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1. Although many plants grow in rock crevices and other regions of small soil volume, including over 20 000 epiphytic and hemi-epiphytic species, analyses of the actual soil volume occupied, the water availability in that soil, the water-storage capacity in the shoots and underground organs, and the photosynthetic pathway utilized have rarely been combined.

2. Dudleya saxosa (M.F. Jones) Britton and Rose (Crassulaceae), growing in the Sonoran Desert, has very shallow roots that occupied soil volumes averaging only 43 x 10(-6) m(3) per medium-sized plant. This volume of soil can hold about the same amount of water (10 g) as can be stored in the leaves, corm and roots combined (11 g), but at a sufficiently high water potential for transfer to the plant for less than 1 week after a substantial rainfall.

3. About 80% of the net carbon dioxide uptake by D. saxosa over a 24-h period occurred during the daytime (C-3) under wet conditions, the daily total decreasing by 34% and the pattern shifting to nocturnal net CO2 uptake (CAM) after 46 days’ drought. Seventy-seven days’ drought eliminated its daily net CO2 uptake.

4. Stomatal frequency was only 67 mm(-2) on the adaxial (upper) surface and twofold lower on the abaxial surface. The cuticle was thick, 34 mu m for the adaxial surface. Leaves had 24 mesophyll cell layers, leading to a high mesophyll cell surface area per unit leaf area of 142.

5. The three leaf anatomical features plus utilization of CAM increased net CO2 uptake per unit of water transpired, and helped D. saxosa thrive in a small soil volume, with the underground corm being a major supplier of water to the succulent leaves during 2.5 months of drought. The maximum water-holding capacity of the soil explored by the roots closely matched the maximum water-holding capacity of the plant, reflecting the conservative strategy used by D. saxosa in a stressful semi-arid environment.