Temporal and spatial patterns of leaf unrolling in Pilea peperomioides

The processes of cellular differentiation and tissue maturation in the early phases of leaf development have been the subject of extensive investigation. During leaf opening, however, a subsequent stage of development, the dynamics of cell and tissue remain poorly understood, particularly in peltate species. In peltate leaves, the petiole merges into the lamina at a more central position, and leaf opening often consists of unrolling. This study focuses on the peltate leaves of Pilea peperomioides and investigates structural changes during unrollingvia histological analysis. Lamina cross-sections were used to examine different tissues at various stages of development. Moreover, investigating leaf curvature across stages allowed the determination of the exact mechanism and behavior. In general, these leaves massively grow in width and thickness as they unroll. Thedominant tissues are the spongy parenchyma and the hypodermal hydrenchyma, a tissue designated for water storage. The latter essentially contributes to the total increase in thickness. From a geometrical perspective, an increase in lamina thickness imposes a spatial constraint and increases the second moment of area and, consequently, the flexural rigidity. These changes lead to curvature reduction and progressive stiffening of the leaf, 'locking' it into a flat configuration.