Pollen’s energy-efficient water conservation

The outer layer of a pollen-grain wallgenerally includes apertures through which the grain can gain or lose water.When in an arid environment, pollen grains avoid becoming dangerously dry byundergoing a process called harmomegathy—the grain’s apertures are effectivelysealed until the pollen lands in a wetter location. For more than a century,scientists have known that wall structure helps determine the form that apollen grain assumes after harmomegathy. Now Harvard University’s Jacques Dumais ,former Harvard student Eleni Katifori, and colleagues have presented the first quantitativemodel of the process and confirmed it with electron micrographs such as shownhere (the scale bars represent 20 µm). The model incorporates the classicresult that stretching a surface costs a lot of energy; instead of stretching,the grain surface bends as the wall folds onto itself to avoid further desiccation.The lily grain in panel a, for example, has an elongated aperture that allowsharmomegathy to proceed somewhat like the way in which one makes a cone byconnecting the edges of a disk that has had a slice removed. Strictly followed,that process yields vertices with high concentrations of bending energy; inreality the lily grain stretches a little at the vertices and ends up lookinglike a US football. The other grains illustrated in the figure have built onthe same simple physics—avoid stretching and kinks—to achieve more intricatebut equally effective harmomegathic responses. (E. Katifori et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0911223107 .) —Steven K. Blau