What determines biological shape? Lessons from the dandelion pappus

The size and shape of biological cells and organs are robustly regulated, but why? To untangle factors determining biological forms, we have embarked on structural-functional studies of functional plant structures. As a pilot model for slender body structures that are universally found in living organisms, the dandelion pappus was chosen. Pappus, or plume, is a bundle of hair-like structures that enhance air drag to aid the flight of the diaspore. Pappus is likely to confer an evolutionary advantage since it has been raised in multiple clades independently.

By examining the air-pappus interactions with a bespoke wind tunnel, we uncovered a previously unobserved fluid behaviour – a vortex ring separated from the body but staying stably at a constant distance. We surveyed different levels of porosity of microfabricated 2D disks and were able to show that 90% porosity is a key design feature underlying the formation of the vortex ring. The dandelion pappus does not always remain open; it closes reversibly when wet. We analyzed the cellular deformation patterns within the flower base, the tissue acting as an actuator. Using a computational model, we found that this actuator acts like a bilayer ring with the internal vasculature ring acting as a relatively lower swelling compared to the surrounding cortex tissue. The pappus morphing is likely to have an evolutionary advantage as it increases dispersal distance by 25%, according to a fluid-dynamic seed dispersal model.

Through investigation like this, one can study the functional pulls determining biological forms during the evolution. However, they are likely not the only drivers; other factors, such as developmental genetic toolkit and energetic cost, are likely at play. A future direction will be discussed to address the why behind biological forms.