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This ontogenetic study conducted at light and electron microscope level, describes several aspects of leaf development in C3 and C4 forms of Alloteropsis semialata. Emphasis is placed on the distribution and anatomy of vascular bundles as seen in transverse sections. The derivation of the specialized photosynthetic tissue of the Kranz sheath, which characterizes the C4 form, is traced in the inner or mestome layer of the double bundle sheath. The lamina and sheath in successive leaves of comparable age in the two forms were used. The proportion of photosynthetic tissue present in sections taken at midlamina is assessed quantitatively using a planimeter. The main conclusions drawn from this study are summarized as follows: 1. lamina and sheath in leaves of A. semialata, in common with other grasses, are derived from different meristems, show different patterns of vascularization and mature at different rates, the tip representing an older portion of the leaf than the base; 2. the differences in bundle sheath anatomy between C3 (non?Kranz) and C4 (Kranz) forms of A. semialata are more obvious in the lamina than in the colourless sheath, and 3. the proportion of chlorenchymatous tissue is higher in the C4 than in the C3 leaf.