Browsing by Author "Mitchell, D T"

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    Certain aspects of the biology of Watsonia SPP
    (1974) Waher, Ivo Gregor; Mitchell, D T
    The flowering responses of two species of Watsonia (W. humilis and W. tabularis) were studied both in the field and in the laboratory. There was no conclusive evidence that heat treatments directly influenced the flowering response. Instead, it appears that flowering may result from a number of factors such as watering, clearing the soil surface of vegetation and transplantation. The effects of temperature on seed germination were also investigated. 15 - 18° are the optimum temperature conditions for germination although viability is not affected by incubation at temperatures up to 35° C. Carbohydrates in the form of mucilaginous polysaccharide are the major storage reserves of the corms. These polysaccharides as well as the ethanol soluble carbohydrates were determined at different stages in the life cycle of Watsonia spp. by means of gas-liquid and paper chromatography. There was little variation in the levels of cold water soluble polysaccharide at different stages in the vegetative development of the corms, but during seed pod formation the monomer components of these polysaccharides declined. The implications of these results are discussed in relation to the ecology of both W. tabularis and W. humilis.
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    Distribution of phosphorus in sandy soils of coastal fynbos
    (1983) Brown, Gary James; Mitchell, D T
    The levels of soil phosphorus (both total and plant available) were investigated at the Pella intensive study site and were found to be low. Resin extractable phosphorus fluctuated seasonally, peaking at the soil surface during summer and declining to a minimum in the winter months. Rates of microbial mineralisation were thought to be the major influencing factor. The profile distribution of all forms of phosphorus were similar to those predicted by Smeck (1973) and were thought to result from the removal of phosphorus by plant roots in the mid-zone (20-70 cm) and its subsequent deposition at the soil surface as litter. The majority of insoluble phosphorus (60%) was organic and the remaining insoluble inorganic compounds were mainly iron bound. The input from precipitation to the site was 194 g ha⁻¹ yr⁻¹ occurring chiefly during the winter months. The different soil forms present at the site contained similar total phosphorus levels and underwent an increase in the size of the resin extractable phosphorus pool with progression from less to more weathered soils. This progressive change was linked directly to the decrease in total iron content in the soils. With decreasing iron content the number of binding sites within the soil declined. Fire returned a large amount of readily available phosphorus to the soil which had the effect of immediately elevating the soil resin extractable phosphorus levels. This effect was a transient one. After twelve months the levels of resin extractable phosphorus had declined to values only slightly above those found in the prefire soil.
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    The physiology and biochemistry of the Laminaria Pallida/Carpoblepharis Minima and Ecklonia Maxima/Suhria Vittata associations from South-Western Cape waters, South Africa
    (1984) Stacey, Vivienne Jean; Mitchell, D T; Lewis, O A M
    The two laminarian brown algae Laminaria pallida Grev. ex. J. Ag. and Eoklonia maxima (Osb.) Papenf. are important both economically and as major components of the South-western Cape waters, South Africa. Growing attached to these brown algae are several different species of red algae two of which, Carpoblepharis minima Bart. and Suhria vittata (L.) J. Ag., were chosen and. the L. pallida/C. minima and. E. maxima/S. vittata associations were studied using physiological and biochemical methods. Carpoblepharis minima has only been observed on L. pallida, whereas S. vittata has been found attached to various substrates as well as to E. maxima.
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    Phytoplankton physiology in a nearshore region of the southern Benguela current
    (1981) Barlow, Raymond Gordon; Mitchell, D T; Field, John G; Strickland, N
    During the summer of 1978-79 diatoms dominated the communities from October to January but microflagellates were dominant in February and March. Low concentrations of ATP, chlorophyll a protein and carbohydrate were measured in Type 1 water with protein/carbohydrate ratios being )1. In Type 2 water concentrations of chlorophyll a ATP and protein were high and the protein/carbohydrate ratio was)l. Concentrations of chlorophyll a and ATP remained high in Type 3 water but the protein/carbohydrate ratio decreased to <1 due to an increase in the concentration of acid-soluble glucan. It was concluded that the communities were in an active phase of growth in Type 1 and Type 2 water when adequate nutrients were available, but were in a slow-growing phase in Type 3 water when nitrate concentrations were low.