In addition to the replicated evolutionary “experiments” provided by the repeated, convergent evolution of C4 photosynthesis, many lineages provide us with another valuable path to discovery: extant C3-C4 intermediate species. When a species with an intermediate phenotype also branches in a phylogenetically intermediate position, it is reasonable to interpret this as representing an ancestral state that has persisted to the present. About one quarter of C4 lineages are known or suspected to include living intermediates, and these all utilize a photorespiratory glycine shuttling pathway termed C2 photosynthesis, after the two carbon atoms in glycine.¹ In C2 species, glycine decarboxylase, the enzyme responsible for the loss of CO2 during photorespiration, is restricted to the vascular sheath cells. This modification promotes re-fixation of CO2, concentrates CO2 in the sheath cells, and establishes the two-cell division of photosynthetic labour characteristic of C4 photosynthesis.² The C2 pathway is thus thought to be an essential evolutionary “stepping stone” between C3 and C4 photosynthesis. The current model of C4 evolution occurs in a stepwise manner, with the earliest stages being an increase in photosynthetic capacity and organelle investment in the vascular sheath cells. Concurrent with these change the mitochondria and some of the chloroplasts localize to the centripetal region of the sheath cells, in an arrangement known as Proto-Kranz. Because glycine decarboxylase is located in the mitochondria, their centripetal aggregation with chloroplasts promotes the scavenging of photorespiratory CO2.³ From a Proto-Kranz state, a gradual downregulation of glycine decarboxylase in the mesophyll establishes the C2 pathway.^4,5 In contrast to the C3-C2 transition, the evolutionary upregulation, tissue-specific compartmentation, and integration of C4 metabolism is more poorly understood.