• Background and Aims: It is well documented that C4 grasses have a shorter distance between longitudinal veins in the leaves than C3 grasses. In grass leaves, however, veins with different structures and functions are differentiated: large longitudinal veins, small longitudinal veins and transverse veins. Thus, the densities of the three types of vein in leaves of C3 and C4 grasses were investigated from a two-dimensional perspective. • Methods: Vein densities in cleared leaves of 15 C3 and 26 C4 grasses representing different taxonomic groups and photosynthetic subtypes were analysed. • Key Results: The C 4 grasses had denser transverse veins and denser small longitudinal veins than the C3 grasses (1.9 and 2.1 times in interveinal distance), but there was no significant difference in large longitudinal veins. The total length of the three vein types per unit area in the C4 grasses was 2.1 times that in the C3 grasses. The ratio of transverse vein length to total vein length was 14.3 % in C3 grasses and 9.9 % in C4 grasses. The C3 grasses generally had greater species variation in the vascular distances than the C4 grasses. The bambusoid and panicoid C3 grasses tended to have a denser vascular system than the festucoid C3 grasses. There were no significant differences in the interveinal distances of the three vein types between C 4 subtypes, although the NADP-malic enzyme grasses tended to have a shorter distance between small longitudinal veins than the NAD-malic enzyme and phosphoenolpyruvate carboxykinase grasses. • Conclusions: It seems that C4 grasses have structurally a superior photosynthate translocation and water distribution system by developing denser networks of small longitudinal and transverse veins, while keeping a constant density of large longitudinal veins. The bambusoid and panicoid C3 grasses have a vascular system that is more similar to that in C4 grasses than to that in the festucoid C3 grasses.
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