Principle for measuring cell and spore hardness using the cantilever of the SPM. (A) Position of the cantilever and cell before measurement. (B) Cantilever moves a fixed distance along the z-axis (measurement). a: Start position of cantilever, b: Position of cantilever after being depressed to apply strain to the cell, ϵ: Physical strain applied by the cantilever over a fixed distance (50 nm), σ: cell physical stress (nN) (C) Diagram of physical stress states for various measurement positions. E: Young's modulus (N/m); σ: cell physical stress (nN); ϵ: physical strain applied by the cantilever (nm) Young’s modulus calculated by E = σ/ϵ. c: Stress in the cell can be accurately measured when the measurement position is the highest point along the x- and y-axes, d: Stress from the slide glass is included in measurements taken at or near the edge of the cell, e: Stress is dispersed if measurement position is slightly away from the highest point.