Science isn't only about explaining what hasn't yet been explained.

Old explanations accumulate their own inertia. It's easy to keep tweaking a familiar and trusted theory, seeking the elusive technical trick that will wrap up its loose ends, rather than making the painful and disorienting break for some really new idea. This is roughly the essence of Thomas Kuhn's rightly celebrated view of the nature of scientific revolutions.

But revolutions needn't only concern fundamental matters with earth-shaking repercussions. Kuhn could have illustrated his point with almost anything: for example, the science of plant leaves and their structure.

The networks of veins in leaves share universal features that demand explanation. All leaves have a hierarchy in their vein structure, with veins branching in a regular way down to smaller scales. Venation networks in all leaves also have a preponderance of vein channels that form complete circuits, and divide a leaf into a patchwork of larger and smaller polygons. Only the tiniest veins have exposed ends.

No theory has ever explained both of these facts successfully. The most widely accepted current 'explanation' of vein structure focuses on the role of the growth hormone, auxin, which is synthesized in growing leaves. Studies find a net flow of auxin towards the base of the leaf, from where it flows out to the rest of the plant. Genetic mutations that influence auxin production also strongly influence the vein pattern, suggesting that auxin transport really is at the heart of the story.

Still, models based on this transport picture — of the co-evolution of auxin flow and network structure, with new veins growing where flow is high — don't give a complete explanation. The resulting networks look fairly realistic, yet conspicuously lack any closed loops.

Hence the suspicion of some researchers that auxin isn't the whole story. In this regard, Yves Couder and colleagues pointed several years ago to a seemingly bizarre similarity between the vein patterns in leaves and the patterns of cracks left over when a slurry dries on a substrate. These patterns occur when one surface shrinks on top of another, leading to a characteristic pattern of stresses that produce the cracks.

This idea enters vein network science from left field, but may in fact be highly relevant. Other researchers now point out (M. F. Laguna et al. http://arxiv.org/abs/0705.0902) that a growing leaf has two epidermal layers separated by a softer tissue called mesophyll. In general, the mesophyll tends to grow faster than the epidermis, creating stresses. Cursory evidence from biological samples suggests that the cell differentiation leading to veins within leaves gets initiated at points of high stress between surfaces. A simplified model of the process shows that it reproduces the statistics of the leaf patterns quite accurately.

This model, too, is incomplete, but may go together with the traditional auxin-based story to produce something much closer to a real explanation, the result of an almost accidental observation of similar patterns in two totally different settings.

A friend of mine once puzzled me by saying that he found a certain research paper uninteresting because “we already have an explanation for that”. But science isn't only about explaining what hasn't yet been explained; it's also about exposing old and largely accepted explanations, or partial explanations, to invigorating new challenges.