On the biological shape of the Polygonaceae Rheum petiole

The petiole is a plant organ that connects the stem to the blade of a leaf. The petiole is made up of a fi brous biomaterial that consists of three integrated tissues. Each of these specializes in different functions, but they work together to provide basic needs, such as nutrient transport, food storage and plant support. From a structural viewpoint, the petiole resembles a cantilever that should resist wind torsion and gravity bending forces acting on the leaf blade. It has a solid cross-section with a grooved fl attened asymmetric shape with size decreasing lengthwise. As all plant organs, the petiole morphology is developed by adaptive growth, which is the plant response to environmental stimuli. Thus, the petiole grows a shape that best optimizes the use of vital resources. This paper focuses on the structural effi ciency of the shape petiole and it examines the capability of the petiole in reducing the wind drag without sagging under gravity forces. Continuum mechanics and dimensionless factors are used to model the twist-to-bend ratio. Twenty specimens of Polygonaceae Rheum rhabarbarum plants were investigated. The results are visualized on maps that contrast the petiole shape properties to those of ideal cross-sections.