Why leaves have stones

Is the leaf a good model for a solar cell? As is so often the case with biomimetics, the answer must be: yes and no. In its favour, the leaf is made at little energy cost with literally 'green' processing; it is fairly robust and flexible and sometimes exhibits origami-like deployable folding; it has a self-repairing capability. But in terms of efficiency and lifetime, semiconductor devices leave it in the shade. As nanocrystal solar cells have demonstrated¹, the trick is not to replicate the leaf but to figure out which principles are worth emulating — in that case, a separation of the source of charge carriers from the agents that transport them, reducing the inefficiencies caused by electron–hole recombination.

It seems quite possible that the new trick identified in leaves by Gal et al.² will also prove worth heeding in synthetic designs. They present evidence that some leaves deploy microscopic mineral nodules as light scatterers to help distribute light through the upper layers of leaf tissue, reducing losses caused by the steep gradient of illumination.

These nodules are called cystoliths, and they are made of amorphous calcium carbonate. With a 'prickly pear' shape that resembles plant spores themselves, they are typically about 50 μm across and sit embedded in the leaves of some flowering plants (angiosperms) from the surface layers (epidermis) to the deeper interior (mesophyll). They have been known since the late nineteenth century, but their function has remained mysterious.

Gal et al. considered the possibility that cystoliths have an optical function, somehow facilitating photosynthesis in the chloroplasts of the mesophyll. The high concentration of chlorophyll in this tissue means that the outermost layers stand at risk of becoming saturated with light, causing some photons to be squandered as heat or chlorophyll fluorescence, while little illumination penetrates to the lower layers.

Credit: PHILIP BALL

The researchers find that less light is 'wasted' as fluorescence for leaf tissue containing cystoliths than for tissue devoid of them. Apparently, scattering from the mineral bodies reduces the light intensity falling on the outer mesophyll and enables it to penetrate deeper into the leaf, redistributing the excess to where it can be better used.

The same seems to be true of a plant (pecan, Carya illinoinensis) that lacks cystoliths but contains instead microcrystals of calcium oxalate, called druses. Previously, druses (found also in roses and onions) have been thought to deter herbivores because of the poisonous nature of oxalate, and perhaps also to be involved in calcium regulation. As indeed they may be — but their particular morphology and distribution might confer this optical benefit too.

The photonic engineering of the leaf probably goes further. There may be a light-channelling function, for example, to the cylindrical shape of the palisade cells of the mesophyll within which the chloroplasts are concentrated. And the spongy mesophyll is already a strong scatterer, reducing light transmission through the leaf. In view of some of the remarkable optical feats known in other organisms³, perhaps none of this sophistication should surprise us. But this method of spreading light throughout an absorbing layer seems well worth copying.