The photosynthetic characteristics of fifty-six monocotyledonous plants belonging to seventeen families were examined. As many as twenty-three plants, hitherto unreported, showed C4 pathway characteristics. C4 species were recorded in ten new genera. The C4 plants were confined to the families, Cyperaceae and Gramineae. The findings confirm the highly evolved nature of the tribes Andropogoneae and Paniceae in contrast to the primitive features of Agrosteae and Festuceae. On the basis of photosynthetic characteristics, it is suggested that Aristida should be included in a separate tribe, Aristideae, whereas Eragrostis resembles more panicoid grasses rather than the members of Festuceae. The importance of the C4 syndrome in relation to the evolution of grasses is stressed.