch concentrations increased withincreasing CO₂ concentration but tended to level off above 500µmolmol^–1 CO₂. Similarly, photosynthesis also increased withCO2 concentrations up to 500µmol mol^–1 and thenreached a plateau at higher concentrations. The ratio of starchto sucrose concentration was positively correlated with theCO₂ concentration. At maturity, increasing CO₂ concentrationresulted in an increase in total non-structural carbohydrate(TNC) concentration in leaf blades, leaf sheaths and culms.Carbohydrates that were stored in vegetative plant parts beforeheading made a smaller contribution to grain dry weight at CO₂concentrations below 330µmol mol^–1 than for treatmentsat concentrations above ambient Increasing CO₂ concentrationhad no effect on the carbohydrate concentration in the grainat maturity Key words: CO₂ enrichment, starch, sucrose