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ItemAspects of the water relations of leaves of eucalyptus globulus(University of Melbourne, 1959)Although it is generally recognised that water is one of the main limiting factors in the development of vegetation in Australia, comparatively little physiological work has been done on the water economy of native plants. Ibis is particularly true of the water relations of trees; yet, due chiefly to the scarcity of water in extensive areas of Australia, the proportion under forest is the smallest for any of the continents, except Antarctica. A large proportion of the published work on ecological aspects of plant water relations has been concerned with determinations of the osmotic pressure of the cell sap, although values of osmotic pressure give no indication of the ability of plant tissues to take up water. Ibis ability is measured by the suction pressure of the tissue. Owing to the difficulty involved in its determination, suction pressure has been measured comparatively rarely in physiological and ecological studies. Most of the work included in this thesis consists of investigations relevant to the suction pressure and water deficit of leaf tissue. Juvenile leaves of Eucalyptus globulus Labill. were used as a source of material in the experiments which will be described. As characteristically Australian trees, the eucalypts are a natural choice for study. This particular species (blue gum) is native to arena of south-east Australia, but it is cultivated widely in other countries, so that material is available to many research workers overseas. In addition, saplings of E. globulus had been planted in a garden at the rear of the Botany Department in Melbourne in 1954, and these provided a good supply of leaves throughout the course of the Investigations detailed below. Much of the work on the water relations of plants ms been concerned with the absorption of water, the ascent of sap, and transpiration. Transpiration, its measurement, and the factors affecting it, have received a particularly large amount of attention. The balance between water uptake and water loss by leaves, however, has not been studied very extensively. investigation of this facet of plant water relations was hampered for a long time by the lack of a suitable common denominator upon which to express either the water content or the water deficit of a tissue. Weatherley (1930), made a critical Investigation of the relative merits of tissue dry weight, cellulose content, and protein content, as possible denominators. He found that, owing to marked seasonal trends, these quantities were unsuitable. In experiments on the uptake of water by floating leaf discs, Weatherley found that water was taken up rapidly at first, but that this was followed by a slow steady uptake of water. Weatherley interpreted the initial rapid uptake as due to the elimination of the water deficit of the tissue. The subsequent slow uptake was attributed to growth. On the basis of these observations, Weatherley developed the concept of "relative turgidity", which he defined as the ratio of the amount of water present in a quantity of tissue when sampled, compared with the amount present in the same quantity of tissue when fully turgid. This ratio has the advantage that it does not include a dry weight component and seasonal variations introduced by the use of this term are avoided. In addition, the determination of relative turgidity is comparatively simple, as it requires only three weighings. No work involving these newer methods of expression of water content has yet been published on species of Eucalyptus and one of the objects of this thesis is to investigate the methods of measuring relative turgidity (and other, related functions of plant cells), and the importance of variations in the water content of the leaves of Eucalyptus globulus.