May 26, 2013
Leaves carry out photosynthesis in clusters of cells locally serviced by end-units of pipes that deliver water and take away sugar. The cells must be within diffusion distance of pores to exchange carbon dioxide and oxygen. Diffusion may set an upper limit on the size of an efficient cluster, and quasi-fractal branching of pipelines may set a lower limit. Accordingly, many species of local woody plants show photosynthetic clusters of approximately uniform size, and these clusters themselves group hierarchically into neighborhoods of successively larger sizes. The photo fields are 4 millimeters wide, except for the aerial photo of human neighborhoods.