Both Kranz anatomy, essential to C4 photosynthesis, and its developmental basis have been intensely studied in the Poaceae and Cyperaceae. In contrast, less is known about the development of Kranz anatomy in C4 dicotyledonous plants. Dense vein spacing appears to be a universal feature of Kranz anatomy, despite multiple independent evolutionary origins, and is required for C4 biochemistry to function. However, the developmental mechanisms of this evolutionary alteration in vein pattern are unknown. This study reports on Kranz anatomy, vein pattern and spacing using the genus Flaveria (Asteraceae), a useful and unusual taxon containing species classified as C3, C4 or C3-C4 (an intermediate). Data show a gradation in many anatomical characters between the C3, C3-C4 and C4 species. C4 Flaveria species show the highest vein density and highest numbers of vein branches and freely ending veinlets, whereas C3 Flaveria species have the lowest vein density, and lowest numbers of branch points and freely ending veinlets. The higher order veins appear to be responsible for the change in vein density by becoming more highly branched. Interveinal distance is lowest with the least number of cells occurring between veins in the C4 species, highest in the C3 species and intermediate in the C3-C4 species. The PCA: PCR (mesophyll: bundle sheath) ratio is lowest in C4 Flaveria species and highest in C3 and C3-C4 Flaveria species. The bundle sheath cells in the C3-C4 intermediate species are undifferentiated from mesophyll cells, suggesting that an alternation in vein spacing occurs earlier in the evolution of C4 photosynthesis than the differentiation of the two photosynthetic cell types in Kranz anatomy or alteration in their ratios to one another.

Key words: Asteraceae, C4 photosynthesis, Flaveria, Kranz anatomy, vein pattern