A variety of methods has been used for determining the volume of intercellular spaces in plant tissues. The most frequently used technique is that based on Unger (1854), in which tissue is infiltrated with water under a partial vaccum (e.g Nius, 1931; Meiri and Anderson, 1970; Byott, 1976; Johnston, 1977). The air spaces in the tissue become filled with water during the evacuation, so their volume can be calculated from the weight difference before and after infiltration. Along similar lines, it is also possible to estimate air-space volume by determining the volume of a given mass of tissue; the volume of air can be computed if it is assumed that the cell components have an average density of 1000 kg m~. In practice, the accuracy of this method may be limited by the precision of the pyenometric devices (e.g. Kohlrausch, 1968) used for measuring the bulk tissue volume. A more refined method has been developed by Czerski (1964), in which the volume of gas extracted from leaves under reduced pressure is measured directly in calibrated capillaries. Finally, it is possible to carry out a stereological analysis using representative cross-sections of tissue (see Turrell, 1936; Weibel, 1969). Precautions are needed with this method to ensure that tissue dimensions are not affected during specimen preparation. In view of the widespread use of the infiltration method, we chose to apply this technique to quantifying the intercellular-space volumes in leaves of plants showing crassulacean acid metabolism (CAM). Because of the succulence of the leaves of most CAM plants and the appearance of such tissue in cross-section (e.g. Steudle, Smith and Liittge, 1980), it would be expected that the intercellular spaces make up only a small proportion of the total volume. Values in the literature for a wide range of species suggest that air spaces can occupy anything from a few per cent to more than 50 per cent of the volume of the leaf (Sifton, 1945; Byott, 1976). There is, however, one potentially serious problem associated with the infiltration method. If pure water is used for infiltration and the leaf cells have a water potential of less than 0 MPa, there will be a tendency for water to enter the cells in addition to filling the intercellular spaces. This was clearly identified by Neger (1912) and by Czerski (1964) as a source of error in the infiltration method, and indeed the latter author subsequently used the fact as a basis for estimating water deficits in leaves (Czerski, 1968). It has, though, been neglected in other studies. The magnitude of the error will be related both to the water potential of the bulk tissue and to the half-time for water uptake by individual cells. As we have recently shown (Steudle et al., 1980) that the half-time in leaf cells ofKalanchoedaigremontiana is very short (about 5 s), special attention was paid to these factors in the present investigation.