Organ development requires coordinated growth at multiple scales, between individual cells, adjacent tissue layers and neighbouring regions within the organ. In plants, “mechanical conflicts” between growing cells or tissues are believed to generate supra-cellular patterns of physical forces, regulating morphogenesis at the organ scale [1]. However, the shape of individual cells also controls force patterns directing cell growth [2]. To resolve this apparent contradiction, we study different plant systems using a multi-disciplinary approach, combining live-microscopy, 3D image analysis [3], genetics, mechanical manipulations [4] and modelling [5]. We propose the concept of “tissue mechanical density”, encapsulating several cellular factors that can trigger differential growth at the organ scale in plants. We demonstrate experimentally how this concept explains the development of complex shapes, such as anthers in Arabidopsis [6] and other systems.