Such an anatomy requires the development of two distinct chloroplast-containing cell types within the leaf; the mesophyll cells, which make up the bulk of the cells within the leaf and lie predominantly between the parallel vascular strands, and the bundle sheath cells, which form a ring around the vascular tissue forming a column of cells encasing each vascular strand throughout the leaf’s length. In C4 plants there is a distinctly different pattern of gene expression in these two cell types
(Hibberd and Covshoff, 2010),
bulk