Plants can respond to different levels of sunlight exposure by altering characteristics of internal leaf structure such as the amount and distribution of palisade and spongy mesophyll tissue, as well as external leaf structure. Recently, many of these structural changes have been associated with the regulation of light and CO2 profiles inside leaves (Parkhurst and Mott, 1990 ; Parkhurst, 1994 ; Terashima and Hikosaka, 1995 ; Vogelmann, Nishio, and Smith, 1996 ), and it has been proposed that these structural changes have evolved in concert with properties of leaf orientation, maximizing photosynthesis per unit leaf biomass (Smith et al., 1997 ; Smith, Bell, and Shepherd, 1998 ). For example, the total surface area of mesophyll cells exposed to intercellular airspace (Ames) beneath a unit area of one leaf surface (A) has been shown to have a strong influence on photosynthesis per unit leaf area (Nobel, Zaragoza, and Smith, 1975 ; Nobel and Walker, 1985 ), although some species do not appear to have a strong association between Ames/A and photosynthetic gas exchange (Araus et al., 1986 ; Syvertson et al., 1995 ). This ratio of the internal to external leaf area (Ames/A) represents the area available for internal CO2 absorption beneath a unit area of leaf surface available for light absorption and CO2 uptake from the atmosphere (Björkman, 1981 ; Nobel and Walker, 1985 ; Nobel, 1991 ). Ames/A can respond strongly to the amount of incident sunlight (Nobel, 1991 ) and other environmental conditions (Smith and Nobel, 1978 ). Other studies have shown contrasting results (Araus, et al., 1986 ; Buisson and Lee, 1993 ; Syvertsen et al., 1995 ).
Leaf anatomy and morphology in Eucalyptus globulus ssp. globulus Labill (Tasmanian blue-gum) change dramatically with development from the seedling to adult tree. This vegetative phase change consists of a shift from juvenile to adult leaf type and generally begins at 1–3 yr of age. Juvenile leaves of young seedlings ( in structure, hypostomatous, and approximately horizontal in orientation (Johnson, 1926 ; Jacobs, 1955 ; Penfold and Willis, 1956 ; Pryor, 1976 ; FAO, 1979 ). In contrast, adult leaves of mature trees (>5 yr old) are dark green in color, isobilateral, amphistomatous, and pendent. Transitional leaves are produced over a range of nodes and have structural features that are intermediate between juvenile and adult leaves.
The present study measured Ames/A, chlorophyll content, and related structural parameters incrementally throughout the mesophyll of all three, ontogenetic leaf types in E. globulus using light microscopy and image analysis. The correspondence between mesophyll structure, particularly Ames/A, and chlorophyll content at specific locations within the mesophyll should provide further evidence for the evolution of a concerted interaction between leaf orientation and internal leaf structure for photosynthetic enhancement (Smith et al., 1997 ).