Presumable mesophyll optics of C3 plants reconsidered

The palisade and spongy chlorenchymata typical of C-3 plant leaves are hypothesized to interact differently with the light penetrating the leaf. The palisade chlorenchyma allegedly transmits the light into the deep mesophyll, whereas the spongy one scatters transmitted light and reflects it back to the palisade tissue. These tissues are concluded to function this way only under artificial experimental conditions which are radically different from the natural environments. Existence of the palisade and spongy chlorenchymata in a leaf is not caused by their alleged differences in optical traits. They jointly optimize the leaf photosynthesis, but in another way. The palisade chlorenchyma maximizes the number of intercellular airspace-exposed chloroplasts per unit of mesophyll volume and can thereby consume up to 95% of the light entering. It also maintains necessary cross-leaf diffusion of gaseous CO2, but it is inefficient in lateral (paradermal) transmitting of CO2 and solutes. The spongy chlorenchyma contains much less chloroplasts per unit of mesophyll volume. It is typically (much) inferior to the palisade counterpart in producing assimilates, but it is specialized in lateral transmitting of solutes and CO2. Haberlandt's old assumptions about functional specificity of the spongy chlorenchyma in a leaf remain speculative. However, they seem to be more consonant to reality.